WIP

crates/nu-protocol/src/block.rs

@@ -0,0 +1,44 @@
+use std::ops::{Index, IndexMut};
+
+use crate::Statement;
+
+#[derive(Debug, Clone)]
+pub struct Block {
+    pub stmts: Vec<Statement>,
+}
+
+impl Block {
+    pub fn len(&self) -> usize {
+        self.stmts.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.stmts.is_empty()
+    }
+}
+
+impl Index<usize> for Block {
+    type Output = Statement;
+
+    fn index(&self, index: usize) -> &Self::Output {
+        &self.stmts[index]
+    }
+}
+
+impl IndexMut<usize> for Block {
+    fn index_mut(&mut self, index: usize) -> &mut Self::Output {
+        &mut self.stmts[index]
+    }
+}
+
+impl Default for Block {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Block {
+    pub fn new() -> Self {
+        Self { stmts: vec![] }
+    }
+}

crates/nu-protocol/src/call.rs

@@ -0,0 +1,27 @@
+use crate::{DeclId, Expression, Span};
+
+#[derive(Debug, Clone)]
+pub struct Call {
+    /// identifier of the declaration to call
+    pub decl_id: DeclId,
+    pub head: Span,
+    pub positional: Vec<Expression>,
+    pub named: Vec<(String, Option<Expression>)>,
+}
+
+impl Default for Call {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Call {
+    pub fn new() -> Call {
+        Self {
+            decl_id: 0,
+            head: Span::unknown(),
+            positional: vec![],
+            named: vec![],
+        }
+    }
+}

crates/nu-protocol/src/command.rs

@@ -0,0 +1,60 @@
+use crate::{Example, Signature};
+
+pub trait Command {
+    fn name(&self) -> &str;
+
+    fn signature(&self) -> Signature {
+        Signature::new(self.name()).desc(self.usage()).filter()
+    }
+
+    fn usage(&self) -> &str;
+
+    fn extra_usage(&self) -> &str {
+        ""
+    }
+
+    // fn run(&self, args: CommandArgs) -> Result<InputStream, ShellError> {
+    //     let context = args.context.clone();
+    //     let stream = self.run_with_actions(args)?;
+
+    //     Ok(Box::new(crate::evaluate::internal::InternalIterator {
+    //         context,
+    //         input: stream,
+    //         leftovers: InputStream::empty(),
+    //     })
+    //     .into_output_stream())
+    // }
+
+    fn is_binary(&self) -> bool {
+        false
+    }
+
+    // Commands that are not meant to be run by users
+    fn is_private(&self) -> bool {
+        false
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        Vec::new()
+    }
+
+    // This is a built-in command
+    fn is_builtin(&self) -> bool {
+        true
+    }
+
+    // Is a sub command
+    fn is_sub(&self) -> bool {
+        self.name().contains(' ')
+    }
+
+    // Is a plugin command
+    fn is_plugin(&self) -> bool {
+        false
+    }
+
+    // Is a custom command i.e. def blah [] { }
+    fn is_custom(&self) -> bool {
+        false
+    }
+}

crates/nu-protocol/src/declaration.rs

@@ -1,6 +0,0 @@
-use crate::{BlockId, Signature};
-
-pub struct Declaration {
-    pub signature: Box<Signature>,
-    pub body: Option<BlockId>,
-}

