From 0514889ba6517f686fb305ac09e0924ffab8306d Mon Sep 17 00:00:00 2001
From: Dustin Swan <dustin@dustinswan.com>
Date: Thu, 26 Mar 2026 21:47:51 -0600
Subject: [PATCH] Type constructors work in typeclass instances

---
 src/cg/01-stdlib.cg | 5 +++++
 src/typechecker.ts  | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/src/cg/01-stdlib.cg b/src/cg/01-stdlib.cg
index 8852925..d712704 100644
--- a/src/cg/01-stdlib.cg
+++ b/src/cg/01-stdlib.cg
@@ -19,10 +19,15 @@ String : Eq;
 
 Semigroup a { cat : a \ a \ a };
 String : Semigroup;
+List : Semigroup;
 
 Maybe a = None | Some a;
 Bool = True | False;
 
+# myTest = cat 3 4;
+# myTest2 = cat [3, 4] [4, 5];
+# myTest3 = debug! "myTest2" myTest2;
+
 # nth : Int \ List a \ Maybe a
 # in host at the moment, until we get typeclasses or something and this can work on strings too
 # nth = i list \ [i, list]
diff --git a/src/typechecker.ts b/src/typechecker.ts
index b1c8994..374538d 100644
--- a/src/typechecker.ts
+++ b/src/typechecker.ts
@@ -168,6 +168,11 @@ function infer(expr: AST, env: TypeEnv, subst: Subst): TypeAST | null {
                             if (!instances || !instances.has(resolved.name)) {
                                 warn(`No instance ${constraint.className} ${resolved.name}`, expr);
                             }
+                        } else if (resolved.kind === 'type-apply' && resolved.constructor.kind === 'type-name') {
+                            const instances = moduleInstances.get(constraint.className);
+                            if (!instances || !instances.has(resolved.constructor.name)) {
+                                warn(`No instance ${constraint.className} ${resolved.constructor.name}`, expr);
+                            }
                         }
                     }
                 }