better type params usage

src/typecheck/typecheck.test.ts

@@ -474,6 +474,32 @@ describe("custom types", () => {
     });
   });
 
+  test("allows using parametric types in constructors", () => {
+    const [types, errs] = tc(
+      `
+        type T<a, b> { C(b) }
+        let a = C
+        let b = C(1)
+  `,
+    );
+
+    expect(errs).toEqual([]);
+    expect(types).toEqual({
+      a: "Fn(t0) -> T<t1, t0>",
+      b: "T<t0, Int>",
+    });
+  });
+
+  test.todo("forbids unbound type params", () => {
+    const [types, errs] = tc(
+      `
+        type T { C(a) }
+  `,
+    );
+
+    expect(errs).not.toEqual([]);
+  });
+
   test.todo("doesn't allow shadowing type params", () => {
     const [types, errs] = tc(
       `

src/typecheck/typecheck.ts

@@ -55,7 +55,7 @@ function unboundTypeError<T extends SpanMeta>(
   };
 }
 
-function castType(ast: TypeAst): Type {
+function castType(ast: TypeAst): Type<Poly> {
   switch (ast.type) {
     case "named":
       return { type: "named", name: ast.name, args: ast.args.map(castType) };
@@ -65,10 +65,11 @@ function castType(ast: TypeAst): Type {
         args: ast.args.map(castType),
         return: castType(ast.return),
       };
-
     case "var":
+      return { type: "quantified", id: ast.ident };
+
     case "any":
-      throw new Error("TODO castType");
+      throw new Error("TODO invalid use of _");
   }
 }