crates/nu-protocol/src/engine_state.rs

@@ -0,0 +1,637 @@
+use crate::{Block, BlockId, Command, DeclId, Span, Type, VarId};
+use core::panic;
+use std::{collections::HashMap, ops::Range, slice::Iter};
+
+pub struct EngineState {
+    files: Vec<(String, usize, usize)>,
+    file_contents: Vec<u8>,
+    vars: Vec<Type>,
+    decls: Vec<Box<dyn Command>>,
+    blocks: Vec<Block>,
+    scope: Vec<ScopeFrame>,
+}
+
+#[derive(Debug)]
+struct ScopeFrame {
+    vars: HashMap<Vec<u8>, VarId>,
+    decls: HashMap<Vec<u8>, DeclId>,
+    aliases: HashMap<Vec<u8>, Vec<Span>>,
+}
+
+impl ScopeFrame {
+    pub fn new() -> Self {
+        Self {
+            vars: HashMap::new(),
+            decls: HashMap::new(),
+            aliases: HashMap::new(),
+        }
+    }
+}
+
+impl Default for EngineState {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl EngineState {
+    pub fn new() -> Self {
+        Self {
+            files: vec![],
+            file_contents: vec![],
+            vars: vec![],
+            decls: vec![],
+            blocks: vec![],
+            scope: vec![ScopeFrame::new()],
+        }
+    }
+
+    pub fn merge_delta(this: &mut EngineState, mut delta: StateDelta) {
+        // Take the mutable reference and extend the permanent state from the working set
+        this.files.extend(delta.files);
+        this.file_contents.extend(delta.file_contents);
+        this.decls.extend(delta.decls);
+        this.vars.extend(delta.vars);
+        this.blocks.extend(delta.blocks);
+
+        if let Some(last) = this.scope.last_mut() {
+            let first = delta.scope.remove(0);
+            for item in first.decls.into_iter() {
+                last.decls.insert(item.0, item.1);
+            }
+            for item in first.vars.into_iter() {
+                last.vars.insert(item.0, item.1);
+            }
+            for item in first.aliases.into_iter() {
+                last.aliases.insert(item.0, item.1);
+            }
+        }
+    }
+
+    pub fn num_files(&self) -> usize {
+        self.files.len()
+    }
+
+    pub fn num_vars(&self) -> usize {
+        self.vars.len()
+    }
+
+    pub fn num_decls(&self) -> usize {
+        self.decls.len()
+    }
+
+    pub fn num_blocks(&self) -> usize {
+        self.blocks.len()
+    }
+
+    pub fn print_vars(&self) {
+        for var in self.vars.iter().enumerate() {
+            println!("var{}: {:?}", var.0, var.1);
+        }
+    }
+
+    pub fn print_decls(&self) {
+        for decl in self.decls.iter().enumerate() {
+            println!("decl{}: {:?}", decl.0, decl.1.signature());
+        }
+    }
+
+    pub fn print_blocks(&self) {
+        for block in self.blocks.iter().enumerate() {
+            println!("block{}: {:?}", block.0, block.1);
+        }
+    }
+
+    pub fn find_decl(&self, name: &[u8]) -> Option<DeclId> {
+        for scope in self.scope.iter().rev() {
+            if let Some(decl_id) = scope.decls.get(name) {
+                return Some(*decl_id);
+            }
+        }
+
+        None
+    }
+
+    pub fn get_var(&self, var_id: VarId) -> &Type {
+        self.vars
+            .get(var_id)
+            .expect("internal error: missing variable")
+    }
+
+    pub fn get_decl(&self, decl_id: DeclId) -> &Box<dyn Command> {
+        self.decls
+            .get(decl_id)
+            .expect("internal error: missing declaration")
+    }
+
+    pub fn get_block(&self, block_id: BlockId) -> &Block {
+        self.blocks
+            .get(block_id)
+            .expect("internal error: missing block")
+    }
+
+    pub fn next_span_start(&self) -> usize {
+        self.file_contents.len()
+    }
+
