Tests pass!

.gitignore

@@ -1,3 +1,4 @@
 /target
 **/*.rs.bk
 history.txt
+tests/fixtures/nuplayground

src/cli.rs

@@ -9,13 +9,11 @@ use crate::commands::plugin::JsonRpc;
 use crate::commands::plugin::{PluginCommand, PluginSink};
 use crate::context::Context;
 crate use crate::errors::ShellError;
-use crate::evaluate::Scope;
 use crate::git::current_branch;
 use crate::object::Value;
 use crate::parser::parse::span::Spanned;
-use crate::parser::registry;
 use crate::parser::registry::CommandConfig;
-use crate::parser::{Pipeline, PipelineElement, TokenNode};
+use crate::parser::{hir, Pipeline, PipelineElement, TokenNode};
 use crate::prelude::*;
 
 use log::{debug, trace};
@@ -150,22 +148,22 @@ pub async fn cli() -> Result<(), Box<dyn Error>> {
         use crate::commands::*;
 
         context.add_commands(vec![
+            command("first", Box::new(first::first)),
+            command("pick", Box::new(pick::pick)),
+            command("from-ini", Box::new(from_ini::from_ini)),
+            command("from-csv", Box::new(from_csv::from_csv)),
+            command("from-json", Box::new(from_json::from_json)),
+            command("from-toml", Box::new(from_toml::from_toml)),
+            command("from-xml", Box::new(from_xml::from_xml)),
             command("ps", Box::new(ps::ps)),
             command("ls", Box::new(ls::ls)),
             command("sysinfo", Box::new(sysinfo::sysinfo)),
             command("cd", Box::new(cd::cd)),
-            command("first", Box::new(first::first)),
             command("size", Box::new(size::size)),
-            command("from-csv", Box::new(from_csv::from_csv)),
-            command("from-ini", Box::new(from_ini::from_ini)),
-            command("from-json", Box::new(from_json::from_json)),
-            command("from-toml", Box::new(from_toml::from_toml)),
-            command("from-xml", Box::new(from_xml::from_xml)),
             command("from-yaml", Box::new(from_yaml::from_yaml)),
             command("get", Box::new(get::get)),
             command("exit", Box::new(exit::exit)),
             command("lines", Box::new(lines::lines)),
-            command("pick", Box::new(pick::pick)),
             command("split-column", Box::new(split_column::split_column)),
             command("split-row", Box::new(split_row::split_row)),
             command("lines", Box::new(lines::lines)),
@@ -348,10 +346,11 @@ async fn process_line(readline: Result<String, ReadlineError>, ctx: &mut Context
                 _ => pipeline.commands.push(ClassifiedCommand::Sink(SinkCommand {
                     command: sink("autoview", Box::new(autoview::autoview)),
                     name_span: None,
-                    args: registry::Args {
-                        positional: None,
-                        named: None,
-                    },
+                    args: hir::Call::new(
+                        Box::new(hir::Expression::synthetic_string("autoview")),
+                        None,
+                        None,
+                    ),
                 })),
             }
 
@@ -386,7 +385,7 @@ async fn process_line(readline: Result<String, ReadlineError>, ctx: &mut Context
 
                     (Some(ClassifiedCommand::Sink(left)), None) => {
                         let input_vec: Vec<Spanned<Value>> = input.objects.into_vec().await;
-                        if let Err(err) = left.run(ctx, input_vec) {
+                        if let Err(err) = left.run(ctx, input_vec, &Text::from(line)) {
                             return LineResult::Error(line.clone(), err);
                         }
                         break;
@@ -395,20 +394,20 @@ async fn process_line(readline: Result<String, ReadlineError>, ctx: &mut Context
                     (
                         Some(ClassifiedCommand::Internal(left)),
                         Some(ClassifiedCommand::External(_)),
-                    ) => match left.run(ctx, input).await {
+                    ) => match left.run(ctx, input, Text::from(line)).await {
                         Ok(val) => ClassifiedInputStream::from_input_stream(val),
                         Err(err) => return LineResult::Error(line.clone(), err),
                     },
 
                     (Some(ClassifiedCommand::Internal(left)), Some(_)) => {
-                        match left.run(ctx, input).await {
+                        match left.run(ctx, input, Text::from(line)).await {
                             Ok(val) => ClassifiedInputStream::from_input_stream(val),
                             Err(err) => return LineResult::Error(line.clone(), err),
                         }
                     }
 
                     (Some(ClassifiedCommand::Internal(left)), None) => {
-                        match left.run(ctx, input).await {
+                        match left.run(ctx, input, Text::from(line)).await {
                             Ok(val) => ClassifiedInputStream::from_input_stream(val),
                             Err(err) => return LineResult::Error(line.clone(), err),
                         }
@@ -487,11 +486,10 @@ fn classify_command(
                 true => {
                     let command = context.get_command(name);
                     let config = command.config();
-                    let scope = Scope::empty();
 
                     trace!(target: "nu::build_pipeline", "classifying {:?}", config);
 
-                    let args = config.evaluate_args(call, context, &scope, source)?;
+                    let args: hir::Call = config.parse_args(call, context.registry(), source)?;
 
                     Ok(ClassifiedCommand::Internal(InternalCommand {
                         command,
@@ -504,9 +502,8 @@ fn classify_command(
                     true => {
                         let command = context.get_sink(name);
                         let config = command.config();
-                        let scope = Scope::empty();
 
-                        let args = config.evaluate_args(call, context, &scope, source)?;
+                        let args = config.parse_args(call, context.registry(), source)?;
 
                         Ok(ClassifiedCommand::Sink(SinkCommand {
                             command,

src/commands.rs

@@ -4,7 +4,6 @@ crate mod macros;
 crate mod args;
 crate mod autoview;
 crate mod cd;
-crate mod rm;
 crate mod classified;
 crate mod clip;
 crate mod command;
@@ -25,6 +24,7 @@ crate mod pick;
 crate mod plugin;
 crate mod ps;
 crate mod reject;
+crate mod rm;
 crate mod save;
 crate mod size;
 crate mod skip_while;
@@ -42,9 +42,9 @@ crate mod trim;
 crate mod vtable;
 crate mod where_;
 
-crate use command::command;
+crate use command::{command, filter, EvaluatedFilterCommandArgs, EvaluatedStaticCommandArgs};
 crate use config::Config;
-crate use rm::Remove;
 crate use open::Open;
+crate use rm::Remove;
 crate use skip_while::SkipWhile;
 crate use where_::Where;

src/commands/cd.rs

@@ -2,8 +2,10 @@ use crate::errors::ShellError;
 use crate::prelude::*;
 use std::env;
 
-pub fn cd(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let env = args.env.lock().unwrap();
+pub fn cd(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let env = args.env.clone();
+    let env = env.lock().unwrap();
+    let args = args.evaluate_once(registry)?;
     let cwd = env.path().to_path_buf();
 
     let path = match args.nth(0) {
@@ -13,7 +15,7 @@ pub fn cd(args: CommandArgs) -> Result<OutputStream, ShellError> {
                 return Err(ShellError::maybe_labeled_error(
                     "Can not change to home directory",
                     "can not go to home",
-                    args.call_info.name_span,
+                    args.name_span(),
                 ))
             }
         },

src/commands/classified.rs

@@ -1,6 +1,7 @@
 use crate::commands::command::Sink;
 use crate::context::SourceMap;
-use crate::parser::{registry::Args, Span, Spanned, TokenNode};
+use crate::evaluate::Scope;
+use crate::parser::{hir, Span, Spanned, TokenNode};
 use crate::prelude::*;
 use bytes::{BufMut, BytesMut};
 use futures::stream::StreamExt;
@@ -101,7 +102,7 @@ impl ClassifiedCommand {
 crate struct SinkCommand {
     crate command: Arc<dyn Sink>,
     crate name_span: Option<Span>,
-    crate args: Args,
+    crate args: hir::Call,
 }
 
 impl SinkCommand {
@@ -109,8 +110,12 @@ impl SinkCommand {
         self,
         context: &mut Context,
         input: Vec<Spanned<Value>>,
+        source: &Text,
     ) -> Result<(), ShellError> {
-        context.run_sink(self.command, self.name_span.clone(), self.args, input)
+        let args = self
+            .args
+            .evaluate(context.registry(), &Scope::empty(), source)?;
+        context.run_sink(self.command, self.name_span.clone(), args, input)
     }
 }
 
@@ -118,7 +123,7 @@ crate struct InternalCommand {
     crate command: Arc<dyn Command>,
     crate name_span: Option<Span>,
     crate source_map: SourceMap,
-    crate args: Args,
+    crate args: hir::Call,
 }
 
 impl InternalCommand {
@@ -126,11 +131,12 @@ impl InternalCommand {
         self,
         context: &mut Context,
         input: ClassifiedInputStream,
+        source: Text,
     ) -> Result<InputStream, ShellError> {
         if log_enabled!(log::Level::Trace) {
             trace!(target: "nu::run::internal", "->");
             trace!(target: "nu::run::internal", "{}", self.command.name());
-            trace!(target: "nu::run::internal", "{:?}", self.args.debug());
+            trace!(target: "nu::run::internal", "{}", self.args.debug(&source));
         }
 
         let objects: InputStream =
@@ -141,6 +147,7 @@ impl InternalCommand {
             self.name_span.clone(),
             self.source_map,
             self.args,
+            source,
             objects,
         )?;
 

src/commands/clip.rs

@@ -16,7 +16,7 @@ pub fn clip(args: SinkCommandArgs) -> Result<(), ShellError> {
             }
 
             let string = i.as_string().map_err(labelled(
-                args.call_info.name_span,
+                args.name_span(),
                 "Given non-string data",
                 "expected strings from pipeline",
             ))?;

src/commands/command.rs

@@ -1,20 +1,62 @@
-use crate::context::SourceMap;
-use crate::context::SpanSource;
+use crate::context::{SourceMap, SpanSource};
 use crate::errors::ShellError;
+use crate::evaluate::Scope;
 use crate::object::Value;
-use crate::parser::{
-    registry::{self, Args},
-    Span, Spanned,
-};
+use crate::parser::hir;
+use crate::parser::{registry, Span, Spanned};
 use crate::prelude::*;
 use getset::Getters;
 use serde::{Deserialize, Serialize};
+use std::fmt;
+use std::ops::Deref;
 use std::path::PathBuf;
 use uuid::Uuid;
 
