Report effectful top-level exprs

crates/compiler/constrain/src/expr.rs

@@ -1845,7 +1845,7 @@ fn constrain_call_fx(
             ))
         }
         None => constraints.push_expected_type(ForReason(
-            Reason::CallInTopLevelDef,
+            Reason::CallInTopLevel,
             // top-level defs are only allowed to call pure functions
             constraints.push_variable(Variable::PURE),
             region,

crates/compiler/load/tests/test_reporting.rs

@@ -14552,6 +14552,44 @@ All branches in an `if` must have the same type!
     "###
     );
 
+    test_report!(
+        effect_in_top_level_value_def,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            hello =
+                Effect.putLine! "calling hello!"
+                "hello"
+
+            main! = \{} ->
+                Effect.putLine! hello
+            "#
+        ),
+        @r###"
+    ── EFFECT IN TOP-LEVEL in /code/proj/Main.roc ──────────────────────────────────
+
+    This top-level expression calls an effectful function:
+
+    6│      Effect.putLine! "calling hello!"
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    However, only functions are allowed to be effectful. This limitation
+    ensures that importing a module never produces a side effect.
+
+    Tip: If you don't need any arguments, use an empty record:
+
+        askName! : {} => Str
+        askName! = \{} ->
+            Stdout.line! "What's your name?"
+            Stdin.line! {}
+
+    This will allow the caller to control when the effect runs.
+    "###
+    );
+
     test_report!(
         aliased_fx_fn,
         indoc!(

crates/compiler/lower_params/src/type_error.rs

@@ -185,7 +185,7 @@ fn remove_for_reason(
         }
         | Reason::CrashArg
         | Reason::CallInFunction(_)
-        | Reason::CallInTopLevelDef
+        | Reason::CallInTopLevel
         | Reason::Stmt
         | Reason::ImportParams(_) => {}
     }

crates/compiler/types/src/types.rs

@@ -3449,7 +3449,7 @@ pub enum Reason {
     },
     Stmt,
     CallInFunction(Option<Region>),
-    CallInTopLevelDef,
+    CallInTopLevel,
     FloatLiteral,
     IntLiteral,
     NumLiteral,

crates/reporting/src/error/type.rs

@@ -1727,7 +1727,26 @@ fn to_expr_report<'b>(
                     severity,
                 }
             }
-            Reason::CallInTopLevelDef => todo!("[purity-inference] CallInTopLevelDef"),
+            Reason::CallInTopLevel => {
+                let lines = [
+                    alloc.reflow("This top-level expression calls an effectful function:"),
+                    alloc.region(lines.convert_region(region), severity),
+                    alloc.reflow("However, only functions are allowed to be effectful. This limitation ensures that importing a module never produces a side effect."),
+                    alloc.concat([
+                        alloc.tip(),
+                        alloc.reflow("If you don't need any arguments, use an empty record:"),
+                    ]),
+                    alloc.parser_suggestion("    askName! : {} => Str\n    askName! = \\{} ->\n        Stdout.line! \"What's your name?\"\n        Stdin.line! {}"),
+                    alloc.reflow("This will allow the caller to control when the effect runs."),
+                ];
+
+                Report {
+                    filename,
+                    title: "EFFECT IN TOP-LEVEL".to_string(),
+                    doc: alloc.stack(lines),
+                    severity,
+                }
+            }
             Reason::Stmt => todo!("[purity-inference] Stmt"),
         },
     }