Module: support defining const and use const variables inside of function (#9773)

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# Description
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Relative: #8248 

After this pr, user can define const variable inside a module.

![image](https://github.com/nushell/nushell/assets/22256154/e3e03e56-c4b5-4144-a944-d1b20bec1cbd)

And user can export const variables, the following screenshot shows how
it works (it follows
https://github.com/nushell/nushell/issues/8248#issuecomment-1637442612):

![image](https://github.com/nushell/nushell/assets/22256154/b2c14760-3f27-41cc-af77-af70a4367f2a)

## About the change
1. To make module support const, we need to change `parse_module_block`
to support `const` keyword.
2. To suport export `const`, we need to make module tracking variables,
so we add `variables` attribute to `Module`
3. During eval, the const variable may not exists in `stack`, because we
don't eval `const` when we define a module, so we need to find variables
which are already registered in `engine_state`

## One more thing to note about the const value.
Consider the following code
```
module foo { const b = 3; export def bar [] { $b } }
use foo bar
const b = 4;
bar
```
The result will be 3 (which is defined in module) rather than 4. I think
it's expected behavior.

It's something like [dynamic
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Dynamic-Binding-Tips.html)
vs [lexical
binding](https://www.gnu.org/software/emacs/manual/html_node/elisp/Lexical-Binding.html)
in lisp like language, and lexical binding should be right behavior
which generates more predicable result, and it doesn't introduce really
subtle bugs in nushell code.

What if user want dynamic-binding?(For example: the example code returns
`4`)
There is no way to do this, user should consider passing the value as
argument to custom command rather than const.

## TODO
- [X] adding tests for the feature.
- [X] support export const out of module to use.

# User-Facing Changes
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# Tests + Formatting
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crates/nu-cmd-lang/src/core_commands/export_const.rs

@@ -0,0 +1,66 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
+};
+
+#[derive(Clone)]
+pub struct ExportConst;
+
+impl Command for ExportConst {
+    fn name(&self) -> &str {
+        "export const"
+    }
+
+    fn usage(&self) -> &str {
+        "Use parse-time constant from a module and export them from this module."
+    }
+
+    fn signature(&self) -> nu_protocol::Signature {
+        Signature::build("export const")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
+            .required("const_name", SyntaxShape::VarWithOptType, "constant name")
+            .required(
+                "initial_value",
+                SyntaxShape::Keyword(b"=".to_vec(), Box::new(SyntaxShape::MathExpression)),
+                "equals sign followed by constant value",
+            )
+            .category(Category::Core)
+    }
+
+    fn extra_usage(&self) -> &str {
+        r#"This command is a parser keyword. For details, check:
+  https://www.nushell.sh/book/thinking_in_nu.html"#
+    }
+
+    fn is_parser_keyword(&self) -> bool {
+        true
+    }
+
+    fn run(
+        &self,
+        _engine_state: &EngineState,
+        _stack: &mut Stack,
+        _call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        Ok(PipelineData::empty())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![Example {
+            description: "Re-export a command from another module",
+            example: r#"module spam { export const foo = 3; }
+    module eggs { export use spam foo }
+    use eggs foo
+    foo
+            "#,
+            result: Some(Value::test_int(3)),
+        }]
+    }
+
+    fn search_terms(&self) -> Vec<&str> {
+        vec!["reexport", "import", "module"]
+    }
+}

crates/nu-cmd-lang/src/core_commands/mod.rs

@@ -11,6 +11,7 @@ mod echo;
 mod error_make;
 mod export;
 mod export_alias;
+mod export_const;
 mod export_def;
 mod export_def_env;
 mod export_extern;
@@ -50,6 +51,7 @@ pub use echo::Echo;
 pub use error_make::ErrorMake;
 pub use export::ExportCommand;
 pub use export_alias::ExportAlias;
+pub use export_const::ExportConst;
 pub use export_def::ExportDef;
 pub use export_def_env::ExportDefEnv;
 pub use export_extern::ExportExtern;

crates/nu-cmd-lang/src/core_commands/use_.rs

@@ -1,8 +1,8 @@
 use nu_engine::{eval_block, find_in_dirs_env, get_dirs_var_from_call, redirect_env};
-use nu_protocol::ast::{Call, Expr, Expression};
+use nu_protocol::ast::{Call, Expr, Expression, ImportPattern, ImportPatternMember};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
+    Category, Example, Module, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -110,6 +110,14 @@ This command is a parser keyword. For details, check:
                 // Merge the block's environment to the current stack
                 redirect_env(engine_state, caller_stack, &callee_stack);
             }
+
+            use_variables(
+                engine_state,
+                import_pattern,
+                module,
+                caller_stack,
+                call.head,
+            );
         } else {
             return Err(ShellError::GenericError(
                 format!(
@@ -162,6 +170,76 @@ This command is a parser keyword. For details, check:
     }
 }
 