+#[derive(Deserialize, Serialize, Debug, Clone)]
+pub struct UnevaluatedCallInfo {
+    pub args: hir::Call,
+    pub source: Text,
+    pub source_map: SourceMap,
+    pub name_span: Option<Span>,
+}
+
+impl ToDebug for UnevaluatedCallInfo {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        self.args.fmt_debug(f, source)
+    }
+}
+
+impl UnevaluatedCallInfo {
+    fn name(&self) -> Result<&str, ShellError> {
+        let head = &self.args.head();
+        match head.item() {
+            hir::RawExpression::Literal(hir::Literal::Bare) => Ok(head.span.slice(&self.source)),
+            hir::RawExpression::Literal(hir::Literal::String(span)) => Ok(span.slice(&self.source)),
+            other => Err(ShellError::type_error(
+                "Command name",
+                head.type_name().spanned(head.span),
+            )),
+        }
+    }
+
+    fn evaluate(
+        self,
+        registry: &registry::CommandRegistry,
+        scope: &Scope,
+    ) -> Result<CallInfo, ShellError> {
+        let args = self.args.evaluate(registry, scope, &self.source)?;
+
+        Ok(CallInfo {
+            args,
+            source_map: self.source_map,
+            name_span: self.name_span,
+        })
+    }
+}
+
 #[derive(Deserialize, Serialize, Debug)]
 pub struct CallInfo {
-    pub args: Args,
+    pub args: registry::EvaluatedArgs,
     pub source_map: SourceMap,
     pub name_span: Option<Span>,
 }
@@ -22,13 +64,137 @@ pub struct CallInfo {
 #[derive(Getters)]
 #[get = "crate"]
 pub struct CommandArgs {
-    pub host: Arc<Mutex<dyn Host + Send>>,
+    pub host: Arc<Mutex<dyn Host>>,
     pub env: Arc<Mutex<Environment>>,
-    pub call_info: CallInfo,
+    pub call_info: UnevaluatedCallInfo,
     pub input: InputStream,
 }
 
+impl ToDebug for CommandArgs {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        self.call_info.fmt_debug(f, source)
+    }
+}
+
 impl CommandArgs {
+    pub fn evaluate_once(
+        self,
+        registry: &registry::CommandRegistry,
+    ) -> Result<EvaluatedStaticCommandArgs, ShellError> {
+        let host = self.host.clone();
+        let env = self.env.clone();
+        let input = self.input;
+        let call_info = self.call_info.evaluate(registry, &Scope::empty())?;
+
+        Ok(EvaluatedStaticCommandArgs::new(host, env, call_info, input))
+    }
+
+    pub fn name_span(&self) -> Option<Span> {
+        self.call_info.name_span
+    }
+}
+
+pub enum EvaluatedInput {
+    Static(InputStream),
+    Filter(Spanned<Value>),
+}
+
+impl EvaluatedInput {
+    pub fn stream(self) -> InputStream {
+        match self {
+            EvaluatedInput::Static(stream) => stream,
+            EvaluatedInput::Filter(value) => vec![value].into(),
+        }
+    }
+}
+
+pub struct EvaluatedStaticCommandArgs {
+    pub args: EvaluatedCommandArgs,
+    pub input: InputStream,
+}
+
+impl Deref for EvaluatedStaticCommandArgs {
+    type Target = EvaluatedCommandArgs;
+    fn deref(&self) -> &Self::Target {
+        &self.args
+    }
+}
+
+impl EvaluatedStaticCommandArgs {
+    pub fn new(
+        host: Arc<Mutex<dyn Host>>,
+        env: Arc<Mutex<Environment>>,
+        call_info: CallInfo,
+        input: impl Into<InputStream>,
+    ) -> EvaluatedStaticCommandArgs {
+        EvaluatedStaticCommandArgs {
+            args: EvaluatedCommandArgs {
+                host,
+                env,
+                call_info,
+            },
+            input: input.into(),
+        }
+    }
+
+    pub fn name_span(&self) -> Option<Span> {
+        self.args.call_info.name_span
+    }
+
+    pub fn parts(self) -> (InputStream, registry::EvaluatedArgs) {
+        let EvaluatedStaticCommandArgs { args, input } = self;
+
+        (input, args.call_info.args)
+    }
+}
+
+#[derive(Getters)]
+#[get = "pub"]
+pub struct EvaluatedFilterCommandArgs {
+    args: EvaluatedCommandArgs,
+    input: Spanned<Value>,
+}
+
+impl Deref for EvaluatedFilterCommandArgs {
+    type Target = EvaluatedCommandArgs;
+    fn deref(&self) -> &Self::Target {
+        &self.args
+    }
+}
+
+impl EvaluatedFilterCommandArgs {
+    pub fn new(
+        host: Arc<Mutex<dyn Host>>,
+        env: Arc<Mutex<Environment>>,
+        call_info: CallInfo,
+        input: Spanned<Value>,
+    ) -> EvaluatedFilterCommandArgs {
+        EvaluatedFilterCommandArgs {
+            args: EvaluatedCommandArgs {
+                host,
+                env,
+                call_info,
+            },
+            input,
+        }
+    }
+}
+
+#[derive(Getters)]
+#[get = "crate"]
+pub struct EvaluatedCommandArgs {
+    pub host: Arc<Mutex<dyn Host>>,
+    pub env: Arc<Mutex<Environment>>,
+    pub call_info: CallInfo,
+}
+
+impl EvaluatedCommandArgs {
+    pub fn parts(self) -> () {}
+
+    pub fn call_args(&self) -> &registry::EvaluatedArgs {
+        &self.call_info.args
+    }
+
     pub fn nth(&self, pos: usize) -> Option<&Spanned<Value>> {
         self.call_info.args.nth(pos)
     }
@@ -61,6 +227,12 @@ pub struct SinkCommandArgs {
     pub input: Vec<Spanned<Value>>,
 }
 
+impl SinkCommandArgs {
+    pub fn name_span(&self) -> Option<Span> {
+        self.call_info.name_span
+    }
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub enum CommandAction {
     ChangePath(PathBuf),
@@ -100,8 +272,12 @@ impl ReturnSuccess {
     }
 }
 
-pub trait Command {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError>;
+pub trait Command: Send + Sync {
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError>;
     fn name(&self) -> &str;
 
     fn config(&self) -> registry::CommandConfig {
@@ -132,14 +308,74 @@ pub trait Sink {
     }
 }
 
-pub struct FnCommand {
+pub struct FnFilterCommand {
     name: String,
-    func: Box<dyn Fn(CommandArgs) -> Result<OutputStream, ShellError>>,
+    func: fn(EvaluatedFilterCommandArgs) -> Result<OutputStream, ShellError>,
 }
 
-impl Command for FnCommand {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
-        (self.func)(args)
+impl Command for FnFilterCommand {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        let CommandArgs {
+            host,
+            env,
+            call_info,
+            input,
+        } = args;
+
+        let host: Arc<Mutex<dyn Host>> = host.clone();
+        let env: Arc<Mutex<Environment>> = env.clone();
+        let registry: registry::CommandRegistry = registry.clone();
+        let func = self.func;
+
+        let result = input.values.map(move |it| {
+            let registry = registry.clone();
+            let call_info = match call_info
+                .clone()
+                .evaluate(&registry, &Scope::it_value(it.clone()))
+            {
+                Err(err) => return OutputStream::from(vec![Err(err)]).values,
+                Ok(args) => args,
+            };
+
+            let args = EvaluatedFilterCommandArgs::new(host.clone(), env.clone(), call_info, it);
+
+            match func(args) {
+                Err(err) => return OutputStream::from(vec![Err(err)]).values,
+                Ok(stream) => stream.values,
+            }
+        });
+
+        let result = result.flatten();
+        let result: BoxStream<ReturnValue> = result.boxed();
+
+        Ok(result.into())
+    }
+}
+
+pub struct FnRawCommand {
+    name: String,
+    func: Box<
+        dyn Fn(CommandArgs, &registry::CommandRegistry) -> Result<OutputStream, ShellError>
+            + Send
+            + Sync,
+    >,
+}
+
+impl Command for FnRawCommand {
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        (self.func)(args, registry)
     }
 
     fn name(&self) -> &str {
@@ -149,9 +385,23 @@ impl Command for FnCommand {
 
 pub fn command(
     name: &str,
-    func: Box<dyn Fn(CommandArgs) -> Result<OutputStream, ShellError>>,
+    func: Box<
+        dyn Fn(CommandArgs, &registry::CommandRegistry) -> Result<OutputStream, ShellError>
+            + Send
+            + Sync,
+    >,
 ) -> Arc<dyn Command> {
-    Arc::new(FnCommand {
+    Arc::new(FnRawCommand {
+        name: name.to_string(),
+        func,
+    })
+}
+
+pub fn filter(
+    name: &str,
+    func: fn(EvaluatedFilterCommandArgs) -> Result<OutputStream, ShellError>,
+) -> Arc<dyn Command> {
+    Arc::new(FnFilterCommand {
         name: name.to_string(),
         func,
     })

src/commands/config.rs

@@ -1,10 +1,10 @@
 use crate::prelude::*;
 
+use crate::commands::EvaluatedStaticCommandArgs;
 use crate::errors::ShellError;
-use crate::object::config;
-use crate::object::Value;
+use crate::object::{config, Value};
 use crate::parser::hir::SyntaxType;
-use crate::parser::registry::{CommandConfig, NamedType};
+use crate::parser::registry::{self, CommandConfig, NamedType};
 use indexmap::IndexMap;
 use log::trace;
 use std::iter::FromIterator;
@@ -12,9 +12,15 @@ use std::iter::FromIterator;
 pub struct Config;
 
 impl Command for Config {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        let args = args.evaluate_once(registry)?;
         config(args)
     }
+
     fn name(&self) -> &str {
         "config"
     }
@@ -38,11 +44,11 @@ impl Command for Config {
     }
 }
 
-pub fn config(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let mut result = crate::object::config::config(args.call_info.name_span)?;
+pub fn config(args: EvaluatedStaticCommandArgs) -> Result<OutputStream, ShellError> {
+    let mut result = crate::object::config::config(args.name_span())?;
 
-    trace!("{:#?}", args.call_info.args.positional);
-    trace!("{:#?}", args.call_info.args.named);
+    trace!("{:#?}", args.call_args().positional);
+    trace!("{:#?}", args.call_args().named);
 
     if let Some(v) = args.get("get") {
         let key = v.as_string()?;
@@ -95,7 +101,7 @@ pub fn config(args: CommandArgs) -> Result<OutputStream, ShellError> {
     }
 
     if args.len() == 0 {
-        return Ok(vec![Value::Object(result.into()).spanned(args.call_info.name_span)].into());
+        return Ok(vec![Value::Object(result.into()).spanned(args.name_span())].into());
     }
 
