From 25c3ac5c0df60e4b56751296d9b9f34a931ebb7f Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Sun, 12 Apr 2026 20:26:16 -0600
Subject: [PATCH] Type Aliases!

---
 src/ast.ts         |  1 +
 src/parser.ts      |  7 +++++++
 src/typechecker.ts | 41 +++++++++++++++++++++++++++++++++++++++--
 3 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/src/ast.ts b/src/ast.ts
index 1ed39e6..4db2ae2 100644
--- a/src/ast.ts
+++ b/src/ast.ts
@@ -173,6 +173,7 @@ export type TypeDefinition = {
     name: string
     params: string[]
     constructors: TypeConstructor[]
+    alias?: TypeAST
     line?: number
     column?: number
     start?: number
diff --git a/src/parser.ts b/src/parser.ts
index ba2b9be..8ede0b6 100644
--- a/src/parser.ts
+++ b/src/parser.ts
@@ -715,6 +715,13 @@ export class Parser {
 
         this.expect('equals');
 
+        // If next token isn't a type-ident, it's a type alias
+        if (this.current().kind !== 'type-ident') {
+            const alias = this.parseType();
+            if (this.current().kind === 'semicolon') this.advance();
+            return { kind: 'type-definition', name, params, constructors: [], alias, ...this.getPos(nameToken) };
+        }
+
         // Parse constructors separated by |
         const constructors: TypeConstructor[] = [];
         constructors.push(this.parseTypeConstructor());
diff --git a/src/typechecker.ts b/src/typechecker.ts
index cd84c84..c494c0f 100644
--- a/src/typechecker.ts
+++ b/src/typechecker.ts
@@ -26,8 +26,8 @@ function applySubst(type: TypeAST, subst: Subst): TypeAST {
 }
 
 function unify(t1: TypeAST, t2: TypeAST, subst: Subst): string | null {
-    const a = applySubst(t1, subst);
-    const b = applySubst(t2, subst);
+    const a = resolveAlias(applySubst(t1, subst));
+    const b = resolveAlias(applySubst(t2, subst));
 
     // Same type name
     if (a.kind === 'type-name' && b.kind === 'type-name' && a.name === b.name) return null;
@@ -376,6 +376,7 @@ let typeConstructors = new Map<string, string[]>();
 
 export function typecheck(defs: Definition[], typeDefs: TypeDefinition[] = [], classDefs: ClassDefinition[] = [], instances: InstanceDeclaration[] = []) {
     const env: TypeEnv = new Map();
+    const typeAliases = new Map<string, { params: string[], type: TypeAST }>();
 
     // Register instances as a lookup: className -> Set of type names
     const instanceMap = new Map<string, Set<string>>();
@@ -395,6 +396,11 @@ export function typecheck(defs: Definition[], typeDefs: TypeDefinition[] = [], c
 
     // Register constructors
     for (const td of typeDefs) {
+        if (td.alias) {
+            typeAliases.set(td.name, { params: td.params, type: td.alias });
+            continue; // don't register constructors for aliases
+        }
+
         typeConstructors.set(td.name, td.constructors.map(c => c.name));
 
         const resultType: TypeAST = td.params.length === 0
@@ -414,6 +420,8 @@ export function typecheck(defs: Definition[], typeDefs: TypeDefinition[] = [], c
         }
     }
 
+    moduleTypeAliases = typeAliases;
+
     // Register all annotated defs in env first so they can ref eachother
     for (const def of defs) {
         if (def.annotation) {
@@ -543,3 +551,32 @@ function checkExhaustiveness(scrutType: TypeAST, cases: { pattern: Pattern }[],
         warn(`Non-exhaustive match, missing: ${missing.join(', ')}`, expr);
     }
 }
+
+let moduleTypeAliases = new Map<string, { params: string[], type: TypeAST }>();
+
+function resolveAlias(t: TypeAST): TypeAST {
+    if (t.kind === 'type-name' && moduleTypeAliases.has(t.name)) {
+        return moduleTypeAliases.get(t.name)!.type;
+    }
+
+    if (t.kind === 'type-apply' && t.constructor.kind === 'type-name' && moduleTypeAliases.has(t.constructor.name)) {
+        const alias = moduleTypeAliases.get(t.constructor.name);
+        // Substitute params: alias.params[i] -> t.args[i]
+        let result = alias.type;
+        for (let i = 0; i < alias.params.length && i < t.args.length; i++) {
+            result = substituteTypeVar(alias.params[i], t.args[i], result);
+        }
+        return result;
+    }
+    return t;
+}
+
+function substituteTypeVar(name: string, replacement: TypeAST, type: TypeAST): TypeAST {
+    switch (type.kind) {
+        case 'type-var': return type.name === name ? replacement : type;
+        case 'type-name': return type;
+        case 'type-function': return { kind: 'type-function', param: substituteTypeVar(name, replacement, type.param), result: substituteTypeVar(name, replacement, type.result) };
+        case 'type-apply': return { kind: 'type-apply', constructor: substituteTypeVar(name, replacement, type.constructor), args: type.args.map(a => substituteTypeVar(name, replacement, a)) };
+        case 'type-record': return { kind: 'type-record', fields: type.fields.map(f => ({ name: f.name, type: substituteTypeVar(name, replacement, f.type) })) };
+    }
+}