+    pub fn files(&self) -> Iter<(String, usize, usize)> {
+        self.files.iter()
+    }
+
+    pub fn get_filename(&self, file_id: usize) -> String {
+        for file in self.files.iter().enumerate() {
+            if file.0 == file_id {
+                return file.1 .0.clone();
+            }
+        }
+
+        "<unknown>".into()
+    }
+
+    pub fn get_file_source(&self, file_id: usize) -> String {
+        for file in self.files.iter().enumerate() {
+            if file.0 == file_id {
+                let output =
+                    String::from_utf8_lossy(&self.file_contents[file.1 .1..file.1 .2]).to_string();
+
+                return output;
+            }
+        }
+
+        "<unknown>".into()
+    }
+
+    #[allow(unused)]
+    pub(crate) fn add_file(&mut self, filename: String, contents: Vec<u8>) -> usize {
+        let next_span_start = self.next_span_start();
+
+        self.file_contents.extend(&contents);
+
+        let next_span_end = self.next_span_start();
+
+        self.files.push((filename, next_span_start, next_span_end));
+
+        self.num_files() - 1
+    }
+}
+
+pub struct StateWorkingSet<'a> {
+    pub permanent_state: &'a EngineState,
+    pub delta: StateDelta,
+}
+
+pub struct StateDelta {
+    files: Vec<(String, usize, usize)>,
+    pub(crate) file_contents: Vec<u8>,
+    vars: Vec<Type>,              // indexed by VarId
+    decls: Vec<Box<dyn Command>>, // indexed by DeclId
+    blocks: Vec<Block>,           // indexed by BlockId
+    scope: Vec<ScopeFrame>,
+}
+
+impl StateDelta {
+    pub fn num_files(&self) -> usize {
+        self.files.len()
+    }
+
+    pub fn num_decls(&self) -> usize {
+        self.decls.len()
+    }
+
+    pub fn num_blocks(&self) -> usize {
+        self.blocks.len()
+    }
+
+    pub fn enter_scope(&mut self) {
+        self.scope.push(ScopeFrame::new());
+    }
+
+    pub fn exit_scope(&mut self) {
+        self.scope.pop();
+    }
+}
+
+impl<'a> StateWorkingSet<'a> {
+    pub fn new(permanent_state: &'a EngineState) -> Self {
+        Self {
+            delta: StateDelta {
+                files: vec![],
+                file_contents: vec![],
+                vars: vec![],
+                decls: vec![],
+                blocks: vec![],
+                scope: vec![ScopeFrame::new()],
+            },
+            permanent_state,
+        }
+    }
+
+    pub fn num_files(&self) -> usize {
+        self.delta.num_files() + self.permanent_state.num_files()
+    }
+
+    pub fn num_decls(&self) -> usize {
+        self.delta.num_decls() + self.permanent_state.num_decls()
+    }
+
+    pub fn num_blocks(&self) -> usize {
+        self.delta.num_blocks() + self.permanent_state.num_blocks()
+    }
+
+    pub fn add_decl(&mut self, decl: Box<dyn Command>) -> DeclId {
+        let name = decl.name().as_bytes().to_vec();
+
+        self.delta.decls.push(decl);
+        let decl_id = self.num_decls() - 1;
+
+        let scope_frame = self
+            .delta
+            .scope
+            .last_mut()
+            .expect("internal error: missing required scope frame");
+        scope_frame.decls.insert(name, decl_id);
+
+        decl_id
+    }
+
+    pub fn add_block(&mut self, block: Block) -> BlockId {
+        self.delta.blocks.push(block);
+
+        self.num_blocks() - 1
+    }
+
+    pub fn next_span_start(&self) -> usize {
+        self.permanent_state.next_span_start() + self.delta.file_contents.len()
+    }
+
+    pub fn global_span_offset(&self) -> usize {
+        self.permanent_state.next_span_start()
+    }
+
+    pub fn files(&'a self) -> impl Iterator<Item = &(String, usize, usize)> {
+        self.permanent_state.files().chain(self.delta.files.iter())
