Aliases (#1589)

* WIP getting scopes right

* finish adding initial support

* Finish with alias and add startup commands

crates/nu-cli/src/cli.rs

@@ -11,7 +11,7 @@ use futures_codec::FramedRead;
 
 use nu_errors::ShellError;
 use nu_protocol::hir::{ClassifiedCommand, ExternalCommand};
-use nu_protocol::{Primitive, ReturnSuccess, Signature, UntaggedValue, Value};
+use nu_protocol::{Primitive, ReturnSuccess, Scope, Signature, UntaggedValue, Value};
 
 use log::{debug, trace};
 use rustyline::error::ReadlineError;
@@ -258,6 +258,7 @@ pub fn create_default_context(
             whole_stream_command(What),
             whole_stream_command(Which),
             whole_stream_command(Debug),
+            per_item_command(Alias),
             // Statistics
             whole_stream_command(Size),
             whole_stream_command(Count),
@@ -442,6 +443,29 @@ pub async fn cli() -> Result<(), Box<dyn Error>> {
     })
     .expect("Error setting Ctrl-C handler");
     let mut ctrlcbreak = false;
+
+    // before we start up, let's run our startup commands
+    if let Ok(config) = crate::data::config::config(Tag::unknown()) {
+        if let Some(commands) = config.get("startup") {
+            match commands {
+                Value {
+                    value: UntaggedValue::Table(pipelines),
+                    ..
+                } => {
+                    for pipeline in pipelines {
+                        if let Ok(pipeline_string) = pipeline.as_string() {
+                            let _ =
+                                run_pipeline_standalone(pipeline_string, false, &mut context).await;
+                        }
+                    }
+                }
+                _ => {
+                    println!("warning: expected a table of pipeline strings as startup commands");
+                }
+            }
+        }
+    }
+
     loop {
         if context.ctrl_c.load(Ordering::SeqCst) {
             context.ctrl_c.store(false, Ordering::SeqCst);
@@ -712,7 +736,7 @@ async fn process_line(
                 None
             };
 
-            match run_pipeline(pipeline, ctx, input_stream).await {
+            match run_pipeline(pipeline, ctx, input_stream, &Scope::empty()).await {
                 Ok(Some(input)) => {
                     // Running a pipeline gives us back a stream that we can then
                     // work through. At the top level, we just want to pull on the

crates/nu-cli/src/commands.rs

@@ -4,6 +4,7 @@ pub(crate) mod macros;
 mod from_delimited_data;
 mod to_delimited_data;
 
+pub(crate) mod alias;
 pub(crate) mod append;
 pub(crate) mod args;
 pub(crate) mod autoview;
@@ -75,6 +76,7 @@ pub(crate) mod reject;
 pub(crate) mod rename;
 pub(crate) mod reverse;
 pub(crate) mod rm;
+pub(crate) mod run_alias;
 pub(crate) mod save;
 pub(crate) mod shells;
 pub(crate) mod shuffle;
@@ -115,6 +117,7 @@ pub(crate) use command::{
     WholeStreamCommand,
 };
 
+pub(crate) use alias::Alias;
 pub(crate) use append::Append;
 pub(crate) use calc::Calc;
 pub(crate) use compact::Compact;

