Improve errors in digiths

src/lang/program.ts

@@ -294,10 +294,18 @@ export namespace Program {
     raw_body: string,
   }
 
+  export function getFunction(program: Program, name: FunctionName): Result<FunctionDefinition> {
+    const fn = program.function_definitions.get(name);
+    if (fn === undefined) {
+      return Result.error({ tag: "FunctionNotFound", name });
+    }
+    return Result.ok(fn);
+  }
+
   export function registerFunction(
     program: Program, 
     { name, body, parameters, raw_parameters, raw_body }: CreateFunction
-  ): Result<void> {
+  ): Result<FunctionName> {
     if (program.function_definitions.has(name)) {
       return Result.error({ tag: "DuplicateFunctionName", name });
     }
@@ -317,13 +325,12 @@ export namespace Program {
     program.function_definitions.set(name, { tag: "user", def: newFunction });
     program.function_definition_order.push(name);
 
-    return Result.ok(undefined);
+    return Result.ok(name);
   }
 
   export type UpdateFunction = {
     parameters: ProductPattern[],
     body: Expr,
-    raw_name: string,
     raw_parameters: string,
     raw_body: string,
   }