conlang: PATTERN MATCHING AND DESTRUCTURED BINDINGS WOOOOO

- Integrate the Match and Pattern nodes into the AST
  - TODO: `let x: T` is ambiguous with `let x: {}`. Currently the latter takes precedence in the parser.

- Implement pattern matching through unification in the interpreter.
  - It's not fast, but it works!

- Refactor destructuring assignments to use the new pattern functionality

compiler/cl-ast/src/ast.rs

@@ -351,6 +351,8 @@ pub enum ExprKind {
     Quote(Quote),
     /// A local bind instruction, `let` [`Sym`] `=` [`Expr`]
     Let(Let),
+    /// A [Match] expression, `match` [Expr] `{` ([MatchArm] `,`)* [MatchArm]? `}`
+    Match(Match),
     /// An [Assign]ment expression: [`Expr`] (`=` [`Expr`])\+
     Assign(Assign),
     /// A [Modify]-assignment expression: [`Expr`] ([`ModifyKind`] [`Expr`])\+
@@ -408,7 +410,7 @@ pub struct Quote {
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct Let {
     pub mutable: Mutability,
-    pub name: Sym,
+    pub name: Pattern,
     pub ty: Option<Box<Ty>>,
     pub init: Option<Box<Expr>>,
 }

compiler/cl-ast/src/ast_impl.rs

@@ -417,6 +417,7 @@ mod display {
                 ExprKind::Empty => "()".fmt(f),
                 ExprKind::Quote(v) => v.fmt(f),
                 ExprKind::Let(v) => v.fmt(f),
+                ExprKind::Match(v) => v.fmt(f),
                 ExprKind::Assign(v) => v.fmt(f),
                 ExprKind::Modify(v) => v.fmt(f),
                 ExprKind::Binary(v) => v.fmt(f),
@@ -814,6 +815,7 @@ mod convert {
         impl From for ExprKind {
             Let => ExprKind::Let,
             Quote => ExprKind::Quote,
+            Match => ExprKind::Match,
             Assign => ExprKind::Assign,
             Modify => ExprKind::Modify,
             Binary => ExprKind::Binary,

compiler/cl-ast/src/ast_visitor/fold.rs

@@ -240,7 +240,7 @@ pub trait Fold {
         let Let { mutable, name, ty, init } = l;
         Let {
             mutable: self.fold_mutability(mutable),
-            name: self.fold_sym(name),
+            name: self.fold_pattern(name),
             ty: ty.map(|t| Box::new(self.fold_ty(*t))),
             init: init.map(|e| Box::new(self.fold_expr(*e))),
         }
@@ -572,6 +572,7 @@ pub fn or_fold_expr_kind<F: Fold + ?Sized>(folder: &mut F, kind: ExprKind) -> Ex
         ExprKind::Empty => ExprKind::Empty,
         ExprKind::Quote(q) => ExprKind::Quote(q), // quoted expressions are left unmodified
         ExprKind::Let(l) => ExprKind::Let(folder.fold_let(l)),
+        ExprKind::Match(m) => ExprKind::Match(folder.fold_match(m)),
         ExprKind::Assign(a) => ExprKind::Assign(folder.fold_assign(a)),
         ExprKind::Modify(m) => ExprKind::Modify(folder.fold_modify(m)),
         ExprKind::Binary(b) => ExprKind::Binary(folder.fold_binary(b)),

compiler/cl-ast/src/ast_visitor/visit.rs

@@ -203,7 +203,7 @@ pub trait Visit<'a>: Sized {
     fn visit_let(&mut self, l: &'a Let) {
         let Let { mutable, name, ty, init } = l;
         self.visit_mutability(mutable);
-        self.visit_sym(name);
+        self.visit_pattern(name);
         if let Some(ty) = ty {
             self.visit_ty(ty);
         }
@@ -492,6 +492,7 @@ pub fn or_visit_expr_kind<'a, V: Visit<'a>>(visitor: &mut V, e: &'a ExprKind) {
         ExprKind::Empty => {}
         ExprKind::Quote(_q) => {} // Quoted expressions are left unvisited
         ExprKind::Let(l) => visitor.visit_let(l),
+        ExprKind::Match(m) => visitor.visit_match(m),
         ExprKind::Assign(a) => visitor.visit_assign(a),
         ExprKind::Modify(m) => visitor.visit_modify(m),
         ExprKind::Binary(b) => visitor.visit_binary(b),