crates/nu-cli/src/commands/alias.rs

@@ -0,0 +1,65 @@
+use crate::commands::PerItemCommand;
+use crate::context::CommandRegistry;
+use crate::prelude::*;
+use nu_errors::ShellError;
+use nu_protocol::{
+    CallInfo, CommandAction, Primitive, ReturnSuccess, Signature, SyntaxShape, UntaggedValue, Value,
+};
+
+pub struct Alias;
+
+impl PerItemCommand for Alias {
+    fn name(&self) -> &str {
+        "alias"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("alias")
+            .required("name", SyntaxShape::String, "the name of the alias")
+            .required("args", SyntaxShape::Table, "the arguments to the alias")
+            .required("block", SyntaxShape::Block, "the block to run on each row")
+    }
+
+    fn usage(&self) -> &str {
+        "Run a block on each row of the table."
+    }
+
+    fn run(
+        &self,
+        call_info: &CallInfo,
+        _registry: &CommandRegistry,
+        _raw_args: &RawCommandArgs,
+        _input: Value,
+    ) -> Result<OutputStream, ShellError> {
+        let call_info = call_info.clone();
+        let stream = async_stream! {
+            match (call_info.args.expect_nth(0)?, call_info.args.expect_nth(1)?, call_info.args.expect_nth(2)?) {
+                (Value {value: UntaggedValue::Primitive(Primitive::String(name)), .. },
+                Value { value: UntaggedValue::Table(list), .. },
+                Value {
+                    value: UntaggedValue::Block(block),
+                    tag
+                }) => {
+                    let mut args: Vec<String> = vec![];
+                    for item in list.iter() {
+                        if let Ok(string) = item.as_string() {
+                            args.push(format!("${}", string));
+                        } else {
+                            yield Err(ShellError::labeled_error("Expected a string", "expected a string", item.tag()));
+                        }
+                    }
+                    yield ReturnSuccess::action(CommandAction::AddAlias(name.to_string(), args, block.clone()))
+                }
+                _ => {
+                    yield Err(ShellError::labeled_error(
+                        "Expected `name [args] {block}",
+                        "needs a name, args, and a block",
+                        call_info.name_tag,
+                    ))
+                }
+            };
+        };
+
+        Ok(stream.to_output_stream())
+    }
+}

crates/nu-cli/src/commands/autoview.rs

@@ -3,7 +3,7 @@ use crate::commands::WholeStreamCommand;
 use crate::prelude::*;
 use nu_errors::ShellError;
 use nu_protocol::{hir, hir::Expression, hir::Literal, hir::SpannedExpression};
-use nu_protocol::{Primitive, ReturnSuccess, Signature, UntaggedValue, Value};
+use nu_protocol::{Primitive, ReturnSuccess, Scope, Signature, UntaggedValue, Value};
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
 
@@ -287,6 +287,7 @@ fn create_default_command_args(context: &RunnableContextWithoutInput) -> RawComm
                 span,
             },
             name_tag: context.name.clone(),
+            scope: Scope::empty(),
         },
     }
 }

crates/nu-cli/src/commands/classified/expr.rs

@@ -11,7 +11,9 @@ pub(crate) fn run_expression_block(
     expr: SpannedExpression,
     context: &mut Context,
     input: Option<InputStream>,
+    scope: &Scope,
 ) -> Result<Option<InputStream>, ShellError> {
+    let scope = scope.clone();
     if log_enabled!(log::Level::Trace) {
         trace!(target: "nu::run::expr", "->");
         trace!(target: "nu::run::expr", "{:?}", expr);
@@ -25,10 +27,11 @@ pub(crate) fn run_expression_block(
             pin_mut!(values);
 
             while let Some(row) = values.next().await {
-                yield evaluate_baseline_expr(&expr, &registry, &Scope::new(row));
+                let scope = scope.clone().set_it(row);
+                yield evaluate_baseline_expr(&expr, &registry, &scope);
             }
         } else {
-            yield evaluate_baseline_expr(&expr, &registry, &Scope::empty());
+            yield evaluate_baseline_expr(&expr, &registry, &scope);
         }
     };
 