     Err(ShellError::string(format!("Unimplemented")))

src/commands/exit.rs

@@ -2,6 +2,6 @@ use crate::commands::command::CommandAction;
 use crate::errors::ShellError;
 use crate::prelude::*;
 
-pub fn exit(_args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn exit(_args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
     Ok(vec![Ok(ReturnSuccess::Action(CommandAction::Exit))].into())
 }

src/commands/first.rs

@@ -1,14 +1,17 @@
 use crate::errors::ShellError;
+use crate::parser::CommandRegistry;
 use crate::prelude::*;
 
 // TODO: "Amount remaining" wrapper
 
-pub fn first(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn first(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+
     if args.len() == 0 {
         return Err(ShellError::maybe_labeled_error(
             "First requires an amount",
             "needs parameter",
-            args.call_info.name_span,
+            args.name_span(),
         ));
     }
 
@@ -25,7 +28,7 @@ pub fn first(args: CommandArgs) -> Result<OutputStream, ShellError> {
         }
     };
 
-    let input = args.input;
-
-    Ok(OutputStream::from_input(input.values.take(amount as u64)))
+    Ok(OutputStream::from_input(
+        args.input.values.take(amount as u64),
+    ))
 }

src/commands/from_csv.rs

@@ -6,7 +6,6 @@ pub fn from_csv_string_to_value(
     s: String,
     span: impl Into<Span>,
 ) -> Result<Spanned<Value>, Box<dyn std::error::Error>> {
-
     let mut reader = ReaderBuilder::new()
         .has_headers(false)
         .from_reader(s.as_bytes());
@@ -49,9 +48,10 @@ pub fn from_csv_string_to_value(
     })
 }
 
-pub fn from_csv(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_csv(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
 
     Ok(out
         .values

src/commands/from_ini.rs

@@ -37,9 +37,11 @@ pub fn from_ini_string_to_value(
     Ok(convert_ini_top_to_nu_value(&v, span))
 }
 
-pub fn from_ini(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_ini(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
+
     Ok(out
         .values
         .map(move |a| match a.item {

src/commands/from_json.rs

@@ -43,9 +43,14 @@ pub fn from_json_string_to_value(
     Ok(convert_json_value_to_nu_value(&v, span))
 }
 
-pub fn from_json(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_json(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
+
     Ok(out
         .values
         .map(move |a| match a.item {

src/commands/from_toml.rs

@@ -41,9 +41,14 @@ pub fn from_toml_string_to_value(
     Ok(convert_toml_value_to_nu_value(&v, span))
 }
 
-pub fn from_toml(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_toml(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
+
     Ok(out
         .values
         .map(move |a| match a.item {

src/commands/from_xml.rs

@@ -59,9 +59,10 @@ pub fn from_xml_string_to_value(
     Ok(from_document_to_value(&parsed, span))
 }
 
-pub fn from_xml(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_xml(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
     Ok(out
         .values
         .map(move |a| match a.item {

src/commands/from_yaml.rs

@@ -48,9 +48,13 @@ pub fn from_yaml_string_to_value(
     Ok(convert_yaml_value_to_nu_value(&v, span))
 }
 
-pub fn from_yaml(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn from_yaml(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let span = args.name_span();
     let out = args.input;
-    let span = args.call_info.name_span;
+
     Ok(out
         .values
         .map(move |a| match a.item {

src/commands/get.rs

@@ -21,36 +21,37 @@ fn get_member(path: &str, span: Span, obj: &Spanned<Value>) -> Result<Spanned<Va
     Ok(current.clone())
 }
 
-pub fn get(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    if args.len() == 0 {
+pub fn get(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
+    let len = args.len();
+
+    if len == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Get requires a field or field path",
             "needs parameter",
-            args.call_info.name_span,
+            span,
         ));
     }
 
     let amount = args.expect_nth(0)?.as_i64();
+    let (input, args) = args.parts();
+    let positional = args.positional;
 
     // If it's a number, get the row instead of the column
     if let Ok(amount) = amount {
-        return Ok(args
-            .input
-            .values
-            .skip(amount as u64)
-            .take(1)
-            .from_input_stream());
+        return Ok(input.values.skip(amount as u64).take(1).from_input_stream());
     }
 
-    let fields: Result<Vec<(String, Span)>, _> = args
-        .positional_iter()
+    let fields: Result<Vec<(String, Span)>, _> = positional
+        .iter()
+        .flatten()
         .map(|a| (a.as_string().map(|x| (x, a.span))))
         .collect();
 
     let fields = fields?;
 
-    let stream = args
-        .input
+    let stream = input
         .values
         .map(move |item| {
             let mut result = VecDeque::new();

src/commands/lines.rs

@@ -5,9 +5,12 @@ use log::trace;
 
 // TODO: "Amount remaining" wrapper
 
-pub fn lines(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn lines(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
     let input = args.input;
-    let span = args.call_info.name_span;
+
+    let input: InputStream = trace_stream!(target: "nu::trace_stream::lines", "input" = input);
 
     let stream = input
         .values

src/commands/ls.rs

@@ -4,8 +4,10 @@ use crate::parser::Spanned;
 use crate::prelude::*;
 use std::path::{Path, PathBuf};
 
-pub fn ls(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let env = args.env.lock().unwrap();
+pub fn ls(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let env = args.env.clone();
+    let env = env.lock().unwrap();
+    let args = args.evaluate_once(registry)?;
     let path = env.path.to_path_buf();
     let mut full_path = PathBuf::from(path);
     match &args.nth(0) {
@@ -30,7 +32,7 @@ pub fn ls(args: CommandArgs) -> Result<OutputStream, ShellError> {
                 return Err(ShellError::maybe_labeled_error(
                     e.to_string(),
                     e.to_string(),
-                    args.call_info.name_span,
+                    args.name_span(),
                 ));
             }
         }
@@ -40,7 +42,7 @@ pub fn ls(args: CommandArgs) -> Result<OutputStream, ShellError> {
     let mut shell_entries = VecDeque::new();
 
     for entry in entries {
-        let value = dir_entry_dict(&entry?, args.call_info.name_span)?;
+        let value = dir_entry_dict(&entry?, args.name_span())?;
         shell_entries.push_back(ReturnSuccess::value(value))
     }
     Ok(shell_entries.to_output_stream())

src/commands/macros.rs

@@ -36,13 +36,14 @@ macro_rules! command {
         pub struct $export;
 
         impl Command for $export {
-            fn run(&self, $args: CommandArgs) -> Result<OutputStream, ShellError> {
-                fn command($args: CommandArgs, ( $($param_name),*, ): ( $($param_type),*, )) -> Result<OutputStream, ShellError> {
+            fn run(&self, $args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+                fn command($args: EvaluatedCommandArgs, ( $($param_name),*, ): ( $($param_type),*, )) -> Result<OutputStream, ShellError> {
                     let output = $body;
 
                     Ok(output.boxed().to_output_stream())
                 }
 
+                let $args = $args.evaluate_once(registry)?;
                 let tuple = ( $($extract ,)* );
                 command( $args, tuple )
             }

src/commands/open.rs

@@ -1,68 +1,151 @@
 use crate::context::SpanSource;
 use crate::errors::ShellError;
 use crate::object::{Primitive, Switch, Value};
+use crate::parser::hir::SyntaxType;
 use crate::parser::parse::span::Span;
+use crate::parser::registry::{self, CommandConfig, NamedType, PositionalType};
 use crate::prelude::*;
+use indexmap::IndexMap;
 use mime::Mime;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use uuid::Uuid;
 
-command! {
-    Open as open(args, path: Spanned<PathBuf>, --raw: Switch,) {
-        let span = args.call_info.name_span;
+pub struct Open;
 
-        let cwd = args
-            .env
-            .lock()
-            .unwrap()
-            .path()
-            .to_path_buf();
+impl Command for Open {
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        let env = args.env.clone();
+        let args = args.evaluate_once(registry)?;
+        let path = <Spanned<PathBuf>>::extract(args.expect_nth(0)?)?;
+        let raw = args.has("raw");
 
+        let span = args.name_span();
+
+        let cwd = env.lock().unwrap().path().to_path_buf();
         let full_path = PathBuf::from(cwd);
 
-        let path_str = path.to_str().ok_or(ShellError::type_error("Path", "invalid path".spanned(path.span)))?;
+        let path_str = path.to_str().ok_or(ShellError::type_error(
+            "Path",
+            "invalid path".spanned(path.span),
+        ))?;
 
-        let (file_extension, contents, contents_span, span_source) = fetch(&full_path, path_str, path.span)?;
+        let (file_extension, contents, contents_span, span_source) =
+            fetch(&full_path, path_str, path.span)?;
 
-        let file_extension = if raw.is_present() {
-            None
-        } else {
-            file_extension
-        };
+        let file_extension = if raw { None } else { file_extension };
 
         let mut stream = VecDeque::new();
 
         if let Some(uuid) = contents_span.source {
             // If we have loaded something, track its source
-            stream.push_back(ReturnSuccess::action(CommandAction::AddSpanSource(uuid, span_source)))
+            stream.push_back(ReturnSuccess::action(CommandAction::AddSpanSource(
+                uuid,
+                span_source,
+            )))
         }
 
         match contents {
             Value::Primitive(Primitive::String(string)) => {
-                let value = parse_as_value(
-                    file_extension,
-                    string,
-                    contents_span,
-                    span,
-                )?;
+                let value = parse_as_value(file_extension, string, contents_span, span)?;
 
                 match value {
-                    Spanned { item: Value::List(list), .. } => {
+                    Spanned {
+                        item: Value::List(list),
+                        ..
+                    } => {
                         for elem in list {
                             stream.push_back(ReturnSuccess::value(elem));
                         }
                     }
-                    x => stream.push_back(ReturnSuccess::value(x))
+                    x => stream.push_back(ReturnSuccess::value(x)),
+                }
             }
-            },
 
             other => stream.push_back(ReturnSuccess::value(other.spanned(contents_span))),
         };
 