+fn use_variables(
+    engine_state: &EngineState,
+    import_pattern: &ImportPattern,
+    module: &Module,
+    caller_stack: &mut Stack,
+    head_span: Span,
+) {
+    if !module.variables.is_empty() {
+        if import_pattern.members.is_empty() {
+            // add a record variable.
+            if let Some(var_id) = import_pattern.module_name_var_id {
+                let mut cols = vec![];
+                let mut vals = vec![];
+                for (var_name, var_id) in module.variables.iter() {
+                    if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                        cols.push(String::from_utf8_lossy(var_name).to_string());
+                        vals.push(val)
+                    }
+                }
+                caller_stack.add_var(
+                    var_id,
+                    Value::record(cols, vals, module.span.unwrap_or(head_span)),
+                )
+            }
+        } else {
+            let mut have_glob = false;
+            for m in &import_pattern.members {
+                if matches!(m, ImportPatternMember::Glob { .. }) {
+                    have_glob = true;
+                    break;
+                }
+            }
+            if have_glob {
+                // bring all variables into scope directly.
+                for (_, var_id) in module.variables.iter() {
+                    if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                        caller_stack.add_var(*var_id, val);
+                    }
+                }
+            } else {
+                let mut members = vec![];
+                for m in &import_pattern.members {
+                    match m {
+                        ImportPatternMember::List { names, .. } => {
+                            for (n, _) in names {
+                                if module.variables.contains_key(n) {
+                                    members.push(n);
+                                }
+                            }
+                        }
+                        ImportPatternMember::Name { name, .. } => {
+                            if module.variables.contains_key(name) {
+                                members.push(name)
+                            }
+                        }
+                        ImportPatternMember::Glob { .. } => continue,
+                    }
+                }
+                for m in members {
+                    if let Some(var_id) = module.variables.get(m) {
+                        if let Some(val) = engine_state.get_var(*var_id).clone().const_val {
+                            caller_stack.add_var(*var_id, val);
+                        }
+                    }
+                }
+            }
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     #[test]

crates/nu-cmd-lang/src/default_context.rs

@@ -29,6 +29,7 @@ pub fn create_default_context() -> EngineState {
             ErrorMake,
             ExportAlias,
             ExportCommand,
+            ExportConst,
             ExportDef,
             ExportDefEnv,
             ExportExtern,

crates/nu-engine/src/eval.rs

@@ -58,6 +58,14 @@ pub fn eval_call(
         let block = engine_state.get_block(block_id);
 
         let mut callee_stack = caller_stack.gather_captures(&block.captures);
+        // When the def is defined in module, relative captured variable doesn't go into stack
+        // so it can't be merged to callee_stack, but the variable is defined in `engine_state`
+        // then, to solve the issue, we also need to try to get relative const from `engine_state`
+        for cap in &block.captures {
+            if let Some(value) = engine_state.get_var(*cap).const_val.clone() {
+                callee_stack.vars.push((*cap, value))
+            }
+        }
 
         for (param_idx, param) in decl
             .signature()

crates/nu-parser/src/parse_keywords.rs

@@ -940,9 +940,8 @@ pub fn parse_export_in_block(
     let full_name = if lite_command.parts.len() > 1 {
         let sub = working_set.get_span_contents(lite_command.parts[1]);
         match sub {
-            b"alias" | b"def" | b"def-env" | b"extern" | b"extern-wrapped" | b"use" | b"module" => {
+            b"alias" | b"def" | b"def-env" | b"extern" | b"extern-wrapped" | b"use" | b"module"
-                [b"export ", sub].concat()
+            | b"const" => [b"export ", sub].concat(),
-            }
             _ => b"export".to_vec(),
         }
     } else {
@@ -1000,6 +999,7 @@ pub fn parse_export_in_block(
     match full_name.as_slice() {
         b"export alias" => parse_alias(working_set, lite_command, None),
         b"export def" | b"export def-env" => parse_def(working_set, lite_command, None),
+        b"export const" => parse_const(working_set, &lite_command.parts[1..]),
         b"export use" => {
             let (pipeline, _) = parse_use(working_set, &lite_command.parts);
             pipeline
@@ -1400,6 +1400,59 @@ pub fn parse_export_in_module(
 