crates/nu-cli/src/commands/classified/external.rs

@@ -7,7 +7,7 @@ use futures_codec::FramedRead;
 use log::trace;
 use nu_errors::ShellError;
 use nu_protocol::hir::{ExternalArg, ExternalCommand};
-use nu_protocol::{ColumnPath, Primitive, ShellTypeName, UntaggedValue, Value};
+use nu_protocol::{ColumnPath, Primitive, Scope, ShellTypeName, UntaggedValue, Value};
 use nu_source::{Tag, Tagged};
 use nu_value_ext::as_column_path;
 use std::io::Write;
@@ -97,6 +97,7 @@ pub(crate) async fn run_external_command(
     command: ExternalCommand,
     context: &mut Context,
     input: Option<InputStream>,
+    _scope: &Scope,
     is_last: bool,
 ) -> Result<Option<InputStream>, ShellError> {
     trace!(target: "nu::run::external", "-> {}", command.name);
@@ -718,6 +719,7 @@ mod tests {
     };
     use futures::executor::block_on;
     use nu_errors::ShellError;
+    use nu_protocol::Scope;
     use nu_test_support::commands::ExternalBuilder;
 
     // async fn read(mut stream: OutputStream) -> Option<Value> {
@@ -738,9 +740,11 @@ mod tests {
 
         let mut ctx = Context::basic().expect("There was a problem creating a basic context.");
 
-        assert!(run_external_command(cmd, &mut ctx, None, false)
+        assert!(
+            run_external_command(cmd, &mut ctx, None, &Scope::empty(), false)
                 .await
-            .is_err());
+                .is_err()
+        );
 
         Ok(())
     }

crates/nu-cli/src/commands/classified/internal.rs

@@ -1,14 +1,17 @@
+use crate::commands::command::per_item_command;
+use crate::commands::run_alias::AliasCommand;
 use crate::commands::UnevaluatedCallInfo;
 use crate::prelude::*;
 use log::{log_enabled, trace};
 use nu_errors::ShellError;
 use nu_protocol::hir::InternalCommand;
-use nu_protocol::{CommandAction, Primitive, ReturnSuccess, UntaggedValue, Value};
+use nu_protocol::{CommandAction, Primitive, ReturnSuccess, Scope, UntaggedValue, Value};
 
 pub(crate) fn run_internal_command(
     command: InternalCommand,
     context: &mut Context,
     input: Option<InputStream>,
+    scope: &Scope,
 ) -> Result<Option<InputStream>, ShellError> {
     if log_enabled!(log::Level::Trace) {
         trace!(target: "nu::run::internal", "->");
@@ -28,6 +31,7 @@ pub(crate) fn run_internal_command(
             internal_command?,
             Tag::unknown_anchor(command.name_span),
             command.args.clone(),
+            scope,
             objects,
         )
     };
@@ -68,6 +72,7 @@ pub(crate) fn run_internal_command(
                                         span: Span::unknown()
                                     },
                                     name_tag: Tag::unknown_anchor(command.name_span),
+                                    scope: Scope::empty(),
                                 }
                             };
                             let mut result = converter.run(new_args.with_input(vec![tagged_contents]), &context.registry);
@@ -120,6 +125,15 @@ pub(crate) fn run_internal_command(
                             FilesystemShell::with_location(location, context.registry().clone()),
                         ));
                     }
+                    CommandAction::AddAlias(name, args, commands) => {
+                        context.add_commands(vec![
+                            per_item_command(AliasCommand::new(
+                                name,
+                                args,
+                                commands,
+                            ))
+                        ]);
+                    }
                     CommandAction::PreviousShell => {
                         context.shell_manager.prev();
                     }

crates/nu-cli/src/commands/classified/pipeline.rs

@@ -5,11 +5,13 @@ use crate::context::Context;
 use crate::stream::InputStream;
 use nu_errors::ShellError;
 use nu_protocol::hir::{ClassifiedCommand, ClassifiedPipeline};
+use nu_protocol::Scope;
 
 pub(crate) async fn run_pipeline(
     pipeline: ClassifiedPipeline,
     ctx: &mut Context,
     mut input: Option<InputStream>,
+    scope: &Scope,
 ) -> Result<Option<InputStream>, ShellError> {
     let mut iter = pipeline.commands.list.into_iter().peekable();
 
@@ -22,18 +24,22 @@ pub(crate) async fn run_pipeline(
                 return Err(ShellError::unimplemented("Dynamic commands"))
             }
 