+    }
+
+    pub fn get_filename(&self, file_id: usize) -> String {
+        for file in self.files().enumerate() {
+            if file.0 == file_id {
+                return file.1 .0.clone();
+            }
+        }
+
+        "<unknown>".into()
+    }
+
+    pub fn get_file_source(&self, file_id: usize) -> String {
+        for file in self.files().enumerate() {
+            if file.0 == file_id {
+                let output = String::from_utf8_lossy(self.get_span_contents(Span {
+                    start: file.1 .1,
+                    end: file.1 .2,
+                }))
+                .to_string();
+
+                return output;
+            }
+        }
+
+        "<unknown>".into()
+    }
+
+    pub fn add_file(&mut self, filename: String, contents: &[u8]) -> usize {
+        let next_span_start = self.next_span_start();
+
+        self.delta.file_contents.extend(contents);
+
+        let next_span_end = self.next_span_start();
+
+        self.delta
+            .files
+            .push((filename, next_span_start, next_span_end));
+
+        self.num_files() - 1
+    }
+
+    pub fn get_span_contents(&self, span: Span) -> &[u8] {
+        let permanent_end = self.permanent_state.next_span_start();
+        if permanent_end <= span.start {
+            &self.delta.file_contents[(span.start - permanent_end)..(span.end - permanent_end)]
+        } else {
+            &self.permanent_state.file_contents[span.start..span.end]
+        }
+    }
+
+    pub fn enter_scope(&mut self) {
+        self.delta.enter_scope();
+    }
+
+    pub fn exit_scope(&mut self) {
+        self.delta.exit_scope();
+    }
+
+    pub fn find_decl(&self, name: &[u8]) -> Option<DeclId> {
+        for scope in self.delta.scope.iter().rev() {
+            if let Some(decl_id) = scope.decls.get(name) {
+                return Some(*decl_id);
+            }
+        }
+
+        for scope in self.permanent_state.scope.iter().rev() {
+            if let Some(decl_id) = scope.decls.get(name) {
+                return Some(*decl_id);
+            }
+        }
+
+        None
+    }
+
+    // pub fn update_decl(&mut self, decl_id: usize, block: Option<BlockId>) {
+    //     let decl = self.get_decl_mut(decl_id);
+    //     decl.body = block;
+    // }
+
+    pub fn contains_decl_partial_match(&self, name: &[u8]) -> bool {
+        for scope in self.delta.scope.iter().rev() {
+            for decl in &scope.decls {
+                if decl.0.starts_with(name) {
+                    return true;
+                }
+            }
+        }
+
+        for scope in self.permanent_state.scope.iter().rev() {
+            for decl in &scope.decls {
+                if decl.0.starts_with(name) {
+                    return true;
+                }
+            }
+        }
+
+        false
+    }
+
+    pub fn next_var_id(&self) -> VarId {
+        let num_permanent_vars = self.permanent_state.num_vars();
+        num_permanent_vars + self.delta.vars.len()
+    }
+
+    pub fn find_variable(&self, name: &[u8]) -> Option<VarId> {
+        for scope in self.delta.scope.iter().rev() {
+            if let Some(var_id) = scope.vars.get(name) {
+                return Some(*var_id);
+            }
+        }
+
+        for scope in self.permanent_state.scope.iter().rev() {
+            if let Some(var_id) = scope.vars.get(name) {
+                return Some(*var_id);
+            }
+        }
+
+        None
+    }
+
+    pub fn find_alias(&self, name: &[u8]) -> Option<&[Span]> {
+        for scope in self.delta.scope.iter().rev() {
+            if let Some(spans) = scope.aliases.get(name) {
+                return Some(spans);