                 result
             }
+            b"const" => {
+                let pipeline = parse_const(working_set, &spans[1..]);
+                let export_def_decl_id = if let Some(id) = working_set.find_decl(b"export const") {
+                    id
+                } else {
+                    working_set.error(ParseError::InternalError(
+                        "missing 'export const' command".into(),
+                        export_span,
+                    ));
+                    return (garbage_pipeline(spans), vec![]);
+                };
+
+                // Trying to warp the 'const' call into the 'export const' in a very clumsy way
+                if let Some(PipelineElement::Expression(
+                    _,
+                    Expression {
+                        expr: Expr::Call(ref def_call),
+                        ..
+                    },
+                )) = pipeline.elements.get(0)
+                {
+                    call = def_call.clone();
+
+                    call.head = span(&spans[0..=1]);
+                    call.decl_id = export_def_decl_id;
+                } else {
+                    working_set.error(ParseError::InternalError(
+                        "unexpected output from parsing a definition".into(),
+                        span(&spans[1..]),
+                    ));
+                };
+
+                let mut result = vec![];
+
+                if let Some(decl_name_span) = spans.get(2) {
+                    let decl_name = working_set.get_span_contents(*decl_name_span);
+                    let decl_name = trim_quotes(decl_name);
+
+                    if let Some(decl_id) = working_set.find_variable(decl_name) {
+                        result.push(Exportable::VarDecl {
+                            name: decl_name.to_vec(),
+                            id: decl_id,
+                        });
+                    } else {
+                        working_set.error(ParseError::InternalError(
+                            "failed to find added variable".into(),
+                            span(&spans[1..]),
+                        ));
+                    }
+                }
+
+                result
+            }
             _ => {
                 working_set.error(ParseError::Expected(
                     "def, def-env, alias, use, module, or extern keyword",
@@ -1589,6 +1642,9 @@ pub fn parse_module_block(
                                 None, // using commands named as the module locally is OK
                             ))
                         }
+                        b"const" => block
+                            .pipelines
+                            .push(parse_const(working_set, &command.parts)),
                         b"extern" | b"extern-wrapped" => {
                             block
                                 .pipelines
@@ -1713,6 +1769,9 @@ pub fn parse_module_block(
                                             module.add_submodule(name, id);
                                         }
                                     }
+                                    Exportable::VarDecl { name, id } => {
+                                        module.add_variable(name, id);
+                                    }
                                 }
                             }
 
@@ -1730,7 +1789,7 @@ pub fn parse_module_block(
                         }
                         _ => {
                             working_set.error(ParseError::ExpectedKeyword(
-                                "def, def-env, extern, alias, use, module, export or export-env keyword".into(),
+                                "def, const, def-env, extern, alias, use, module, export or export-env keyword".into(),
                                 command.parts[0],
                             ));
 
@@ -2208,7 +2267,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
         return (garbage_pipeline(spans), vec![]);
     };
 
-    let (import_pattern, module, module_id) = if let Some(module_id) = import_pattern.head.id {
+    let (mut import_pattern, module, module_id) = if let Some(module_id) = import_pattern.head.id {
         let module = working_set.get_module(module_id).clone();
         (
             ImportPattern {
@@ -2219,6 +2278,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                 },
                 members: import_pattern.members,
                 hidden: HashSet::new(),
+                module_name_var_id: None,
             },
             module,
             module_id,
@@ -2239,6 +2299,7 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                 },
                 members: import_pattern.members,
                 hidden: HashSet::new(),
+                module_name_var_id: None,
             },
             module,
             module_id,
@@ -2276,12 +2337,29 @@ pub fn parse_use(working_set: &mut StateWorkingSet, spans: &[Span]) -> (Pipeline
                     id: *module_id,
                 }),
         )
+        .chain(
+            definitions
+                .variables
+                .iter()
+                .map(|(name, variable_id)| Exportable::VarDecl {
+                    name: name.clone(),
+                    id: *variable_id,
+                }),
+        )
         .collect();
 
     // Extend the current scope with the module's exportables
     working_set.use_decls(definitions.decls);
     working_set.use_modules(definitions.modules);
+    working_set.use_variables(definitions.variables);
 
