From b8fd8f0672c289ff43e501f430b76de1c7b0440a Mon Sep 17 00:00:00 2001
From: Kai Stevenson <kai@kaistevenson.com>
Date: Tue, 4 Nov 2025 20:53:14 -0800
Subject: constrained args for createFn

---
 src/lang/ks-lang/index.ts | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/src/lang/ks-lang/index.ts b/src/lang/ks-lang/index.ts
index c9c7405..e5a78b9 100644
--- a/src/lang/ks-lang/index.ts
+++ b/src/lang/ks-lang/index.ts
@@ -14,19 +14,29 @@ export type TransformArgs<Args extends readonly ASTNode[]> = {
   [Idx in keyof Args]: any;
 };
 
-export const createFn = <Program extends string>(
-  program: Program
-): Evaluate<Parse<Lex<Program>>> extends [infer ProgramFn extends FnPrim]
-  ? <const Args extends TransformArgs<ProgramFn["args"]>>(
-      ...args: Args
-    ) => CallFn<ProgramFn, Args, EmptyStackFrame>
-  : EvalError<"Cannot create a function from a program that does not eval to a function"> => {
-  const [programFn] = evaluate(parse(lex(program))) as Array<FnPrim>;
-  if (!programFn.fn) {
-    throw new Error(
-      "Cannot create a function from a program that does not eval to a function"
-    );
-  }
+export type AssertArgs<
+  Args extends readonly ASTNode[],
+  ArgsConstraint extends readonly any[]
+> = TransformArgs<Args> extends ArgsConstraint ? ArgsConstraint : never;
 
-  return ((...args: any[]) => callFn(programFn, args, emptyStackFrame)) as any;
-};
+export const createFn =
+  <ArgsConstraint extends any[]>() =>
+  <Program extends string>(
+    program: Program
+  ): Evaluate<Parse<Lex<Program>>> extends [infer ProgramFn extends FnPrim]
+    ? TransformArgs<ProgramFn["args"]> extends ArgsConstraint
+      ? <const Args extends ArgsConstraint>(
+          ...args: Args
+        ) => CallFn<ProgramFn, Args, EmptyStackFrame>
+      : EvalError<`Program's args do not extend args constraint`>
+    : EvalError<"Cannot create a function from a program that does not eval to a function"> => {
+    const [programFn] = evaluate(parse(lex(program))) as Array<FnPrim>;
+    if (!programFn.fn) {
+      throw new Error(
+        "Cannot create a function from a program that does not eval to a function"
+      );
+    }
+
+    return ((...args: any[]) =>
+      callFn(programFn, args, emptyStackFrame)) as any;
+  };
-- 
cgit v1.2.3-70-g09d2