-            (Some(ClassifiedCommand::Expr(expr)), _) => run_expression_block(*expr, ctx, input)?,
+            (Some(ClassifiedCommand::Expr(expr)), _) => {
+                run_expression_block(*expr, ctx, input, scope)?
+            }
             (Some(ClassifiedCommand::Error(err)), _) => return Err(err.into()),
             (_, Some(ClassifiedCommand::Error(err))) => return Err(err.clone().into()),
 
-            (Some(ClassifiedCommand::Internal(left)), _) => run_internal_command(left, ctx, input)?,
+            (Some(ClassifiedCommand::Internal(left)), _) => {
+                run_internal_command(left, ctx, input, scope)?
+            }
 
             (Some(ClassifiedCommand::External(left)), None) => {
-                run_external_command(left, ctx, input, true).await?
+                run_external_command(left, ctx, input, scope, true).await?
             }
 
             (Some(ClassifiedCommand::External(left)), _) => {
-                run_external_command(left, ctx, input, false).await?
+                run_external_command(left, ctx, input, scope, false).await?
             }
 
             (None, _) => break,

crates/nu-cli/src/commands/command.rs

@@ -16,15 +16,27 @@ use std::sync::atomic::AtomicBool;
 pub struct UnevaluatedCallInfo {
     pub args: hir::Call,
     pub name_tag: Tag,
+    pub scope: Scope,
 }
 
 impl UnevaluatedCallInfo {
-    pub fn evaluate(
+    pub fn evaluate(self, registry: &CommandRegistry) -> Result<CallInfo, ShellError> {
+        let args = evaluate_args(&self.args, registry, &self.scope)?;
+
+        Ok(CallInfo {
+            args,
+            name_tag: self.name_tag,
+        })
+    }
+
+    pub fn evaluate_with_new_it(
         self,
         registry: &CommandRegistry,
-        scope: &Scope,
+        it: &Value,
     ) -> Result<CallInfo, ShellError> {
-        let args = evaluate_args(&self.args, registry, scope)?;
+        let mut scope = self.scope.clone();
+        scope = scope.set_it(it.clone());
+        let args = evaluate_args(&self.args, registry, &scope)?;
 
         Ok(CallInfo {
             args,
@@ -113,7 +125,7 @@ impl CommandArgs {
         let ctrl_c = self.ctrl_c.clone();
         let shell_manager = self.shell_manager.clone();
         let input = self.input;
-        let call_info = self.call_info.evaluate(registry, &Scope::empty())?;
+        let call_info = self.call_info.evaluate(registry)?;
 
         Ok(EvaluatedWholeStreamCommandArgs::new(
             host,
@@ -133,7 +145,12 @@ impl CommandArgs {
         let ctrl_c = self.ctrl_c.clone();
         let shell_manager = self.shell_manager.clone();
         let input = self.input;
-        let call_info = self.call_info.evaluate(registry, scope)?;
+        let call_info = UnevaluatedCallInfo {
+            name_tag: self.call_info.name_tag,
+            args: self.call_info.args,
+            scope: scope.clone(),
+        };
+        let call_info = call_info.evaluate(registry)?;
 
         Ok(EvaluatedWholeStreamCommandArgs::new(
             host,
@@ -515,10 +532,16 @@ impl Command {
             .input
             .values
             .map(move |x| {
-                let call_info = raw_args
+                let call_info = UnevaluatedCallInfo {
-                    .clone()
+                    args: raw_args.call_info.args.clone(),
-                    .call_info
+                    name_tag: raw_args.call_info.name_tag.clone(),
-                    .evaluate(&registry, &Scope::it_value(x.clone()));
+                    scope: raw_args.call_info.scope.clone().set_it(x.clone()),
+                }
+                .evaluate(&registry);
+                // let call_info = raw_args
+                //     .clone()
+                //     .call_info
+                //     .evaluate(&registry, &Scope::it_value(x.clone()));
 