+    let module_name_var_id = working_set.add_variable(
+        module.name(),
+        module.span.unwrap_or(Span::unknown()),
+        Type::Any,
+        false,
+    );
+    import_pattern.module_name_var_id = Some(module_name_var_id);
     // Create a new Use command call to pass the import pattern as parser info
     let import_pattern_expr = Expression {
         expr: Expr::ImportPattern(import_pattern),
@@ -2703,7 +2781,7 @@ pub fn parse_overlay_use(working_set: &mut StateWorkingSet, call: Box<Call>) ->
             )
         }
     } else {
-        (ResolvedImportPattern::new(vec![], vec![]), vec![])
+        (ResolvedImportPattern::new(vec![], vec![], vec![]), vec![])
     };
 
     if errors.is_empty() {

crates/nu-parser/src/parser.rs

@@ -2976,6 +2976,7 @@ pub fn parse_import_pattern(working_set: &mut StateWorkingSet, spans: &[Span]) -
         },
         members: vec![],
         hidden: HashSet::new(),
+        module_name_var_id: None,
     };
 
     if spans.len() > 1 {

crates/nu-protocol/src/ast/import_pattern.rs

@@ -1,6 +1,6 @@
 use serde::{Deserialize, Serialize};
 
-use crate::{span, ModuleId, Span};
+use crate::{span, ModuleId, Span, VarId};
 use std::collections::HashSet;
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
@@ -24,6 +24,7 @@ pub struct ImportPattern {
     // communicate to eval which decls/aliases were hidden during `parse_hide()` so it does not
     // interpret these as env var names:
     pub hidden: HashSet<Vec<u8>>,
+    pub module_name_var_id: Option<VarId>,
 }
 
 impl ImportPattern {
@@ -36,6 +37,7 @@ impl ImportPattern {
             },
             members: vec![],
             hidden: HashSet::new(),
+            module_name_var_id: None,
         }
     }
 
@@ -62,6 +64,7 @@ impl ImportPattern {
             head: self.head,
             members: self.members,
             hidden,
+            module_name_var_id: self.module_name_var_id,
         }
     }
 }

crates/nu-protocol/src/engine/engine_state.rs

@@ -1161,6 +1161,17 @@ impl<'a> StateWorkingSet<'a> {
         }
     }
 
+    pub fn use_variables(&mut self, variables: Vec<(Vec<u8>, VarId)>) {
+        let overlay_frame = self.last_overlay_mut();
+
+        for (mut name, var_id) in variables {
+            if !name.starts_with(b"$") {
+                name.insert(0, b'$');
+            }
+            overlay_frame.insert_variable(name, var_id);
+        }
+    }
+
     pub fn add_predecl(&mut self, decl: Box<dyn Command>) -> Option<DeclId> {
         let name = decl.name().as_bytes().to_vec();
 
@@ -1530,11 +1541,15 @@ impl<'a> StateWorkingSet<'a> {
     }
 
     pub fn find_variable(&self, name: &[u8]) -> Option<VarId> {
+        let mut name = name.to_vec();
+        if !name.starts_with(b"$") {
+            name.insert(0, b'$');
+        }
         let mut removed_overlays = vec![];
 
         for scope_frame in self.delta.scope.iter().rev() {
             for overlay_frame in scope_frame.active_overlays(&mut removed_overlays).rev() {
-                if let Some(var_id) = overlay_frame.vars.get(name) {
+                if let Some(var_id) = overlay_frame.vars.get(&name) {
                     return Some(*var_id);
                 }
             }
@@ -1545,7 +1560,7 @@ impl<'a> StateWorkingSet<'a> {
             .active_overlays(&removed_overlays)
             .rev()
         {
-            if let Some(var_id) = overlay_frame.vars.get(name) {
+            if let Some(var_id) = overlay_frame.vars.get(&name) {
                 return Some(*var_id);
             }
         }

crates/nu-protocol/src/engine/overlay.rs

@@ -206,6 +206,10 @@ impl OverlayFrame {
         self.modules.insert(name, module_id)
     }
 
+    pub fn insert_variable(&mut self, name: Vec<u8>, variable_id: VarId) -> Option<VarId> {
+        self.vars.insert(name, variable_id)
+    }
+
     pub fn get_decl(&self, name: &[u8]) -> Option<DeclId> {
         self.decls.get(name).cloned()
     }

crates/nu-protocol/src/exportable.rs

@@ -1,6 +1,7 @@
-use crate::{DeclId, ModuleId};
+use crate::{DeclId, ModuleId, VarId};
 
 pub enum Exportable {
     Decl { name: Vec<u8>, id: DeclId },
     Module { name: Vec<u8>, id: ModuleId },
+    VarDecl { name: Vec<u8>, id: VarId },
 }