-        stream
+        Ok(stream.boxed().to_output_stream())
+    }
+
+    fn name(&self) -> &str {
+        "open"
+    }
+
+    fn config(&self) -> CommandConfig {
+        let mut named = IndexMap::default();
+        named.insert("raw".to_string(), NamedType::Switch);
+
+        CommandConfig {
+            name: self.name().to_string(),
+            positional: vec![PositionalType::mandatory("path", SyntaxType::Block)],
+            rest_positional: false,
+            named,
+            is_sink: true,
+            is_filter: false,
         }
     }
+}
+
+// command! {
+//     Open as open(args, path: Spanned<PathBuf>, --raw: Switch,) {
+//         let span = args.name_span();
+//         let env = args.env.clone();
+
+//         let path = env
+//             .lock()
+//             .unwrap()
+//             .path()
+//             .to_path_buf();
+
+//         let full_path = PathBuf::from(cwd);
+
+//         let path_str = path.to_str().ok_or(ShellError::type_error("Path", "invalid path".spanned(path.span)))?;
+
+//         let (file_extension, contents, contents_span, span_source) = fetch(&full_path, path_str, path.span)?;
+
+//         let file_extension = if raw.is_present() {
+//             None
+//         } else {
+//             file_extension
+//         };
+
+//         let mut stream = VecDeque::new();
+
+//         if let Some(uuid) = contents_span.source {
+//             // If we have loaded something, track its source
+//             stream.push_back(ReturnSuccess::action(CommandAction::AddSpanSource(uuid, span_source)))
+//         }
+
+//         match contents {
+//             Value::Primitive(Primitive::String(string)) => {
+//                 let value = parse_as_value(
+//                     file_extension,
+//                     string,
+//                     contents_span,
+//                     span,
+//                 )?;
+
+//                 match value {
+//                     Spanned { item: Value::List(list), .. } => {
+//                         for elem in list {
+//                             stream.push_back(ReturnSuccess::value(elem));
+//                         }
+//                     }
+//                     x => stream.push_back(ReturnSuccess::value(x))
+//                 }
+//             },
+
+//             other => stream.push_back(ReturnSuccess::value(other.spanned(contents_span))),
+//         };
+
+//         stream
+//     }
+// }
 
 pub fn fetch(
     cwd: &PathBuf,

src/commands/pick.rs

@@ -1,19 +1,30 @@
+use crate::context::CommandRegistry;
 use crate::errors::ShellError;
 use crate::object::base::select_fields;
 use crate::prelude::*;
 
-pub fn pick(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    if args.len() == 0 {
+pub fn pick(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let len = args.len();
+    let span = args.name_span();
+    let (input, args) = args.parts();
+
+    if len == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Pick requires fields",
             "needs parameter",
-            args.call_info.name_span,
+            span,
         ));
     }
 
-    let fields: Result<Vec<String>, _> = args.positional_iter().map(|a| a.as_string()).collect();
+    let fields: Result<Vec<String>, _> = args
+        .positional
+        .iter()
+        .flatten()
+        .map(|a| a.as_string())
+        .collect();
+
     let fields = fields?;
-    let input = args.input;
 
     let objects = input
         .values

src/commands/plugin.rs

@@ -41,8 +41,12 @@ pub struct PluginCommand {
 }
 
 impl Command for PluginCommand {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
-        filter_plugin(self.path.clone(), args)
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        filter_plugin(self.path.clone(), args, registry)
     }
     fn name(&self) -> &str {
         &self.name
@@ -71,7 +75,13 @@ impl Sink for PluginSink {
     }
 }
 
-pub fn filter_plugin(path: String, args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn filter_plugin(
+    path: String,
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+
     let mut child = std::process::Command::new(path)
         .stdin(std::process::Stdio::piped())
         .stdout(std::process::Stdio::piped())
@@ -84,7 +94,7 @@ pub fn filter_plugin(path: String, args: CommandArgs) -> Result<OutputStream, Sh
 
         let mut reader = BufReader::new(stdout);
 
-        let request = JsonRpc::new("begin_filter", args.call_info);
+        let request = JsonRpc::new("begin_filter", args.args.call_info);
         let request_raw = serde_json::to_string(&request).unwrap();
         stdin.write(format!("{}\n", request_raw).as_bytes())?;
         let mut input = String::new();

src/commands/ps.rs

@@ -3,14 +3,14 @@ use crate::object::process::process_dict;
 use crate::prelude::*;
 use sysinfo::{RefreshKind, SystemExt};
 
-pub fn ps(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn ps(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
     let mut system = sysinfo::System::new_with_specifics(RefreshKind::new().with_processes());
     system.refresh_processes();
     let list = system.get_process_list();
 
     let list = list
         .into_iter()
-        .map(|(_, process)| process_dict(process, args.call_info.name_span))
+        .map(|(_, process)| process_dict(process, args.name_span()))
         .collect::<VecDeque<_>>();
 
     Ok(list.from_input_stream())

src/commands/reject.rs

@@ -2,21 +2,31 @@ use crate::errors::ShellError;
 use crate::object::base::reject_fields;
 use crate::prelude::*;
 
-pub fn reject(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let name_span = args.call_info.name_span;
+pub fn reject(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let name_span = args.name_span();
+    let args = args.evaluate_once(registry)?;
+    let len = args.len();
+    let span = args.name_span();
+    let (input, args) = args.parts();
 
-    if args.len() == 0 {
+    if len == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Reject requires fields",
             "needs parameter",
-            args.call_info.name_span,
+            span,
         ));
     }
 
-    let fields: Result<Vec<String>, _> = args.positional_iter().map(|a| a.as_string()).collect();
+    let fields: Result<Vec<String>, _> = args
+        .positional
+        .iter()
+        .flatten()
+        .map(|a| a.as_string())
+        .collect();
+
     let fields = fields?;
 
-    let stream = args.input.values.map(move |item| {
+    let stream = input.values.map(move |item| {
         reject_fields(&item, &fields, item.span)
             .into_spanned_value()
             .spanned(name_span)

src/commands/rm.rs

@@ -1,3 +1,4 @@
+use crate::commands::EvaluatedStaticCommandArgs;
 use crate::errors::ShellError;
 use crate::parser::hir::SyntaxType;
 use crate::parser::registry::{CommandConfig, NamedType, PositionalType};
@@ -7,8 +8,13 @@ use indexmap::IndexMap;
 pub struct Remove;
 
 impl Command for Remove {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
-        rm(args)
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        let env = args.env.clone();
+        rm(args.evaluate_once(registry)?, env)
     }
 
     fn name(&self) -> &str {
@@ -30,8 +36,11 @@ impl Command for Remove {
     }
 }
 
-pub fn rm(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let mut full_path = args.env.lock().unwrap().path().to_path_buf();
+pub fn rm(
+    args: EvaluatedStaticCommandArgs,
+    env: Arc<Mutex<Environment>>,
+) -> Result<OutputStream, ShellError> {
+    let mut full_path = env.lock().unwrap().path().to_path_buf();
 
     match args
         .nth(0)
@@ -48,7 +57,7 @@ pub fn rm(args: CommandArgs) -> Result<OutputStream, ShellError> {
             return Err(ShellError::labeled_error(
                 "is a directory",
                 "",
-                args.call_info.name_span.unwrap(),
+                args.name_span().unwrap(),
             ));
         }
         std::fs::remove_dir_all(&full_path).expect("can not remove directory");

src/commands/save.rs

@@ -13,7 +13,7 @@ pub fn save(args: SinkCommandArgs) -> Result<(), ShellError> {
         return Err(ShellError::maybe_labeled_error(
             "Save requires a filepath",
             "needs path",
-            args.call_info.name_span,
+            args.name_span(),
         ));
     }
 

src/commands/size.rs

@@ -2,7 +2,7 @@ use crate::errors::ShellError;
 use crate::object::{SpannedDictBuilder, Value};
 use crate::prelude::*;
 
-pub fn size(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn size(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
     let input = args.input;
     Ok(input
         .values

src/commands/skip_while.rs

@@ -6,8 +6,12 @@ use crate::prelude::*;
 pub struct SkipWhile;
 
 impl Command for SkipWhile {
-    fn run(&self, args: CommandArgs) -> Result<OutputStream, ShellError> {
-        skip_while(args)
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        skip_while(args, registry)
     }
     fn name(&self) -> &str {
         "skip-while"
@@ -25,18 +29,24 @@ impl Command for SkipWhile {
     }
 }
 
-pub fn skip_while(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    if args.len() == 0 {
+pub fn skip_while(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let block = args.expect_nth(0)?.as_block()?;
+    let span = args.name_span();
+    let len = args.len();
+    let input = args.input;
+
+    if len == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Where requires a condition",
             "needs condition",
-            args.call_info.name_span,
+            span,
         ));
     }
 
-    let block = args.nth(0).unwrap().as_block()?;
-    let input = args.input;
-
     let objects = input.values.skip_while(move |item| {
         let result = block.invoke(&item);
 

src/commands/sort_by.rs

@@ -1,11 +1,20 @@
 use crate::errors::ShellError;
 use crate::prelude::*;
 
-pub fn sort_by(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let fields: Result<Vec<_>, _> = args.positional_iter().map(|a| a.as_string()).collect();
+pub fn sort_by(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let (input, args) = args.parts();
+
+    let fields: Result<Vec<_>, _> = args
+        .positional
+        .iter()
+        .flatten()
+        .map(|a| a.as_string())
+        .collect();
+
     let fields = fields?;
 
-    let output = args.input.values.collect::<Vec<_>>();
+    let output = input.values.collect::<Vec<_>>();
 
     let output = output.map(move |mut vec| {
         vec.sort_by_key(|item| {

src/commands/split_column.rs

@@ -3,20 +3,24 @@ use crate::object::{Primitive, SpannedDictBuilder, Value};
 use crate::prelude::*;
 use log::trace;
 
-pub fn split_column(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let positional: Vec<_> = args.positional_iter().cloned().collect();
-    let span = args.call_info.name_span;
+pub fn split_column(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
+    let (input, args) = args.parts();
+
+    let positional: Vec<_> = args.positional.iter().flatten().cloned().collect();
 
     if positional.len() == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Split-column needs more information",
             "needs parameter (eg split-column \",\")",
-            args.call_info.name_span,
+            span,
         ));
     }
 
-    let input = args.input;
-
     Ok(input
         .values
         .map(move |v| match v.item {

src/commands/split_row.rs

@@ -4,20 +4,25 @@ use crate::parser::Spanned;
 use crate::prelude::*;
 use log::trace;
 
-pub fn split_row(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let positional: Vec<Spanned<Value>> = args.positional_iter().cloned().collect();
-    let span = args.call_info.name_span;
+pub fn split_row(
+    args: CommandArgs,
+    registry: &CommandRegistry,
+) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let span = args.name_span();
+    let len = args.len();
+    let (input, args) = args.parts();
 
