Report effect call in pure function

crates/compiler/load/tests/test_reporting.rs

@@ -14543,37 +14543,6 @@ All branches in an `if` must have the same type!
     "
     );
 
-    test_report!(
-        leftover_statement,
-        indoc!(
-            r#"
-            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
-
-            import pf.Effect
-
-            main! = \{} ->
-                identity {}
-
-                Effect.putLine! "hello"
-
-            identity = \x -> x
-            "#
-        ),
-        @r###"
-    ── LEFTOVER STATEMENT in /code/proj/Main.roc ───────────────────────────────────
-
-    This statement does not produce any effects:
-
-    6│      identity {}
-            ^^^^^^^^^^^
-
-    Standalone statements are only useful if they call effectful
-    functions.
-
-    Did you forget to use its result? If not, feel free to remove it.
-    "###
-    );
-
     test_report!(
         return_outside_of_function,
         indoc!(
@@ -14691,7 +14660,7 @@ All branches in an `if` must have the same type!
     );
 
     test_report!(
-        function_def_fx_no_bang,
+        leftover_statement,
         indoc!(
             r#"
             app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
@@ -14699,25 +14668,63 @@ All branches in an `if` must have the same type!
             import pf.Effect
 
             main! = \{} ->
-                printHello {}
+                identity {}
 
-            printHello = \{} ->
                 Effect.putLine! "hello"
+
+            identity = \x -> x
             "#
         ),
         @r###"
-    ── MISSING EXCLAMATION in /code/proj/Main.roc ──────────────────────────────────
+    ── LEFTOVER STATEMENT in /code/proj/Main.roc ───────────────────────────────────
 
-    This function is effectful, but its name does not indicate so:
+    This statement does not produce any effects:
 
-    8│  printHello = \{} ->
+    6│      identity {}
-        ^^^^^^^^^^
+            ^^^^^^^^^^^
 
-    Add an exclamation mark at the end of its name, like:
+    Standalone statements are only useful if they call effectful
+    functions.
 
-        printHello!
+    Did you forget to use its result? If not, feel free to remove it.
+    "###
+    );
 
-    This will help readers identify it as a source of effects.
+    test_report!(
+        fx_fn_annotated_as_pure,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            main! = \{} ->
+                Effect.putLine! (getCheer "hello")
+
+            getCheer : Str -> Str
+            getCheer = \msg ->
+                name = Effect.getLine! {}
+
+                "$(msg), $(name)!"
+            "#
+        ),
+        @r###"
+    ── EFFECT IN PURE FUNCTION in /code/proj/Main.roc ──────────────────────────────
+
+    This expression calls an effectful function:
+
+    10│      name = Effect.getLine! {}
+                    ^^^^^^^^^^^^^^^^^^
+
+    However, the type of the enclosing function indicates it must be pure:
+
+    8│  getCheer : Str -> Str
+                   ^^^^^^^^^^
+
+    Tip: Replace `->` with `=>` to annotate it as effectful.
+
+    You can still run the program with this error, which can be helpful
+    when you're debugging.
     "###
     );
 
@@ -14809,4 +14816,6 @@ All branches in an `if` must have the same type!
     This will help readers identify it as a source of effects.
     "###
     );
+
+    // [purity-inference] TODO: check ! in records, tuples, tags, opaques, and arguments
 }

crates/reporting/src/error/type.rs

@@ -351,7 +351,9 @@ pub fn type_problem<'b>(
             let stack = [
                 alloc.reflow("This function is pure, but its name suggests otherwise:"),
                 alloc.region(lines.convert_region(region), severity),
-                alloc.reflow("Remove the exclamation mark to give an accurate impression of its behavior."),
+                alloc.reflow(
+                    "Remove the exclamation mark to give an accurate impression of its behavior.",
+                ),
             ];
 
             Some(Report {
@@ -1692,6 +1694,7 @@ fn to_expr_report<'b>(
                 unimplemented!("record default field is not implemented yet")
             }
             Reason::ImportParams(_) => unreachable!(),
+
             Reason::FunctionOutput => {
                 let problem = alloc.concat([
                     alloc.text("This "),
@@ -1726,7 +1729,40 @@ fn to_expr_report<'b>(
                     severity,
                 }
             }
-            Reason::CallInFunction(_) => todo!("[purity-inference] CallInFunction"),
+
+            Reason::CallInFunction(ann_region) => {
+                let lines = [
+                    alloc.reflow("This expression calls an effectful function:"),
+                    alloc.region(lines.convert_region(region), severity),
+                    match ann_region {
+                        Some(ann_region) => alloc.stack([
+                            alloc.reflow(
+                                "However, the type of the enclosing function indicates it must be pure:",
+                            ),
+                            alloc.region(lines.convert_region(ann_region), Severity::Warning),
+                            alloc.concat([
+                                alloc.tip(),
+                                alloc.text("Replace "),
+                                alloc.keyword("->"),
+                                alloc.text(" with "),
+                                alloc.keyword("=>"),
+                                alloc.text(" to annotate it as effectful."),
+                            ]),
+                        ]),
+                        None => {
+                            alloc.reflow("However, the enclosing function is required to be pure.")
+                        }
+                    },
+                    alloc.reflow("You can still run the program with this error, which can be helpful when you're debugging."),
+                ];
+
+                Report {
+                    filename,
+                    title: "EFFECT IN PURE FUNCTION".to_string(),
+                    doc: alloc.stack(lines),
+                    severity,
+                }
+            }
             Reason::CallInTopLevelDef => todo!("[purity-inference] CallInTopLevelDef"),
             Reason::Stmt => todo!("[purity-inference] Stmt"),
         },