                 match call_info {
                     Ok(call_info) => match command.run(&call_info, &registry, &raw_args, x) {
@@ -576,7 +599,7 @@ impl WholeStreamCommand for FnFilterCommand {
 
         let result = input.values.map(move |it| {
             let registry = registry.clone();
-            let call_info = match call_info.clone().evaluate(&registry, &Scope::it_value(it)) {
+            let call_info = match call_info.clone().evaluate_with_new_it(&registry, &it) {
                 Err(err) => return OutputStream::from(vec![Err(err)]).values,
                 Ok(args) => args,
             };

crates/nu-cli/src/commands/each.rs

@@ -5,7 +5,8 @@ use crate::context::CommandRegistry;
 use crate::prelude::*;
 use nu_errors::ShellError;
 use nu_protocol::{
-    hir::ClassifiedPipeline, CallInfo, ReturnSuccess, Signature, SyntaxShape, UntaggedValue, Value,
+    hir::ClassifiedPipeline, CallInfo, ReturnSuccess, Scope, Signature, SyntaxShape, UntaggedValue,
+    Value,
 };
 
 pub struct Each;
@@ -52,6 +53,7 @@ impl PerItemCommand for Each {
                         ClassifiedPipeline::new(block.clone(), None),
                         &mut context,
                         Some(input_stream),
+                        &Scope::empty(),
                     ).await;
 
                     match result {

crates/nu-cli/src/commands/enter.rs

@@ -104,6 +104,7 @@ impl PerItemCommand for Enter {
                                                     span: Span::unknown()
                                                 },
                                                 name_tag: raw_args.call_info.name_tag,
+                                                scope: raw_args.call_info.scope.clone()
                                             },
                                         };
                                         let mut result = converter.run(

crates/nu-cli/src/commands/plugin.rs

@@ -3,7 +3,7 @@ use crate::prelude::*;
 use derive_new::new;
 use log::trace;
 use nu_errors::ShellError;
-use nu_protocol::{Primitive, ReturnSuccess, ReturnValue, Scope, Signature, UntaggedValue, Value};
+use nu_protocol::{Primitive, ReturnSuccess, ReturnValue, Signature, UntaggedValue, Value};
 use serde::{self, Deserialize, Serialize};
 use std::io::prelude::*;
 use std::io::BufReader;
@@ -71,10 +71,13 @@ pub fn filter_plugin(
 ) -> Result<OutputStream, ShellError> {
     trace!("filter_plugin :: {}", path);
 
-    let args = args.evaluate_once_with_scope(
+    let scope = &args
-        registry,
+        .call_info
-        &Scope::it_value(UntaggedValue::string("$it").into_untagged_value()),
+        .scope
-    )?;
+        .clone()
+        .set_it(UntaggedValue::string("$it").into_untagged_value());
+
+    let args = args.evaluate_once_with_scope(registry, &scope)?;
 
     let mut child = std::process::Command::new(path)
         .stdin(std::process::Stdio::piped())