-    if positional.len() == 0 {
+    let positional: Vec<Spanned<Value>> = args.positional.iter().flatten().cloned().collect();
+
+    if len == 0 {
         return Err(ShellError::maybe_labeled_error(
             "Split-row needs more information",
             "needs parameter (eg split-row \"\\n\")",
-            args.call_info.name_span,
+            span,
         ));
     }
 
-    let input = args.input;
-
     let stream = input
         .values
         .map(move |v| match v.item {

src/commands/sysinfo.rs

@@ -6,8 +6,8 @@ use crate::prelude::*;
 use sys_info::*;
 use sysinfo::{ComponentExt, DiskExt, NetworkExt, RefreshKind, SystemExt};
 
-pub fn sysinfo(args: CommandArgs) -> Result<OutputStream, ShellError> {
-    let name_span = args.call_info.name_span;
+pub fn sysinfo(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let name_span = args.name_span();
     let mut idx = SpannedDictBuilder::new(name_span);
 
     if let (Ok(name), Ok(version)) = (os_type(), os_release()) {

src/commands/to_array.rs

@@ -1,7 +1,7 @@
 use crate::object::Value;
 use crate::prelude::*;
 
-pub fn to_array(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn to_array(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
     let out = args.input.values.collect();
 
     Ok(out

src/commands/to_csv.rs

@@ -1,10 +1,9 @@
 use crate::object::{Primitive, Value};
 use crate::prelude::*;
-use log::debug;
 use csv::WriterBuilder;
+use log::debug;
 
 pub fn value_to_csv_value(v: &Value) -> Value {
-
     debug!("value_to_csv_value(Value::Object(v)) where v = {:?}", v);
 
     match v {
@@ -13,7 +12,7 @@ pub fn value_to_csv_value(v: &Value) -> Value {
         Value::Object(o) => Value::Object(o.clone()),
         Value::List(l) => Value::List(l.clone()),
         Value::Block(_) => Value::Primitive(Primitive::Nothing),
-        _ => Value::Primitive(Primitive::Nothing)
+        _ => Value::Primitive(Primitive::Nothing),
     }
 }
 
@@ -21,7 +20,6 @@ pub fn to_string(v: &Value) -> Result<String, Box<dyn std::error::Error>> {
     match v {
         Value::List(_l) => return Ok(String::from("[list list]")),
         Value::Object(o) => {
-
             debug!("to_csv:to_string(Value::Object(v)) where v = {:?}", v);
 
             let mut wtr = WriterBuilder::new().from_writer(vec![]);
@@ -36,21 +34,21 @@ pub fn to_string(v: &Value) -> Result<String, Box<dyn std::error::Error>> {
             wtr.write_record(fields).expect("can not write.");
             wtr.write_record(values).expect("can not write.");
 
-            return Ok(String::from_utf8(wtr.into_inner()?)?)
-        },
+            return Ok(String::from_utf8(wtr.into_inner()?)?);
+        }
         Value::Primitive(Primitive::String(s)) => return Ok(s.to_string()),
-        _ => return Err("Bad input".into())
+        _ => return Err("Bad input".into()),
     }
 }
 
-pub fn to_csv(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn to_csv(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let name_span = args.name_span();
     let out = args.input;
-    let name_span = args.call_info.name_span;
+
     Ok(out
         .values
-        .map(
-            move |a| match to_string(&value_to_csv_value(&a.item)) {
-
+        .map(move |a| match to_string(&value_to_csv_value(&a.item)) {
             Ok(x) => {
                 ReturnSuccess::value(Value::Primitive(Primitive::String(x)).spanned(name_span))
             }
@@ -59,7 +57,6 @@ pub fn to_csv(args: CommandArgs) -> Result<OutputStream, ShellError> {
                 "can not convert piped data to CSV string",
                 name_span,
             )),
-            },
-        )
+        })
         .to_output_stream())
 }

src/commands/to_json.rs

@@ -40,9 +40,11 @@ pub fn value_to_json_value(v: &Value) -> serde_json::Value {
     }
 }
 
-pub fn to_json(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn to_json(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let name_span = args.name_span();
     let out = args.input;
-    let name_span = args.call_info.name_span;
+
     Ok(out
         .values
         .map(

src/commands/to_toml.rs

@@ -30,9 +30,10 @@ pub fn value_to_toml_value(v: &Value) -> toml::Value {
     }
 }
 
-pub fn to_toml(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn to_toml(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let name_span = args.name_span();
     let out = args.input;
-    let name_span = args.call_info.name_span;
 
     Ok(out
         .values

src/commands/to_yaml.rs

@@ -38,9 +38,10 @@ pub fn value_to_yaml_value(v: &Value) -> serde_yaml::Value {
     }
 }
 
-pub fn to_yaml(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn to_yaml(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
+    let args = args.evaluate_once(registry)?;
+    let name_span = args.name_span();
     let out = args.input;
-    let name_span = args.call_info.name_span;
     Ok(out
         .values
         .map(

src/commands/trim.rs

@@ -2,7 +2,7 @@ use crate::errors::ShellError;
 use crate::object::Value;
 use crate::prelude::*;
 
-pub fn trim(args: CommandArgs) -> Result<OutputStream, ShellError> {
+pub fn trim(args: CommandArgs, registry: &CommandRegistry) -> Result<OutputStream, ShellError> {
     let input = args.input;
 
     Ok(input

src/commands/where_.rs

@@ -1,14 +1,31 @@
 use crate::errors::ShellError;
-use crate::object::Block;
+use crate::object::base as value;
+use crate::object::{types, Value};
+use crate::parser::hir::SyntaxType;
+use crate::parser::registry::{self, CommandConfig, PositionalType};
 use crate::prelude::*;
+
 use futures::future::ready;
+use indexmap::IndexMap;
 use log::trace;
 
-command! {
-    Where as where(args, condition: Block,) {
-        let input: InputStream = trace_stream!(target: "nu::trace_stream::where", "where input" = args.input);
+pub struct Where;
 
-        input.values.filter_map(move |item| {
+impl Command for Where {
+    fn run(
+        &self,
+        args: CommandArgs,
+        registry: &registry::CommandRegistry,
+    ) -> Result<OutputStream, ShellError> {
+        let args = args.evaluate_once(registry)?;
+        let condition = value::Block::extract(args.expect_nth(0)?)?;
+        let input = args.input;
+        let input: InputStream =
+            trace_stream!(target: "nu::trace_stream::where", "where input" = input);
+
+        Ok(input
+            .values
+            .filter_map(move |item| {
                 let result = condition.invoke(&item);
 
                 let return_value = match result {
@@ -19,5 +36,41 @@ command! {
 
                 ready(return_value)
             })
+            .boxed()
+            .to_output_stream())
+    }
+
+    fn name(&self) -> &str {
+        "where"
+    }
+
+    fn config(&self) -> CommandConfig {
+        CommandConfig {
+            name: self.name().to_string(),
+            positional: vec![PositionalType::mandatory("condition", SyntaxType::Block)],
+            rest_positional: false,
+            named: IndexMap::default(),
+            is_sink: true,
+            is_filter: false,
         }
     }
+}
+
+// command! {
+//     Where as where(args, condition: Block,) {
+//         let input = args.input;
+//         let input: InputStream = trace_stream!(target: "nu::trace_stream::where", "where input" = input);
+
+//         input.values.filter_map(move |item| {
+//             let result = condition.invoke(&item);
+
+//             let return_value = match result {
+//                 Err(err) => Some(Err(err)),
+//                 Ok(v) if v.is_true() => Some(Ok(ReturnSuccess::Value(item.clone()))),
+//                 _ => None,
+//             };
+
+//            ready(return_value)
+//         })
+//     }
+// }

src/context.rs

@@ -1,21 +1,24 @@
-use crate::commands::command::{CallInfo, Sink, SinkCommandArgs};
+use crate::commands::command::{CallInfo, Sink, SinkCommandArgs, UnevaluatedCallInfo};
 use crate::parser::{
-    registry::{Args, CommandConfig, CommandRegistry},
+    hir,
+    registry::{self, CommandConfig},
     Span,
 };
 use crate::prelude::*;
-use serde::{Deserialize, Serialize};
-use uuid::Uuid;
 
+use derive_new::new;
 use indexmap::IndexMap;
+use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 use std::error::Error;
 use std::sync::Arc;
+use uuid::Uuid;
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
 pub enum SpanSource {
     Url(String),
     File(String),
+    Source(Text),
 }
 
 #[derive(Clone, Debug, Serialize, Deserialize)]
@@ -35,9 +38,51 @@ impl SourceMap {
     }
 }
 
+#[derive(Clone, new)]
+pub struct CommandRegistry {
+    #[new(value = "Arc::new(Mutex::new(IndexMap::default()))")]
+    registry: Arc<Mutex<IndexMap<String, Arc<dyn Command>>>>,
+}
+
+impl CommandRegistry {
+    crate fn empty() -> CommandRegistry {
+        CommandRegistry {
+            registry: Arc::new(Mutex::new(IndexMap::default())),
+        }
+    }
+
+    fn get_config(&self, name: &str) -> Option<CommandConfig> {
+        let registry = self.registry.lock().unwrap();
+
+        registry.get(name).map(|c| c.config())
+    }
+
+    fn get_command(&self, name: &str) -> Option<Arc<dyn Command>> {
+        let registry = self.registry.lock().unwrap();
+
+        registry.get(name).map(|c| c.clone())
+    }
+
+    fn has(&self, name: &str) -> bool {
+        let registry = self.registry.lock().unwrap();
+
+        registry.contains_key(name)
+    }
+
+    fn insert(&mut self, name: impl Into<String>, command: Arc<dyn Command>) {
+        let mut registry = self.registry.lock().unwrap();
+        registry.insert(name.into(), command);
+    }
+
+    crate fn names(&self) -> Vec<String> {
+        let mut registry = self.registry.lock().unwrap();
+        registry.keys().cloned().collect()
+    }
+}
+
 #[derive(Clone)]
 pub struct Context {
-    commands: IndexMap<String, Arc<dyn Command>>,
+    registry: CommandRegistry,
     sinks: IndexMap<String, Arc<dyn Sink>>,
     crate source_map: SourceMap,
     crate host: Arc<Mutex<dyn Host + Send>>,
@@ -45,9 +90,13 @@ pub struct Context {
 }
 
 impl Context {
+    crate fn registry(&self) -> &CommandRegistry {
+        &self.registry
+    }
+
     crate fn basic() -> Result<Context, Box<dyn Error>> {
         Ok(Context {
-            commands: indexmap::IndexMap::new(),
+            registry: CommandRegistry::new(),
             sinks: indexmap::IndexMap::new(),
             source_map: SourceMap::new(),
             host: Arc::new(Mutex::new(crate::env::host::BasicHost)),
@@ -57,7 +106,7 @@ impl Context {
 
     pub fn add_commands(&mut self, commands: Vec<Arc<dyn Command>>) {
         for command in commands {
-            self.commands.insert(command.name().to_string(), command);
+            self.registry.insert(command.name().to_string(), command);
         }
     }
 