+            }
+        }
+
+        for scope in self.permanent_state.scope.iter().rev() {
+            if let Some(spans) = scope.aliases.get(name) {
+                return Some(spans);
+            }
+        }
+
+        None
+    }
+
+    pub fn add_variable(&mut self, mut name: Vec<u8>, ty: Type) -> VarId {
+        let next_id = self.next_var_id();
+
+        // correct name if necessary
+        if !name.starts_with(b"$") {
+            name.insert(0, b'$');
+        }
+
+        let last = self
+            .delta
+            .scope
+            .last_mut()
+            .expect("internal error: missing stack frame");
+
+        last.vars.insert(name, next_id);
+
+        self.delta.vars.push(ty);
+
+        next_id
+    }
+
+    pub fn add_alias(&mut self, name: Vec<u8>, replacement: Vec<Span>) {
+        let last = self
+            .delta
+            .scope
+            .last_mut()
+            .expect("internal error: missing stack frame");
+
+        last.aliases.insert(name, replacement);
+    }
+
+    pub fn set_variable_type(&mut self, var_id: VarId, ty: Type) {
+        let num_permanent_vars = self.permanent_state.num_vars();
+        if var_id < num_permanent_vars {
+            panic!("Internal error: attempted to set into permanent state from working set")
+        } else {
+            self.delta.vars[var_id - num_permanent_vars] = ty;
+        }
+    }
+
+    pub fn get_variable(&self, var_id: VarId) -> &Type {
+        let num_permanent_vars = self.permanent_state.num_vars();
+        if var_id < num_permanent_vars {
+            self.permanent_state.get_var(var_id)
+        } else {
+            self.delta
+                .vars
+                .get(var_id - num_permanent_vars)
+                .expect("internal error: missing variable")
+        }
+    }
+
+    pub fn get_decl(&self, decl_id: DeclId) -> &Box<dyn Command> {
+        let num_permanent_decls = self.permanent_state.num_decls();
+        if decl_id < num_permanent_decls {
+            self.permanent_state.get_decl(decl_id)
+        } else {
+            self.delta
+                .decls
+                .get(decl_id - num_permanent_decls)
+                .expect("internal error: missing declaration")
+        }
+    }
+
+    pub fn get_decl_mut(&mut self, decl_id: DeclId) -> &mut Box<dyn Command> {
+        let num_permanent_decls = self.permanent_state.num_decls();
+        if decl_id < num_permanent_decls {
+            panic!("internal error: can only mutate declarations in working set")
+        } else {
+            self.delta
+                .decls
+                .get_mut(decl_id - num_permanent_decls)
+                .expect("internal error: missing declaration")
+        }
+    }
+
+    pub fn get_block(&self, block_id: BlockId) -> &Block {
+        let num_permanent_blocks = self.permanent_state.num_blocks();
+        if block_id < num_permanent_blocks {
+            self.permanent_state.get_block(block_id)
+        } else {
+            self.delta
+                .blocks
+                .get(block_id - num_permanent_blocks)
+                .expect("internal error: missing block")
+        }
+    }
+
+    pub fn render(self) -> StateDelta {
+        self.delta
+    }
+}
+
+impl<'a> codespan_reporting::files::Files<'a> for StateWorkingSet<'a> {
+    type FileId = usize;
+
+    type Name = String;
+
+    type Source = String;
+
+    fn name(&'a self, id: Self::FileId) -> Result<Self::Name, codespan_reporting::files::Error> {
+        Ok(self.get_filename(id))
+    }
+
+    fn source(
+        &'a self,
+        id: Self::FileId,
+    ) -> Result<Self::Source, codespan_reporting::files::Error> {