crates/nu-cli/src/commands/run_alias.rs

@@ -0,0 +1,93 @@
+use crate::commands::classified::pipeline::run_pipeline;
+use crate::prelude::*;
+
+use derive_new::new;
+use nu_errors::ShellError;
+use nu_protocol::{
+    hir::ClassifiedPipeline, hir::Commands, CallInfo, ReturnSuccess, Scope, Signature, SyntaxShape,
+    Value,
+};
+
+#[derive(new)]
+pub struct AliasCommand {
+    name: String,
+    args: Vec<String>,
+    block: Commands,
+}
+
+impl PerItemCommand for AliasCommand {
+    fn name(&self) -> &str {
+        &self.name
+    }
+
+    fn signature(&self) -> Signature {
+        let mut alias = Signature::build(&self.name);
+
+        for arg in &self.args {
+            alias = alias.required(arg, SyntaxShape::Any, "");
+        }
+
+        alias
+    }
+
+    fn usage(&self) -> &str {
+        ""
+    }
+
+    fn run(
+        &self,
+        call_info: &CallInfo,
+        registry: &CommandRegistry,
+        raw_args: &RawCommandArgs,
+        input: Value,
+    ) -> Result<OutputStream, ShellError> {
+        let tag = call_info.name_tag.clone();
+        let call_info = call_info.clone();
+        let registry = registry.clone();
+        let raw_args = raw_args.clone();
+        let block = self.block.clone();
+
+        let mut scope = Scope::empty();
+        if let Some(positional) = &call_info.args.positional {
+            for (pos, arg) in positional.iter().enumerate() {
+                scope = scope.set_var(self.args[pos].to_string(), arg.clone());
+            }
+        }
+
+        let stream = async_stream! {
+            let mut context = Context::from_raw(&raw_args, &registry);
+            let input_stream = async_stream! {
+                yield Ok(input.clone())
+            }.to_input_stream();
+
+            let result = run_pipeline(
+                ClassifiedPipeline::new(block.clone(), None),
+                &mut context,
+                Some(input_stream),
+                &scope
+            ).await;
+
+            match result {
+                Ok(Some(v)) => {
+                    let results: Vec<Value> = v.collect().await;
+
+                    for result in results {
+                        yield Ok(ReturnSuccess::Value(result));
+                    }
+                }
+                Ok(None) => {
+                    yield Err(ShellError::labeled_error(
+                        "Expected a block",
+                        "each needs a block",
+                        tag,
+                    ));
+                }
+                Err(e) => {
+                    yield Err(e);
+                }
+            }
+        };
+
+        Ok(stream.to_output_stream())
+    }
+}

crates/nu-cli/src/commands/save.rs

@@ -1,7 +1,7 @@
 use crate::commands::{UnevaluatedCallInfo, WholeStreamCommand};
 use crate::prelude::*;
 use nu_errors::ShellError;
-use nu_protocol::{Primitive, ReturnSuccess, Signature, SyntaxShape, UntaggedValue, Value};
+use nu_protocol::{Primitive, ReturnSuccess, Scope, Signature, SyntaxShape, UntaggedValue, Value};
 use nu_source::Tagged;
 use std::path::{Path, PathBuf};
 
@@ -235,6 +235,7 @@ fn save(
                                     span: Span::unknown()
                                 },
                                 name_tag: raw_args.call_info.name_tag,
+                                scope: Scope::empty(), // FIXME?
                             }
                         };
                         let mut result = converter.run(new_args.with_input(input), &registry);

crates/nu-cli/src/commands/where_.rs

@@ -68,6 +68,7 @@ impl PerItemCommand for Where {
             }
         };
 
+        //FIXME: should we use the scope that's brought in as well?
         let condition = evaluate_baseline_expr(&condition, registry, &Scope::new(input.clone()))?;
 
         let stream = match condition.as_bool() {

crates/nu-cli/src/context.rs

@@ -7,7 +7,7 @@ use crate::stream::{InputStream, OutputStream};
 use indexmap::IndexMap;
 use nu_errors::ShellError;
 use nu_parser::SignatureRegistry;
-use nu_protocol::{hir, Signature};
+use nu_protocol::{hir, Scope, Signature};
 use nu_source::{Tag, Text};
 use parking_lot::Mutex;
 use std::error::Error;
@@ -196,22 +196,33 @@ impl Context {
         command: Arc<Command>,
         name_tag: Tag,
         args: hir::Call,
+        scope: &Scope,
         input: InputStream,
     ) -> OutputStream {
-        let command_args = self.command_args(args, input, name_tag);
+        let command_args = self.command_args(args, input, name_tag, scope);
         command.run(command_args, self.registry())
     }
 