@@ -83,7 +132,7 @@ impl Context {
         &mut self,
         command: Arc<dyn Sink>,
         name_span: Option<Span>,
-        args: Args,
+        args: registry::EvaluatedArgs,
         input: Vec<Spanned<Value>>,
     ) -> Result<(), ShellError> {
         let command_args = SinkCommandArgs {
@@ -99,16 +148,16 @@ impl Context {
         command.run(command_args)
     }
 
-    pub fn clone_commands(&self) -> indexmap::IndexMap<String, Arc<dyn Command>> {
-        self.commands.clone()
+    pub fn clone_commands(&self) -> CommandRegistry {
+        self.registry.clone()
     }
 
     crate fn has_command(&self, name: &str) -> bool {
-        self.commands.contains_key(name)
+        self.registry.has(name)
     }
 
     crate fn get_command(&self, name: &str) -> Arc<dyn Command> {
-        self.commands.get(name).unwrap().clone()
+        self.registry.get_command(name).unwrap()
     }
 
     crate fn run_command(
@@ -116,26 +165,43 @@ impl Context {
         command: Arc<dyn Command>,
         name_span: Option<Span>,
         source_map: SourceMap,
-        args: Args,
+        args: hir::Call,
+        source: Text,
         input: InputStream,
     ) -> Result<OutputStream, ShellError> {
-        let command_args = CommandArgs {
+        let command_args = self.command_args(args, input, source, source_map, name_span);
+
+        command.run(command_args, self.registry())
+    }
+
+    fn call_info(
+        &self,
+        args: hir::Call,
+        source: Text,
+        source_map: SourceMap,
+        name_span: Option<Span>,
+    ) -> UnevaluatedCallInfo {
+        UnevaluatedCallInfo {
+            args,
+            source,
+            source_map,
+            name_span,
+        }
+    }
+
+    fn command_args(
+        &self,
+        args: hir::Call,
+        input: InputStream,
+        source: Text,
+        source_map: SourceMap,
+        name_span: Option<Span>,
+    ) -> CommandArgs {
+        CommandArgs {
             host: self.host.clone(),
             env: self.env.clone(),
-            call_info: CallInfo {
-                name_span,
-                source_map,
-                args,
-            },
+            call_info: self.call_info(args, source, source_map, name_span),
             input,
-        };
-
-        command.run(command_args)
         }
     }
-
-impl CommandRegistry for Context {
-    fn get(&self, name: &str) -> Option<CommandConfig> {
-        self.commands.get(name).map(|c| c.config())
-    }
 }

src/env/host.rs

@@ -2,7 +2,7 @@ use crate::prelude::*;
 use language_reporting::termcolor;
 use std::fmt::Debug;
 
-pub trait Host: Debug {
+pub trait Host: Debug + Send {
     fn out_terminal(&self) -> Box<term::StdoutTerminal>;
     fn err_terminal(&self) -> Box<term::StderrTerminal>;
 

src/evaluate/evaluator.rs

@@ -9,7 +9,7 @@ use derive_new::new;
 use indexmap::IndexMap;
 
 #[derive(new)]
-crate struct Scope {
+pub struct Scope {
     it: Spanned<Value>,
     #[new(default)]
     vars: IndexMap<String, Spanned<Value>>,
@@ -22,16 +22,26 @@ impl Scope {
             vars: IndexMap::new(),
         }
     }
+
+    crate fn it_value(value: Spanned<Value>) -> Scope {
+        Scope {
+            it: value,
+            vars: IndexMap::new(),
+        }
+    }
 }
 
 crate fn evaluate_baseline_expr(
     expr: &Expression,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     scope: &Scope,
     source: &Text,
 ) -> Result<Spanned<Value>, ShellError> {
     match &expr.item {
         RawExpression::Literal(literal) => Ok(evaluate_literal(expr.copy_span(*literal), source)),
+        RawExpression::Synthetic(hir::Synthetic::String(s)) => {
+            Ok(Value::string(s).spanned_unknown())
+        }
         RawExpression::Variable(var) => evaluate_reference(var, scope, source),
         RawExpression::Binary(binary) => {
             let left = evaluate_baseline_expr(binary.left(), registry, scope, source)?;

src/lib.rs

@@ -23,6 +23,7 @@ mod parser;
 mod plugin;
 mod shell;
 mod stream;
+mod traits;
 
 pub use crate::commands::command::{CallInfo, ReturnSuccess, ReturnValue};
 pub use crate::context::SpanSource;
@@ -34,4 +35,4 @@ pub use cli::cli;
 pub use errors::ShellError;
 pub use object::base::{Primitive, Value};
 pub use parser::parse::text::Text;
-pub use parser::registry::{Args, CommandConfig, NamedType, PositionalType};
+pub use parser::registry::{CommandConfig, EvaluatedArgs, NamedType, PositionalType};

src/object/base.rs

@@ -1,3 +1,4 @@
+use crate::context::CommandRegistry;
 use crate::errors::ShellError;
 use crate::evaluate::{evaluate_baseline_expr, Scope};
 use crate::object::SpannedDictBuilder;
@@ -169,7 +170,12 @@ impl Block {
         let mut last = None;
 
         for expr in self.expressions.iter() {
-            last = Some(evaluate_baseline_expr(&expr, &(), &scope, &self.source)?)
+            last = Some(evaluate_baseline_expr(
+                &expr,
+                &CommandRegistry::empty(),
+                &scope,
+                &self.source,
+            )?)
         }
 
         Ok(last.unwrap())

src/parser/hir.rs

@@ -4,9 +4,13 @@ crate mod binary;
 crate mod named;
 crate mod path;
 
-use crate::parser::{Span, Spanned, Unit};
+use crate::evaluate::Scope;
+use crate::parser::{registry, Span, Spanned, Unit};
+use crate::prelude::*;
 use derive_new::new;
 use getset::Getters;
+use serde_derive::{Deserialize, Serialize};
+use std::fmt;
 
 crate use baseline_parse::{baseline_parse_single_token, baseline_parse_token_as_string};
 crate use baseline_parse_tokens::{baseline_parse_next_expr, SyntaxType, TokensIterator};
@@ -23,7 +27,7 @@ pub fn path(head: impl Into<Expression>, tail: Vec<Spanned<impl Into<String>>>)
     )
 }
 
-#[derive(Debug, Clone, Eq, PartialEq, Getters, new)]
+#[derive(Debug, Clone, Eq, PartialEq, Getters, Serialize, Deserialize, new)]
 pub struct Call {
     #[get = "crate"]
     head: Box<Expression>,
@@ -33,9 +37,42 @@ pub struct Call {
     named: Option<NamedArguments>,
 }
 
-#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+impl Call {
+    pub fn evaluate(
+        &self,
+        registry: &registry::CommandRegistry,
+        scope: &Scope,
+        source: &Text,
+    ) -> Result<registry::EvaluatedArgs, ShellError> {
+        registry::evaluate_args(self, registry, scope, source)
+    }
+}
+
+impl ToDebug for Call {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        write!(f, "({}", self.head.debug(source))?;
+
+        if let Some(positional) = &self.positional {
+            write!(f, " ")?;
+            write!(
+                f,
+                "{}",
+                &itertools::join(positional.iter().map(|p| p.debug(source)), " ")
+            )?;
+        }
+
+        if let Some(named) = &self.named {
+            write!(f, "{}", named.debug(source))?;
+        }
+
+        Ok(())
+    }
+}
+
+#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
 pub enum RawExpression {
     Literal(Literal),
+    Synthetic(Synthetic),
     Variable(Variable),
     Binary(Box<Binary>),
     Block(Vec<Expression>),
@@ -45,10 +82,24 @@ pub enum RawExpression {
     Boolean(bool),
 }
 
+#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
+pub enum Synthetic {
+    String(String),
+}
+
+impl Synthetic {
+    pub fn type_name(&self) -> &'static str {
+        match self {
+            Synthetic::String(_) => "string",
+        }
+    }
+}
+
 impl RawExpression {
     pub fn type_name(&self) -> &'static str {
         match self {
             RawExpression::Literal(literal) => literal.type_name(),
+            RawExpression::Synthetic(synthetic) => synthetic.type_name(),
             RawExpression::Variable(..) => "variable",
             RawExpression::Binary(..) => "binary",
             RawExpression::Block(..) => "block",
@@ -61,36 +112,40 @@ impl RawExpression {
 pub type Expression = Spanned<RawExpression>;
 
 impl Expression {
-    fn int(i: impl Into<i64>, span: impl Into<Span>) -> Expression {
+    crate fn int(i: impl Into<i64>, span: impl Into<Span>) -> Expression {
         Spanned::from_item(RawExpression::Literal(Literal::Integer(i.into())), span)
     }
 
-    fn size(i: impl Into<i64>, unit: impl Into<Unit>, span: impl Into<Span>) -> Expression {
+    crate fn size(i: impl Into<i64>, unit: impl Into<Unit>, span: impl Into<Span>) -> Expression {
         Spanned::from_item(
             RawExpression::Literal(Literal::Size(i.into(), unit.into())),
             span,
         )
     }
 
-    fn string(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
+    crate fn synthetic_string(s: impl Into<String>) -> Expression {
+        RawExpression::Synthetic(Synthetic::String(s.into())).spanned_unknown()
+    }
+
+    crate fn string(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
         Spanned::from_item(
             RawExpression::Literal(Literal::String(inner.into())),
             outer.into(),
         )
     }
 
-    fn bare(span: impl Into<Span>) -> Expression {
+    crate fn bare(span: impl Into<Span>) -> Expression {
         Spanned::from_item(RawExpression::Literal(Literal::Bare), span.into())
     }
 
-    fn variable(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
+    crate fn variable(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
         Spanned::from_item(
             RawExpression::Variable(Variable::Other(inner.into())),
             outer.into(),
         )
     }
 
-    fn it_variable(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
+    crate fn it_variable(inner: impl Into<Span>, outer: impl Into<Span>) -> Expression {
         Spanned::from_item(
             RawExpression::Variable(Variable::It(inner.into())),
             outer.into(),
@@ -98,13 +153,37 @@ impl Expression {
     }
 }
 
