feat(lang): support kwargs in cell and fn decls

README.md

@@ -70,6 +70,27 @@ arc run --cell 'top(pitch * 4., -width / 2.)'
 arc run --cell 'array(cons(250., cons(350., [])), Mode::Fast)'
 ```
 
+Parameters declared with a default value are keyword parameters. They are
+passed by name, may be omitted, and must follow the positional parameters. A
+default is an ordinary expression evaluated at each call; it may refer to the
+parameters declared before it and to module-level items, and its type must match
+the declared type exactly:
+
+```rust
+cell via(layer: String, w: Float, h: Float = w, n: Int = 1) {
+    // ...
+}
+
+cell top() {
+    let square = inst(via("met1", 100.));
+    let stack = inst(via("met1", 100., h=300., n=3));
+}
+```
+
+Positional parameters cannot be passed by name, and keyword parameters cannot be
+passed positionally. Keyword arguments also work in cell invocations:
+`arc run --cell 'via("met1", 100., n=3)'`.
+
 GDS imports are zero-argument cells. A module-qualified entry such as
 `"macros::sram"` can be referenced as `lib::macros::sram()` or imported with
 `use lib::macros::sram;`. Paths in the manifest are relative to `Argon.toml`,

crates/compiler/src/ast/annotated.rs

@@ -303,6 +303,7 @@ impl<S, T: AstMetadata> AstTransformer for AstAnnotationPass<S, T> {
         input: &super::ArgDecl<Self::InputS, Self::InputMetadata>,
         _name: &super::Ident<Self::OutputS, Self::OutputMetadata>,
         _ty: &super::TySpec<Self::OutputS, Self::OutputMetadata>,
+        _default: &Option<super::Expr<Self::OutputS, Self::OutputMetadata>>,
     ) -> <Self::OutputMetadata as AstMetadata>::ArgDecl {
         input.metadata.clone()
     }

crates/compiler/src/ast/mod.rs

@@ -366,10 +366,12 @@ pub struct KwArgValue<S, T: AstMetadata> {
     pub metadata: T::KwArgValue,
 }
 
+/// A parameter of a cell or function; `default` makes it a keyword parameter.
 #[derive_where(Debug, Clone, Serialize, Deserialize; S)]
 pub struct ArgDecl<S, T: AstMetadata> {
     pub name: Ident<S, T>,
     pub ty: TySpec<S, T>,
+    pub default: Option<Expr<S, T>>,
     pub metadata: T::ArgDecl,
 }
 
@@ -580,6 +582,7 @@ pub trait AstTransformer {
         input: &ArgDecl<Self::InputS, Self::InputMetadata>,
         name: &Ident<Self::OutputS, Self::OutputMetadata>,
         ty: &TySpec<Self::OutputS, Self::OutputMetadata>,
+        default: &Option<Expr<Self::OutputS, Self::OutputMetadata>>,
     ) -> <Self::OutputMetadata as AstMetadata>::ArgDecl;
     fn dispatch_scope(
         &mut self,
@@ -972,8 +975,19 @@ pub trait AstTransformer {
     ) -> ArgDecl<Self::OutputS, Self::OutputMetadata> {
         let name = self.transform_ident(&input.name);
         let ty = self.transform_ty_spec(&input.ty);
-        let metadata = self.dispatch_arg_decl(input, &name, &ty);
-        ArgDecl { name, ty, metadata }
+        // The default is visited before the parameter is dispatched, so it can
+        // see earlier parameters but not this one.
+        let default = input
+            .default
+            .as_ref()
+            .map(|default| self.transform_expr(default));
+        let metadata = self.dispatch_arg_decl(input, &name, &ty, &default);
+        ArgDecl {
+            name,
+            ty,
+            default,
+            metadata,
+        }
     }
 
     fn transform_scope(

crates/compiler/src/cli.rs

@@ -596,6 +596,20 @@ mod tests {
         assert_eq!(rect.x1.0, 20.);
     }
 
+    #[test]
+    fn execution_accepts_keyword_cell_arguments() {
+        let source = temp_source(
+            "kwargs",
+            "cell top(w: Float = 100., h: Float = w) {\n\
+             let r = rect(\"met1\", x0=0., y0=0., x1=w, y1=h);\n\
+             }\n",
+        );
+        let rect = compiled_rect("kwargs-default", source.clone(), "top()");
+        assert_eq!((rect.x1.0, rect.y1.0), (100., 100.));
+        let rect = compiled_rect("kwargs-explicit", source, "top(w=300.)");
+        assert_eq!((rect.x1.0, rect.y1.0), (300., 300.));
+    }
+
     #[test]
     fn out_of_range_cell_argument_is_reported_cleanly() {
         let source = temp_source("out-of-range-arg", "cell top(n: Int) {}\n");