+        Ok(self.get_file_source(id))
+    }
+
+    fn line_index(
+        &'a self,
+        id: Self::FileId,
+        byte_index: usize,
+    ) -> Result<usize, codespan_reporting::files::Error> {
+        let source = self.get_file_source(id);
+
+        let mut count = 0;
+
+        for byte in source.bytes().enumerate() {
+            if byte.0 == byte_index {
+                // println!("count: {} for file: {} index: {}", count, id, byte_index);
+                return Ok(count);
+            }
+            if byte.1 == b'\n' {
+                count += 1;
+            }
+        }
+
+        // println!("count: {} for file: {} index: {}", count, id, byte_index);
+        Ok(count)
+    }
+
+    fn line_range(
+        &'a self,
+        id: Self::FileId,
+        line_index: usize,
+    ) -> Result<Range<usize>, codespan_reporting::files::Error> {
+        let source = self.get_file_source(id);
+
+        let mut count = 0;
+
+        let mut start = Some(0);
+        let mut end = None;
+
+        for byte in source.bytes().enumerate() {
+            #[allow(clippy::comparison_chain)]
+            if count > line_index {
+                let start = start.expect("internal error: couldn't find line");
+                let end = end.expect("internal error: couldn't find line");
+
+                // println!(
+                //     "Span: {}..{} for fileid: {} index: {}",
+                //     start, end, id, line_index
+                // );
+                return Ok(start..end);
+            } else if count == line_index {
+                end = Some(byte.0 + 1);
+            }
+
+            #[allow(clippy::comparison_chain)]
+            if byte.1 == b'\n' {
+                count += 1;
+                if count > line_index {
+                    break;
+                } else if count == line_index {
+                    start = Some(byte.0 + 1);
+                }
+            }
+        }
+
+        match (start, end) {
+            (Some(start), Some(end)) => {
+                // println!(
+                //     "Span: {}..{} for fileid: {} index: {}",
+                //     start, end, id, line_index
+                // );
+                Ok(start..end)
+            }
+            _ => Err(codespan_reporting::files::Error::FileMissing),
+        }
+    }
+}
+
+#[cfg(test)]
+mod engine_state_tests {
+    use super::*;
+
+    #[test]
+    fn add_file_gives_id() {
+        let engine_state = EngineState::new();
+        let mut engine_state = StateWorkingSet::new(&engine_state);
+        let id = engine_state.add_file("test.nu".into(), &[]);
+
+        assert_eq!(id, 0);
+    }
+
+    #[test]
+    fn add_file_gives_id_including_parent() {
+        let mut engine_state = EngineState::new();
+        let parent_id = engine_state.add_file("test.nu".into(), vec![]);
+
+        let mut working_set = StateWorkingSet::new(&engine_state);
+        let working_set_id = working_set.add_file("child.nu".into(), &[]);
+
+        assert_eq!(parent_id, 0);
+        assert_eq!(working_set_id, 1);
+    }
+
+    #[test]
+    fn merge_states() {
+        let mut engine_state = EngineState::new();
+        engine_state.add_file("test.nu".into(), vec![]);
+
+        let delta = {
+            let mut working_set = StateWorkingSet::new(&engine_state);
+            working_set.add_file("child.nu".into(), &[]);
+            working_set.render()
+        };
+
+        EngineState::merge_delta(&mut engine_state, delta);
+
+        assert_eq!(engine_state.num_files(), 2);
+        assert_eq!(&engine_state.files[0].0, "test.nu");
+        assert_eq!(&engine_state.files[1].0, "child.nu");
+    }
+}