+impl ToDebug for Expression {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        match self.item() {
+            RawExpression::Literal(l) => write!(f, "{}", l.spanned(self.span()).debug(source)),
+            RawExpression::Synthetic(Synthetic::String(s)) => write!(f, "{:?}", s),
+            RawExpression::Variable(Variable::It(_)) => write!(f, "$it"),
+            RawExpression::Variable(Variable::Other(s)) => write!(f, "${}", s.slice(source)),
+            RawExpression::Binary(b) => write!(f, "{}", b.debug(source)),
+            RawExpression::Block(exprs) => {
+                write!(f, "{{ ")?;
+
+                for expr in exprs {
+                    write!(f, "{} ", expr.debug(source))?;
+                }
+
+                write!(f, "}}")
+            }
+            RawExpression::Path(p) => write!(f, "{}", p.debug(source)),
+            RawExpression::Boolean(true) => write!(f, "$yes"),
+            RawExpression::Boolean(false) => write!(f, "$no"),
+        }
+    }
+}
+
 impl From<Spanned<Path>> for Expression {
     fn from(path: Spanned<Path>) -> Expression {
         path.map(|p| RawExpression::Path(Box::new(p)))
     }
 }
 
-#[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
 pub enum Literal {
     Integer(i64),
     Size(i64, Unit),
@@ -112,6 +191,17 @@ pub enum Literal {
     Bare,
 }
 
+impl ToDebug for Spanned<&Literal> {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        match self.item() {
+            Literal::Integer(int) => write!(f, "{}", *int),
+            Literal::Size(int, unit) => write!(f, "{}{:?}", *int, unit),
+            Literal::String(span) => write!(f, "{}", span.slice(source)),
+            Literal::Bare => write!(f, "{}", self.span().slice(source)),
+        }
+    }
+}
+
 impl Literal {
     fn type_name(&self) -> &'static str {
         match self {
@@ -123,7 +213,7 @@ impl Literal {
     }
 }
 
-#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
 pub enum Variable {
     It(Span),
     Other(Span),

src/parser/hir/baseline_parse_tokens.rs

@@ -12,7 +12,7 @@ use serde_derive::{Deserialize, Serialize};
 
 pub fn baseline_parse_tokens(
     token_nodes: &mut TokensIterator<'_>,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     source: &Text,
 ) -> Result<Vec<hir::Expression>, ShellError> {
     let mut exprs: Vec<hir::Expression> = vec![];
@@ -43,7 +43,7 @@ pub enum SyntaxType {
 
 pub fn baseline_parse_next_expr(
     tokens: &mut TokensIterator,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     source: &Text,
     syntax_type: SyntaxType,
 ) -> Result<hir::Expression, ShellError> {
@@ -176,7 +176,7 @@ pub fn baseline_parse_next_expr(
 
 pub fn baseline_parse_semantic_token(
     token: &TokenNode,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     source: &Text,
 ) -> Result<hir::Expression, ShellError> {
     match token {
@@ -197,7 +197,7 @@ pub fn baseline_parse_semantic_token(
 
 pub fn baseline_parse_delimited(
     token: &Spanned<DelimitedNode>,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     source: &Text,
 ) -> Result<hir::Expression, ShellError> {
     match token.delimiter() {
@@ -216,7 +216,7 @@ pub fn baseline_parse_delimited(
 
 pub fn baseline_parse_path(
     token: &Spanned<PathNode>,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     source: &Text,
 ) -> Result<hir::Expression, ShellError> {
     let head = baseline_parse_semantic_token(token.head(), registry, source)?;

src/parser/hir/binary.rs

@@ -1,11 +1,26 @@
 use crate::parser::{hir::Expression, Operator, Spanned};
+use crate::prelude::*;
 use derive_new::new;
 use getset::Getters;
+use serde_derive::{Deserialize, Serialize};
+use std::fmt;
 
-#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Getters, new)]
+#[derive(
+    Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Getters, Serialize, Deserialize, new,
+)]
 #[get = "crate"]
 pub struct Binary {
     left: Expression,
     op: Spanned<Operator>,
     right: Expression,
 }
+
+impl ToDebug for Binary {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        write!(f, "{}", self.left.debug(source))?;
+        write!(f, " {} ", self.op.debug(source))?;
+        write!(f, "{}", self.right.debug(source))?;
+
+        Ok(())
+    }
+}

src/parser/hir/named.rs

@@ -1,10 +1,13 @@
 use crate::parser::hir::Expression;
 use crate::parser::{Flag, Span};
+use crate::prelude::*;
 use derive_new::new;
 use indexmap::IndexMap;
 use log::trace;
+use serde_derive::{Deserialize, Serialize};
+use std::fmt;
 
-#[derive(Debug, Clone, Eq, PartialEq)]
+#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
 pub enum NamedValue {
     AbsentSwitch,
     PresentSwitch(Span),
@@ -12,12 +15,27 @@ pub enum NamedValue {
     Value(Expression),
 }
 
-#[derive(Debug, Clone, Eq, PartialEq, new)]
+#[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize, new)]
 pub struct NamedArguments {
     #[new(default)]
     crate named: IndexMap<String, NamedValue>,
 }
 
+impl ToDebug for NamedArguments {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        for (name, value) in &self.named {
+            match value {
+                NamedValue::AbsentSwitch => continue,
+                NamedValue::PresentSwitch(span) => write!(f, " {}", span.slice(source))?,
+                NamedValue::AbsentValue => continue,
+                NamedValue::Value(expr) => write!(f, " --{} {}", name, expr.debug(source))?,
+            }
+        }
+
+        Ok(())
+    }
+}
+
 impl NamedArguments {
     pub fn insert_switch(&mut self, name: impl Into<String>, switch: Option<Flag>) {
         let name = name.into();

src/parser/hir/path.rs

@@ -1,10 +1,27 @@
 use crate::parser::{hir::Expression, Spanned};
+use crate::prelude::*;
 use derive_new::new;
 use getset::Getters;
+use serde_derive::{Deserialize, Serialize};
+use std::fmt;
 
-#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Getters, new)]
+#[derive(
+    Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash, Getters, Serialize, Deserialize, new,
+)]
 #[get = "crate"]
 pub struct Path {
     head: Expression,
     tail: Vec<Spanned<String>>,
 }
+
+impl ToDebug for Path {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result {
+        write!(f, "{}", self.head.debug(source))?;
+
+        for part in &self.tail {
+            write!(f, ".{}", part.item())?;
+        }
+
+        Ok(())
+    }
+}

src/parser/parse/operator.rs

@@ -1,4 +1,6 @@
+use crate::prelude::*;
 use serde_derive::{Deserialize, Serialize};
+use std::fmt;
 use std::str::FromStr;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Deserialize, Serialize)]
@@ -11,6 +13,12 @@ pub enum Operator {
     GreaterThanOrEqual,
 }
 
+impl ToDebug for Operator {
+    fn fmt_debug(&self, f: &mut fmt::Formatter, _source: &str) -> fmt::Result {
+        write!(f, "{}", self.as_str())
+    }
+}
+
 impl Operator {
     #[allow(unused)]
     pub fn print(&self) -> String {

src/parser/parse/span.rs

@@ -1,3 +1,4 @@
+use crate::prelude::*;
 use crate::Text;
 use derive_new::new;
 use getset::Getters;
@@ -14,6 +15,12 @@ pub struct Spanned<T> {
     pub item: T,
 }
 
+impl<T> HasSpan for Spanned<T> {
+    fn span(&self) -> Span {
+        self.span
+    }
+}
+
 impl<T> Spanned<T> {
     pub fn spanned(self, span: impl Into<Span>) -> Spanned<T> {
         Spanned::from_item(self.item, span.into())

src/parser/parse/text.rs

@@ -1,3 +1,4 @@
+use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std::cmp::Ordering;
 use std::hash::Hash;
 use std::hash::Hasher;
@@ -202,3 +203,21 @@ where
         self.partial_cmp(*other)
     }
 }
+
+impl Serialize for Text {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        self.as_ref().serialize(serializer)
+    }
+}
+
+impl Deserialize<'de> for Text {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        Ok(Text::from(String::deserialize(deserializer)?))
+    }
+}

src/parser/parse_command.rs

@@ -10,7 +10,7 @@ use log::trace;
 
 pub fn parse_command(
     config: &CommandConfig,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     call: &Spanned<CallNode>,
     source: &Text,
 ) -> Result<hir::Call, ShellError> {
@@ -63,7 +63,7 @@ fn parse_command_head(head: &TokenNode) -> Result<hir::Expression, ShellError> {
 
 fn parse_command_tail(
     config: &CommandConfig,
-    registry: &dyn CommandRegistry,
+    registry: &CommandRegistry,
     tail: Option<Vec<TokenNode>>,
     source: &Text,
     command_span: Span,

src/parser/registry.rs

@@ -1,3 +1,5 @@
+// TODO: Temporary redirect
+crate use crate::context::CommandRegistry;
 use crate::evaluate::{evaluate_baseline_expr, Scope};
 use crate::parser::{hir, hir::SyntaxType, parse_command, CallNode, Spanned};
 use crate::prelude::*;
@@ -80,17 +82,17 @@ pub struct CommandConfig {
 }
 
 #[derive(Debug, Default, new, Serialize, Deserialize)]
-pub struct Args {
+pub struct EvaluatedArgs {
     pub positional: Option<Vec<Spanned<Value>>>,
     pub named: Option<IndexMap<String, Spanned<Value>>>,
 }
 
 #[derive(new)]
-pub struct DebugPositional<'a> {
+pub struct DebugEvaluatedPositional<'a> {
     positional: &'a Option<Vec<Spanned<Value>>>,
 }
 
-impl fmt::Debug for DebugPositional<'a> {
+impl fmt::Debug for DebugEvaluatedPositional<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match &self.positional {
             None => write!(f, "None"),
@@ -103,11 +105,11 @@ impl fmt::Debug for DebugPositional<'a> {
 }
 
 #[derive(new)]
-pub struct DebugNamed<'a> {
+pub struct DebugEvaluatedNamed<'a> {
     named: &'a Option<IndexMap<String, Spanned<Value>>>,
 }
 
-impl fmt::Debug for DebugNamed<'a> {
+impl fmt::Debug for DebugEvaluatedNamed<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match &self.named {
             None => write!(f, "None"),
@@ -119,24 +121,27 @@ impl fmt::Debug for DebugNamed<'a> {
     }
 }
 
-pub struct DebugArgs<'a> {
-    args: &'a Args,
+pub struct DebugEvaluatedArgs<'a> {
+    args: &'a EvaluatedArgs,
 }
 