crates/nu-protocol/src/module.rs

@@ -1,5 +1,6 @@
 use crate::{
     ast::ImportPatternMember, engine::StateWorkingSet, BlockId, DeclId, ModuleId, ParseError, Span,
+    VarId,
 };
 
 use indexmap::IndexMap;
@@ -7,11 +8,20 @@ use indexmap::IndexMap;
 pub struct ResolvedImportPattern {
     pub decls: Vec<(Vec<u8>, DeclId)>,
     pub modules: Vec<(Vec<u8>, ModuleId)>,
+    pub variables: Vec<(Vec<u8>, VarId)>,
 }
 
 impl ResolvedImportPattern {
-    pub fn new(decls: Vec<(Vec<u8>, DeclId)>, modules: Vec<(Vec<u8>, ModuleId)>) -> Self {
+    pub fn new(
-        ResolvedImportPattern { decls, modules }
+        decls: Vec<(Vec<u8>, DeclId)>,
+        modules: Vec<(Vec<u8>, ModuleId)>,
+        variables: Vec<(Vec<u8>, VarId)>,
+    ) -> Self {
+        ResolvedImportPattern {
+            decls,
+            modules,
+            variables,
+        }
     }
 }
 
@@ -21,6 +31,7 @@ pub struct Module {
     pub name: Vec<u8>,
     pub decls: IndexMap<Vec<u8>, DeclId>,
     pub submodules: IndexMap<Vec<u8>, ModuleId>,
+    pub variables: IndexMap<Vec<u8>, VarId>,
     pub env_block: Option<BlockId>, // `export-env { ... }` block
     pub main: Option<DeclId>,       // `export def main`
     pub span: Option<Span>,
@@ -32,6 +43,7 @@ impl Module {
             name,
             decls: IndexMap::new(),
             submodules: IndexMap::new(),
+            variables: IndexMap::new(),
             env_block: None,
             main: None,
             span: None,
@@ -43,6 +55,7 @@ impl Module {
             name,
             decls: IndexMap::new(),
             submodules: IndexMap::new(),
+            variables: IndexMap::new(),
             env_block: None,
             main: None,
             span: Some(span),
@@ -61,6 +74,10 @@ impl Module {
         self.submodules.insert(name, module_id)
     }
 
+    pub fn add_variable(&mut self, name: Vec<u8>, var_id: VarId) -> Option<VarId> {
+        self.variables.insert(name, var_id)
+    }
+
     pub fn add_env_block(&mut self, block_id: BlockId) {
         self.env_block = Some(block_id);
     }
@@ -87,7 +104,8 @@ impl Module {
             (head, rest)
         } else {
             // Import pattern was just name without any members
-            let mut results = vec![];
+            let mut decls = vec![];
+            let mut vars = vec![];
             let mut errors = vec![];
 
             for (_, id) in &self.submodules {
@@ -101,14 +119,19 @@ impl Module {
                     new_name.push(b' ');
                     new_name.extend(sub_name);
 
-                    results.push((new_name, sub_decl_id));
+                    decls.push((new_name, sub_decl_id));
+                }
+
+                for (sub_name, sub_var_id) in sub_results.variables {
+                    vars.push((sub_name, sub_var_id));
                 }
             }
 
-            results.extend(self.decls_with_head(&final_name));
+            decls.extend(self.decls_with_head(&final_name));
+            vars.extend(self.vars());
 
             return (
-                ResolvedImportPattern::new(results, vec![(final_name, self_id)]),
+                ResolvedImportPattern::new(decls, vec![(final_name, self_id)], vars),
                 errors,
             );
         };
@@ -118,18 +141,27 @@ impl Module {
                 if name == b"main" {
                     if let Some(main_decl_id) = self.main {
                         (
-                            ResolvedImportPattern::new(vec![(final_name, main_decl_id)], vec![]),
+                            ResolvedImportPattern::new(
+                                vec![(final_name, main_decl_id)],
+                                vec![],
+                                vec![],
+                            ),
                             vec![],
                         )
                     } else {
                         (
-                            ResolvedImportPattern::new(vec![], vec![]),
+                            ResolvedImportPattern::new(vec![], vec![], vec![]),
                             vec![ParseError::ExportNotFound(*span)],
                         )
                     }
                 } else if let Some(decl_id) = self.decls.get(name) {
                     (
-                        ResolvedImportPattern::new(vec![(name.clone(), *decl_id)], vec![]),
+                        ResolvedImportPattern::new(vec![(name.clone(), *decl_id)], vec![], vec![]),
+                        vec![],
+                    )
+                } else if let Some(var_id) = self.variables.get(name) {
+                    (
+                        ResolvedImportPattern::new(vec![], vec![], vec![(name.clone(), *var_id)]),
                         vec![],
                     )
                 } else if let Some(submodule_id) = self.submodules.get(name) {
@@ -137,7 +169,7 @@ impl Module {
                     submodule.resolve_import_pattern(working_set, *submodule_id, rest, None)
                 } else {
                     (
-                        ResolvedImportPattern::new(vec![], vec![]),
+                        ResolvedImportPattern::new(vec![], vec![], vec![]),
                         vec![ParseError::ExportNotFound(*span)],
                     )
                 }
@@ -145,6 +177,7 @@ impl Module {
             ImportPatternMember::Glob { .. } => {
                 let mut decls = vec![];
                 let mut submodules = vec![];
+                let mut variables = vec![];
                 let mut errors = vec![];
 