crates/nu-protocol/src/example.rs

@@ -0,0 +1,7 @@
+use crate::Value;
+
+pub struct Example {
+    pub example: &'static str,
+    pub description: &'static str,
+    pub result: Option<Vec<Value>>,
+}

crates/nu-protocol/src/expr.rs

@@ -0,0 +1,21 @@
+use crate::{BlockId, Call, Expression, Operator, Signature, Span, VarId};
+
+#[derive(Debug, Clone)]
+pub enum Expr {
+    Bool(bool),
+    Int(i64),
+    Float(f64),
+    Var(VarId),
+    Call(Box<Call>),
+    ExternalCall(Vec<u8>, Vec<Vec<u8>>),
+    Operator(Operator),
+    BinaryOp(Box<Expression>, Box<Expression>, Box<Expression>), //lhs, op, rhs
+    Subexpression(BlockId),
+    Block(BlockId),
+    List(Vec<Expression>),
+    Table(Vec<Expression>, Vec<Vec<Expression>>),
+    Keyword(Vec<u8>, Span, Box<Expression>),
+    String(String), // FIXME: improve this in the future?
+    Signature(Box<Signature>),
+    Garbage,
+}

crates/nu-protocol/src/expression.rs

@@ -0,0 +1,85 @@
+use crate::{BlockId, Expr, Operator, Signature, Span, Type, VarId};
+
+#[derive(Debug, Clone)]
+pub struct Expression {
+    pub expr: Expr,
+    pub span: Span,
+    pub ty: Type,
+}
+impl Expression {
+    pub fn garbage(span: Span) -> Expression {
+        Expression {
+            expr: Expr::Garbage,
+            span,
+            ty: Type::Unknown,
+        }
+    }
+    pub fn precedence(&self) -> usize {
+        match &self.expr {
+            Expr::Operator(operator) => {
+                // Higher precedence binds tighter
+
+                match operator {
+                    Operator::Pow => 100,
+                    Operator::Multiply | Operator::Divide | Operator::Modulo => 95,
+                    Operator::Plus | Operator::Minus => 90,
+                    Operator::NotContains
+                    | Operator::Contains
+                    | Operator::LessThan
+                    | Operator::LessThanOrEqual
+                    | Operator::GreaterThan
+                    | Operator::GreaterThanOrEqual
+                    | Operator::Equal
+                    | Operator::NotEqual
+                    | Operator::In
+                    | Operator::NotIn => 80,
+                    Operator::And => 50,
+                    Operator::Or => 40, // TODO: should we have And and Or be different precedence?
+                }
+            }
+            _ => 0,
+        }
+    }
+
+    pub fn as_block(&self) -> Option<BlockId> {
+        match self.expr {
+            Expr::Block(block_id) => Some(block_id),
+            _ => None,
+        }
+    }
+
+    pub fn as_signature(&self) -> Option<Box<Signature>> {
+        match &self.expr {
+            Expr::Signature(sig) => Some(sig.clone()),
+            _ => None,
+        }
+    }
+
+    pub fn as_list(&self) -> Option<Vec<Expression>> {
+        match &self.expr {
+            Expr::List(list) => Some(list.clone()),
+            _ => None,
+        }
+    }
+
+    pub fn as_keyword(&self) -> Option<&Expression> {
+        match &self.expr {
+            Expr::Keyword(_, _, expr) => Some(expr),
+            _ => None,
+        }
+    }
+
+    pub fn as_var(&self) -> Option<VarId> {
+        match self.expr {
+            Expr::Var(var_id) => Some(var_id),
+            _ => None,
+        }
+    }
+
+    pub fn as_string(&self) -> Option<String> {
+        match &self.expr {
+            Expr::String(string) => Some(string.clone()),
+            _ => None,
+        }
+    }
+}

crates/nu-protocol/src/lib.rs

@@ -1,17 +1,35 @@
-mod declaration;
+mod block;
+mod call;
+mod command;
+mod engine_state;
+mod example;
+mod expr;
+mod expression;
 mod id;
+mod operator;
+mod pipeline;
 mod shell_error;
 mod signature;
 mod span;
+mod statement;
 mod syntax_shape;
 mod ty;
 mod value;
 
-pub use declaration::*;
+pub use block::*;
+pub use call::*;
+pub use command::*;
+pub use engine_state::*;
+pub use example::*;
+pub use expr::*;
+pub use expression::*;
 pub use id::*;
+pub use operator::*;
+pub use pipeline::*;
 pub use shell_error::*;
 pub use signature::*;
 pub use span::*;
+pub use statement::*;
 pub use syntax_shape::*;
 pub use ty::*;
 pub use value::*;