-impl fmt::Debug for DebugArgs<'a> {
+impl fmt::Debug for DebugEvaluatedArgs<'a> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         let mut s = f.debug_struct("Args");
 
-        s.field("positional", &DebugPositional::new(&self.args.positional));
-        s.field("named", &DebugNamed::new(&self.args.named));
+        s.field(
+            "positional",
+            &DebugEvaluatedPositional::new(&self.args.positional),
+        );
+        s.field("named", &DebugEvaluatedNamed::new(&self.args.named));
 
         s.finish()
     }
 }
 
-impl Args {
-    pub fn debug(&'a self) -> DebugArgs<'a> {
-        DebugArgs { args: self }
+impl EvaluatedArgs {
+    pub fn debug(&'a self) -> DebugEvaluatedArgs<'a> {
+        DebugEvaluatedArgs { args: self }
     }
 
     pub fn nth(&self, pos: usize) -> Option<&Spanned<Value>> {
@@ -205,92 +210,17 @@ impl Iterator for PositionalIter<'a> {
 }
 
 impl CommandConfig {
-    crate fn evaluate_args(
+    crate fn parse_args(
         &self,
         call: &Spanned<CallNode>,
-        registry: &dyn CommandRegistry,
-        scope: &Scope,
+        registry: &CommandRegistry,
         source: &Text,
-    ) -> Result<Args, ShellError> {
+    ) -> Result<hir::Call, ShellError> {
         let args = parse_command(self, registry, call, source)?;
 
         trace!("parsed args: {:?}", args);
 
-        evaluate_args(args, registry, scope, source)
-
-        // let mut positional: Vec<Spanned<Value>> = vec![];
-        // let mut named: IndexMap<String, Value> = IndexMap::default();
-
-        // let mut args: Vec<TokenNode> = args.cloned().collect();
-
-        // for (key, ty) in self.named.iter() {
-        //     let index = args.iter().position(|a| a.is_flag(&key, source));
-
-        //     match (index, ty) {
-        //         (Some(i), NamedType::Switch) => {
-        //             args.remove(i);
-        //             named.insert(key.clone(), Value::boolean(true));
-        //         }
-
-        //         (None, NamedType::Switch) => {}
-
-        //         (Some(i), NamedType::Optional(v)) => {
-        //             args.remove(i);
-        //             named.insert(key.clone(), extract_named(&mut args, i, v)?);
-        //         }
-
-        //         (None, NamedType::Optional(_)) => {}
-
-        //         (Some(i), NamedType::Mandatory(v)) => {
-        //             args.remove(i);
-        //             named.insert(key.clone(), extract_named(&mut args, i, v)?);
-        //         }
-
-        //         (None, NamedType::Mandatory(_)) => {
-        //             return Err(ShellError::string(&format!(
-        //                 "Expected mandatory argument {}, but it was missing",
-        //                 key
-        //             )))
-        //         }
-        //     }
-        // }
-
-        // let mut args = args.into_iter();
-
-        // for param in &self.mandatory_positional {
-        //     let arg = args.next();
-
-        //     let value = match arg {
-        //         None => {
-        //             return Err(ShellError::string(format!(
-        //                 "expected mandatory positional argument {}",
-        //                 param.name()
-        //             )))
-        //         }
-
-        //         Some(arg) => param.evaluate(arg.clone(), scope, source)?,
-        //     };
-
-        //     positional.push(value);
-        // }
-
-        // if self.rest_positional {
-        //     let rest: Result<Vec<Spanned<Value>>, _> = args
-        //         .map(|i| evaluate_baseline_expr(&i, &Scope::empty(), source))
-        //         .collect();
-        //     positional.extend(rest?);
-        // } else {
-        //     let rest: Vec<TokenNode> = args.collect();
-
-        //     if rest.len() > 0 {
-        //         return Err(ShellError::string(&format!(
-        //             "Too many arguments, extras: {:?}",
-        //             rest
-        //         )));
-        //     }
-        // }
-
-        // Ok(Args { positional, named })
+        Ok(args)
     }
 
     #[allow(unused)]
@@ -299,25 +229,25 @@ impl CommandConfig {
     }
 }
 
-fn evaluate_args(
-    args: hir::Call,
-    registry: &dyn CommandRegistry,
+crate fn evaluate_args(
+    call: &hir::Call,
+    registry: &CommandRegistry,
     scope: &Scope,
     source: &Text,
-) -> Result<Args, ShellError> {
-    let positional: Result<Option<Vec<_>>, _> = args
+) -> Result<EvaluatedArgs, ShellError> {
+    let positional: Result<Option<Vec<_>>, _> = call
         .positional()
         .as_ref()
         .map(|p| {
             p.iter()
-                .map(|e| evaluate_baseline_expr(e, &(), scope, source))
+                .map(|e| evaluate_baseline_expr(e, &CommandRegistry::empty(), scope, source))
                 .collect()
         })
         .transpose();
 
     let positional = positional?;
 
-    let named: Result<Option<IndexMap<String, Spanned<Value>>>, ShellError> = args
+    let named: Result<Option<IndexMap<String, Spanned<Value>>>, ShellError> = call
         .named()
         .as_ref()
         .map(|n| {
@@ -348,15 +278,5 @@ fn evaluate_args(
 
     let named = named?;
 
-    Ok(Args::new(positional, named))
-}
-
-pub trait CommandRegistry {
-    fn get(&self, name: &str) -> Option<CommandConfig>;
-}
-
-impl CommandRegistry for () {
-    fn get(&self, _name: &str) -> Option<CommandConfig> {
-        None
-    }
+    Ok(EvaluatedArgs::new(positional, named))
 }

src/prelude.rs

@@ -34,9 +34,10 @@ macro_rules! trace_stream {
 
 crate use crate::cli::MaybeOwned;
 crate use crate::commands::command::{
-    Command, CommandAction, CommandArgs, ReturnSuccess, ReturnValue, Sink, SinkCommandArgs,
+    Command, CommandAction, CommandArgs, EvaluatedCommandArgs, ReturnSuccess, ReturnValue, Sink,
+    SinkCommandArgs,
 };
-crate use crate::context::Context;
+crate use crate::context::{CommandRegistry, Context};
 crate use crate::env::host::handle_unexpected;
 crate use crate::env::{Environment, Host};
 crate use crate::errors::ShellError;
@@ -44,10 +45,10 @@ crate use crate::object::types::ExtractType;
 crate use crate::object::{Primitive, Value};
 crate use crate::parser::{Span, Spanned, SpannedItem};
 crate use crate::stream::{InputStream, OutputStream};
+crate use crate::traits::{HasSpan, ToDebug};
 crate use crate::Text;
 crate use futures::stream::BoxStream;
-crate use futures::Stream;
-crate use futures::{FutureExt, StreamExt};
+crate use futures::{FutureExt, Stream, StreamExt};
 crate use std::collections::VecDeque;
 crate use std::future::Future;
 crate use std::sync::{Arc, Mutex};

src/shell/completer.rs

@@ -1,4 +1,6 @@
+use crate::context::CommandRegistry;
 use crate::prelude::*;
+
 use derive_new::new;
 use rustyline::completion::Completer;
 use rustyline::completion::{self, FilenameCompleter};
@@ -7,7 +9,7 @@ use rustyline::line_buffer::LineBuffer;
 #[derive(new)]
 crate struct NuCompleter {
     pub file_completer: FilenameCompleter,
-    pub commands: indexmap::IndexMap<String, Arc<dyn Command>>,
+    pub commands: CommandRegistry,
 }
 
 impl Completer for NuCompleter {
@@ -19,7 +21,7 @@ impl Completer for NuCompleter {
         pos: usize,
         context: &rustyline::Context,
     ) -> rustyline::Result<(usize, Vec<completion::Pair>)> {
-        let commands: Vec<String> = self.commands.keys().cloned().collect();
+        let commands: Vec<String> = self.commands.names();
 
         let mut completions = self.file_completer.complete(line, pos, context)?.1;
 

src/shell/helper.rs

@@ -18,7 +18,7 @@ crate struct Helper {
 }
 
 impl Helper {
-    crate fn new(commands: indexmap::IndexMap<String, Arc<dyn Command>>) -> Helper {
+    crate fn new(commands: CommandRegistry) -> Helper {
         Helper {
             completer: NuCompleter {
                 file_completer: FilenameCompleter::new(),

src/traits.rs

@@ -0,0 +1,28 @@
+use crate::prelude::*;
+use std::fmt;
+
+pub struct Debuggable<'a, T: ToDebug> {
+    inner: &'a T,
+    source: &'a str,
+}
+
+impl<T: ToDebug> fmt::Display for Debuggable<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.inner.fmt_debug(f, self.source)
+    }
+}
+
+pub trait HasSpan {
+    fn span(&self) -> Span;
+}
+
+pub trait ToDebug: Sized {
+    fn debug(&'a self, source: &'a str) -> Debuggable<'a, Self> {
+        Debuggable {
+            inner: self,
+            source,
+        }
+    }
+
+    fn fmt_debug(&self, f: &mut fmt::Formatter, source: &str) -> fmt::Result;
+}

tests/fixtures/nuplayground/.gitignore

@@ -1,2 +0,0 @@
-*_test
-*.txt

tests/helpers/mod.rs

@@ -7,9 +7,9 @@ use std::io::Read;
 #[macro_export]
 macro_rules! nu {
     ($out:ident, $cwd:expr, $commands:expr) => {
+        pub use std::error::Error;
         pub use std::io::prelude::*;
         pub use std::process::{Command, Stdio};
-        pub use std::error::Error;
 
         let commands = &*format!(
             "
@@ -95,7 +95,8 @@ pub fn setup_playground_for(topic: &str) -> (String, String) {
 pub fn file_contents(full_path: &str) -> String {
     let mut file = std::fs::File::open(full_path).expect("can not open file");
     let mut contents = String::new();
-  file.read_to_string(&mut contents).expect("can not read file");
+    file.read_to_string(&mut contents)
+        .expect("can not read file");
     contents
 }
 
@@ -112,7 +113,14 @@ pub fn delete_directory_at(full_path: &str) {
 }
 
 pub fn create_directory_at(full_path: &str) {
-    std::fs::create_dir(PathBuf::from(full_path)).expect("can not create directory");
+    let path = PathBuf::from(full_path);
+
+    println!("{:?} - is_dir: {:?}", path, path.is_dir());
+
+    if !path.is_dir() {
+        std::fs::create_dir_all(PathBuf::from(full_path))
+            .expect(&format!("can not create directory {:?}", full_path));
+    }
 }
 
 pub fn executable_path() -> PathBuf {