-    fn call_info(&self, args: hir::Call, name_tag: Tag) -> UnevaluatedCallInfo {
+    fn call_info(&self, args: hir::Call, name_tag: Tag, scope: &Scope) -> UnevaluatedCallInfo {
-        UnevaluatedCallInfo { args, name_tag }
+        UnevaluatedCallInfo {
+            args,
+            name_tag,
+            scope: scope.clone(),
+        }
     }
 
-    fn command_args(&self, args: hir::Call, input: InputStream, name_tag: Tag) -> CommandArgs {
+    fn command_args(
+        &self,
+        args: hir::Call,
+        input: InputStream,
+        name_tag: Tag,
+        scope: &Scope,
+    ) -> CommandArgs {
         CommandArgs {
             host: self.host.clone(),
             ctrl_c: self.ctrl_c.clone(),
             shell_manager: self.shell_manager.clone(),
-            call_info: self.call_info(args, name_tag),
+            call_info: self.call_info(args, name_tag, scope),
             input,
         }
     }

crates/nu-cli/tests/commands/alias.rs

@@ -0,0 +1,17 @@
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+#[test]
+fn alias_args_work() {
+    Playground::setup("append_test_1", |dirs, _| {
+        let actual = nu!(
+            cwd: dirs.root(),
+            r#"
+                alias double_echo [a b] {echo $a $b}
+                double_echo 1 2 | to-json
+            "#
+        );
+
+        assert_eq!(actual, "[1,2]");
+    })
+}

crates/nu-cli/tests/commands/mod.rs

@@ -1,3 +1,4 @@
+mod alias;
 mod append;
 mod calc;
 mod cd;

crates/nu-protocol/src/return_value.rs

@@ -1,3 +1,4 @@
+use crate::hir::Commands;
 use crate::value::Value;
 use nu_errors::ShellError;
 use nu_source::{b, DebugDocBuilder, PrettyDebug};
@@ -20,6 +21,8 @@ pub enum CommandAction {
     EnterValueShell(Value),
     /// Enter the help shell, which allows exploring the help system
     EnterHelpShell(Value),
+    /// Enter the help shell, which allows exploring the help system
+    AddAlias(String, Vec<String>, Commands),
     /// Go to the previous shell in the shell ring buffer
     PreviousShell,
     /// Go to the next shell in the shell ring buffer
@@ -41,6 +44,7 @@ impl PrettyDebug for CommandAction {
             CommandAction::EnterShell(s) => b::typed("enter shell", b::description(s)),
             CommandAction::EnterValueShell(v) => b::typed("enter value shell", v.pretty()),
             CommandAction::EnterHelpShell(v) => b::typed("enter help shell", v.pretty()),
+            CommandAction::AddAlias(..) => b::description("add alias"),
             CommandAction::PreviousShell => b::description("previous shell"),
             CommandAction::NextShell => b::description("next shell"),
             CommandAction::LeaveShell => b::description("leave shell"),

crates/nu-protocol/src/value/evaluate.rs

@@ -1,11 +1,12 @@
 use crate::value::{Primitive, UntaggedValue, Value};
 use indexmap::IndexMap;
+use serde::{Deserialize, Serialize};
 use std::fmt::Debug;
 
 /// An evaluation scope. Scopes map variable names to Values and aid in evaluating blocks and expressions.
 /// Additionally, holds the value for the special $it variable, a variable used to refer to the value passing
 /// through the pipeline at that moment
-#[derive(Debug)]
+#[derive(Deserialize, Serialize, Debug, Clone)]
 pub struct Scope {
     pub it: Value,
     pub vars: IndexMap<String, Value>,
@@ -37,4 +38,20 @@ impl Scope {
             vars: IndexMap::new(),
         }
     }
+
+    pub fn set_it(self, value: Value) -> Scope {
+        Scope {
+            it: value,
+            vars: self.vars,
+        }
+    }
+
+    pub fn set_var(self, name: String, value: Value) -> Scope {
+        let mut new_vars = self.vars.clone();
+        new_vars.insert(name, value);
+        Scope {
+            it: self.it,
+            vars: new_vars,
+        }
+    }
 }