crates/nu-protocol/src/operator.rs

@@ -0,0 +1,48 @@
+use std::fmt::Display;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Operator {
+    Equal,
+    NotEqual,
+    LessThan,
+    GreaterThan,
+    LessThanOrEqual,
+    GreaterThanOrEqual,
+    Contains,
+    NotContains,
+    Plus,
+    Minus,
+    Multiply,
+    Divide,
+    In,
+    NotIn,
+    Modulo,
+    And,
+    Or,
+    Pow,
+}
+
+impl Display for Operator {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Operator::Equal => write!(f, "=="),
+            Operator::NotEqual => write!(f, "!="),
+            Operator::LessThan => write!(f, "<"),
+            Operator::GreaterThan => write!(f, ">"),
+            Operator::Contains => write!(f, "=~"),
+            Operator::NotContains => write!(f, "!~"),
+            Operator::Plus => write!(f, "+"),
+            Operator::Minus => write!(f, "-"),
+            Operator::Multiply => write!(f, "*"),
+            Operator::Divide => write!(f, "/"),
+            Operator::In => write!(f, "in"),
+            Operator::NotIn => write!(f, "not-in"),
+            Operator::Modulo => write!(f, "mod"),
+            Operator::And => write!(f, "&&"),
+            Operator::Or => write!(f, "||"),
+            Operator::Pow => write!(f, "**"),
+            Operator::LessThanOrEqual => write!(f, "<="),
+            Operator::GreaterThanOrEqual => write!(f, ">="),
+        }
+    }
+}

crates/nu-protocol/src/pipeline.rs

@@ -0,0 +1,20 @@
+use crate::Expression;
+
+#[derive(Debug, Clone)]
+pub struct Pipeline {
+    pub expressions: Vec<Expression>,
+}
+
+impl Default for Pipeline {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Pipeline {
+    pub fn new() -> Self {
+        Self {
+            expressions: vec![],
+        }
+    }
+}

crates/nu-protocol/src/signature.rs

@@ -1,5 +1,7 @@
+use crate::BlockId;
+use crate::Command;
+use crate::SyntaxShape;
 use crate::VarId;
-use crate::{Declaration, SyntaxShape};
 
 #[derive(Debug, Clone)]
 pub struct Flag {
@@ -273,22 +275,62 @@ impl Signature {
         }
         None
     }
-}
 
-impl From<Box<Signature>> for Declaration {
-    fn from(val: Box<Signature>) -> Self {
-        Declaration {
-            signature: val,
-            body: None,
-        }
+    /// Set the filter flag for the signature
+    pub fn filter(mut self) -> Signature {
+        self.is_filter = true;
+        self
+    }
+
+    /// Create a placeholder implementation of Command as a way to predeclare a definition's
+    /// signature so other definitions can see it. This placeholder is later replaced with the
+    /// full definition in a second pass of the parser.
+    pub fn predeclare(self) -> Box<dyn Command> {
+        Box::new(Predeclaration { signature: self })
+    }
+
+    /// Combines a signature and a block into a runnable block
+    pub fn into_block_command(self, block_id: BlockId) -> Box<dyn Command> {
+        Box::new(BlockCommand {
+            signature: self,
+            block_id,
+        })
     }
 }
 
-impl From<Signature> for Declaration {
-    fn from(val: Signature) -> Self {
-        Declaration {
-            signature: Box::new(val),
-            body: None,
-        }
+struct Predeclaration {
+    signature: Signature,
+}
+
+impl Command for Predeclaration {
+    fn name(&self) -> &str {
+        &self.signature.name
+    }
+
+    fn signature(&self) -> Signature {
+        self.signature.clone()
+    }
+
+    fn usage(&self) -> &str {
+        &self.signature.usage
+    }
+}
+
+struct BlockCommand {
+    signature: Signature,
+    block_id: BlockId,
+}
+
+impl Command for BlockCommand {
+    fn name(&self) -> &str {
+        &self.signature.name
+    }
+
+    fn signature(&self) -> Signature {
+        self.signature.clone()
+    }
+
+    fn usage(&self) -> &str {
+        &self.signature.usage
     }
 }

crates/nu-protocol/src/statement.rs

@@ -0,0 +1,8 @@
+use crate::{DeclId, Expression, Pipeline};
+
+#[derive(Debug, Clone)]
+pub enum Statement {
+    Declaration(DeclId),
+    Pipeline(Pipeline),
+    Expression(Expression),
+}