                 for (_, id) in &self.submodules {
@@ -152,18 +185,25 @@ impl Module {
                     let (sub_results, sub_errors) =
                         submodule.resolve_import_pattern(working_set, *id, &[], None);
                     decls.extend(sub_results.decls);
+
                     submodules.extend(sub_results.modules);
+                    variables.extend(sub_results.variables);
                     errors.extend(sub_errors);
                 }
 
                 decls.extend(self.decls());
+                variables.extend(self.variables.clone());
                 submodules.extend(self.submodules());
 
-                (ResolvedImportPattern::new(decls, submodules), errors)
+                (
+                    ResolvedImportPattern::new(decls, submodules, variables),
+                    errors,
+                )
             }
             ImportPatternMember::List { names } => {
                 let mut decls = vec![];
                 let mut submodules = vec![];
+                let mut variables = vec![];
                 let mut errors = vec![];
 
                 for (name, span) in names {
@@ -175,6 +215,8 @@ impl Module {
                         }
                     } else if let Some(decl_id) = self.decls.get(name) {
                         decls.push((name.clone(), *decl_id));
+                    } else if let Some(var_id) = self.variables.get(name) {
+                        variables.push((name.clone(), *var_id));
                     } else if let Some(submodule_id) = self.submodules.get(name) {
                         submodules.push((name.clone(), *submodule_id));
                     } else {
@@ -182,7 +224,10 @@ impl Module {
                     }
                 }
 
-                (ResolvedImportPattern::new(decls, submodules), errors)
+                (
+                    ResolvedImportPattern::new(decls, submodules, variables),
+                    errors,
+                )
             }
         }
     }
@@ -217,6 +262,13 @@ impl Module {
         result
     }
 
+    pub fn vars(&self) -> Vec<(Vec<u8>, VarId)> {
+        self.variables
+            .iter()
+            .map(|(name, id)| (name.to_vec(), *id))
+            .collect()
+    }
+
     pub fn decl_names_with_head(&self, head: &[u8]) -> Vec<Vec<u8>> {
         let mut result: Vec<Vec<u8>> = self
             .decls

src/tests/test_modules.rs

@@ -111,3 +111,47 @@ fn export_alias() -> TestResult {
         "hello",
     )
 }
+
+#[test]
+fn export_consts() -> TestResult {
+    run_test(
+        r#"module spam { export const b = 3; }; use spam b; $b"#,
+        "3",
+    )
+}
+
+#[test]
+fn func_use_consts() -> TestResult {
+    run_test(
+        r#"module spam { const b = 3; export def c [] { $b } }; use spam; spam c"#,
+        "3",
+    )
+}
+
+#[test]
+fn export_module_which_defined_const() -> TestResult {
+    run_test(
+        r#"module spam { export const b = 3; export const c = 4 }; use spam; $spam.b + $spam.c"#,
+        "7",
+    )
+}
+
+#[test]
+fn cannot_export_private_const() -> TestResult {
+    fail_test(
+        r#"module spam { const b = 3; export const c = 4 }; use spam; $spam.b + $spam.c"#,
+        "cannot find column 'b'",
+    )
+}
+
+#[test]
+fn test_lexical_binding() -> TestResult {
+    run_test(
+        r#"module spam { const b = 3; export def c [] { $b } }; use spam c; const b = 4; c"#,
+        "3",
+    )?;
+    run_test(
+        r#"const b = 4; module spam { const b = 3; export def c [] { $b } }; use spam; spam c"#,
+        "3",
+    )
+}