repl: show error for partial application

cli/tests/repl_eval.rs

@@ -476,6 +476,26 @@ mod repl_eval {
         expect_success("Num.abs", "<function> : Num a -> Num a");
     }
 
+    #[test]
+    fn too_few_args() {
+        expect_failure(
+            "Num.add 2",
+            indoc!(
+                r#"
+                ── TOO FEW ARGS ────────────────────────────────────────────────────────────────
+
+                The add function expects 2 arguments, but it got only 1:
+
+                4│      Num.add 2
+                        ^^^^^^^
+
+                Roc does not allow functions to be partially applied. Use a closure to
+                make partial application explicit.
+                "#
+            ),
+        );
+    }
+
     #[test]
     fn type_problem() {
         expect_failure(

compiler/load/src/file.rs

@@ -3548,9 +3548,29 @@ fn add_def_to_module<'a>(
                     // get specialized!
                     if is_exposed {
                         let annotation = def.expr_var;
-                        let layout = layout_cache.from_var(mono_env.arena, annotation, mono_env.subs).unwrap_or_else(|err|
+
-                                        todo!("TODO gracefully handle the situation where we expose a function to the host which doesn't have a valid layout (e.g. maybe the function wasn't monomorphic): {:?}", err)
+                        let layout = match layout_cache.from_var(
-                                    );
+                            mono_env.arena,
+                            annotation,
+                            mono_env.subs,
+                        ) {
+                            Ok(l) => l,
+                            Err(LayoutProblem::Erroneous) => {
+                                let message = "top level function has erroneous type";
+                                procs.runtime_errors.insert(symbol, message);
+                                return;
+                            }
+                            Err(LayoutProblem::UnresolvedTypeVar(v)) => {
+                                let message = format!(
+                                    "top level function has unresolved type variable {:?}",
+                                    v
+                                );
+                                procs
+                                    .runtime_errors
+                                    .insert(symbol, mono_env.arena.alloc(message));
+                                return;
+                            }
+                        };
 
                         procs.insert_exposed(
                             symbol,