Refactor UntypedKind::Parameter to Identifier

The `Parameter` kind in `UntypedKind` was only used for identifiers that
were being declared or referenced. This commit renames it to
`Identifier` and updates all the necessary code to reflect this change.
This simplifies the AST and makes it more consistent.

Additionally, a new macro `repeat` has been added to
`src/ast/system.myc`.

src/ast/compiler/binder.rs

@@ -197,11 +197,6 @@ impl Binder {
                 }
             }
 
-            UntypedKind::Parameter(_) => {
-                diag.push_error("Unexpected 'Parameter' node in general binder context. This should be handled via 'bind_pattern'.", Some(node.identity.clone()));
-                self.make_node(node.identity.clone(), BoundKind::Error)
-            }
-
             UntypedKind::FieldAccessor(k) => {
                 self.make_node(node.identity.clone(), BoundKind::FieldAccessor(*k))
             }
@@ -230,7 +225,7 @@ impl Binder {
 
             UntypedKind::Def { target, value } => {
                 // Special case: Single identifier (to support recursion)
-                if let UntypedKind::Parameter(ref name) = target.kind {
+                if let UntypedKind::Identifier(ref name) = target.kind {
                     let addr_opt = self.declare_variable(
                         name,
                         node.identity.clone(), // Identity of the Def node
@@ -543,7 +538,7 @@ impl Binder {
         diag: &mut Diagnostics,
     ) -> BoundNode {
         match &node.kind {
-            UntypedKind::Parameter(sym) => {
+            UntypedKind::Identifier(sym) => {
                 if let Some(addr) = self.declare_variable(sym, node.identity.clone(), kind, diag) {
                     self.make_node(
                         node.identity.clone(),

src/ast/compiler/macros.rs

@@ -344,7 +344,7 @@ impl<E: MacroEvaluator> MacroExpander<E> {
 
     fn extract_param_names(&self, node: &Node<UntypedKind>) -> Result<Vec<Rc<str>>, String> {
         match &node.kind {
-            UntypedKind::Parameter(sym) => Ok(vec![sym.name.clone()]),
+            UntypedKind::Identifier(sym) => Ok(vec![sym.name.clone()]),
             UntypedKind::Tuple { elements } => {
                 let mut names = Vec::new();
                 for el in elements {
@@ -372,15 +372,6 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
-            UntypedKind::Parameter(mut sym) => {
-                sym.context = Some(state.expansion_id.clone());
-                Ok(Node {
-                    identity: node.identity,
-                    kind: UntypedKind::Parameter(sym),
-                    ty: (),
-                })
-            }
-
             UntypedKind::Placeholder(inner) => {
                 // Break out of template for substitution/evaluation
                 if let UntypedKind::Identifier(ref sym) = inner.kind
@@ -654,11 +645,11 @@ mod tests {
             } = &exprs[1].kind
         {
             if let UntypedKind::Def { target, .. } = &result.kind {
-                if let UntypedKind::Parameter(sym) = &target.kind {
+                if let UntypedKind::Identifier(sym) = &target.kind {
                     assert_eq!(sym.context, Some(call.identity.clone()));
                     assert_eq!(sym.name.as_ref(), "y");
                 } else {
-                    panic!("Expected Parameter target, got {:?}", target.kind);
+                    panic!("Expected Identifier target, got {:?}", target.kind);
                 }
             } else {
                 panic!("Expected Def result, got {:?}", result.kind);

src/ast/environment.rs

@@ -381,7 +381,7 @@ impl Environment {
     fn discover_globals(&self, node: &Node<UntypedKind>) {
         match &node.kind {
             UntypedKind::Def { target, .. } => {
-                if let UntypedKind::Parameter(sym) = &target.kind {
+                if let UntypedKind::Identifier(sym) = &target.kind {
                     let mut names = self.global_names.borrow_mut();
                     if !names.contains_key(sym) {
                         let idx = GlobalIdx(names.len() as u32);
@@ -391,12 +391,11 @@ impl Environment {
             }
             UntypedKind::MacroDecl { name, params, body } => {
                 let mut registry = self.macro_registry.borrow_mut();
-                
+
                 fn extract_names(node: &Node<UntypedKind>) -> Vec<Rc<str>> {
                     match &node.kind {
-                        UntypedKind::Parameter(sym) => vec![sym.name.clone()],
-                        UntypedKind::Tuple { elements } => {
-                            elements.iter().flat_map(extract_names).collect()
+                        UntypedKind::Identifier(sym) => vec![sym.name.clone()],
+                        UntypedKind::Tuple { elements } => {                            elements.iter().flat_map(extract_names).collect()
                         }
                         _ => vec![],
                     }

src/ast/nodes.rs

@@ -63,8 +63,8 @@ impl Clone for Box<dyn CustomNode> {
 pub enum UntypedKind {
     Nop,
     Constant(Value),
+    /// A general identifier (used for both references and declarations in the untyped AST).
     Identifier(Symbol),
-    Parameter(Symbol),
     /// A first-class field accessor (e.g. .name)
     FieldAccessor(crate::ast::types::Keyword),
     If {

src/ast/parser.rs

@@ -358,7 +358,7 @@ impl<'a> Parser<'a> {
                 };
                 Node {
                     identity: NodeIdentity::new(token.location),
-                    kind: UntypedKind::Parameter(sym),
+                    kind: UntypedKind::Identifier(sym),
                     ty: (),
                 }
             }

src/ast/system.myc

@@ -8,6 +8,17 @@
              )))
   )
 
+  (macro repeat [var limit body]
+    `((fn [~var __limit]
+        (if (< ~var __limit)
+          (do
+            ~body
+            (again (+ ~var 1) __limit)
+          )
+        ))
+      0 ~limit)
+  )
+
   ;; Creates a stateful cache (Series) from a stateless stream.
   (macro cache [lookback type src]
     `(do