diff --git a/crates/compiler/load/tests/test_reporting.rs b/crates/compiler/load/tests/test_reporting.rs
index e98eae796e..a349f25252 100644
--- a/crates/compiler/load/tests/test_reporting.rs
+++ b/crates/compiler/load/tests/test_reporting.rs
@@ -14547,11 +14547,11 @@ All branches in an `if` must have the same type!
         leftover_statement,
         indoc!(
             r#"
-            app [main] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
 
             import pf.Effect
 
-            main = \{} ->
+            main! = \{} ->
                 identity {}
 
                 Effect.putLine! "hello"
@@ -14689,4 +14689,66 @@ All branches in an `if` must have the same type!
             Num *
         "###
     );
+
+    test_report!(
+        function_def_fx_no_bang,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            main! = \{} ->
+                printHello {}
+
+            printHello = \{} ->
+                Effect.putLine! "hello"
+            "#
+        ),
+        @r###"
+    ── MISSING EXCLAMATION in /code/proj/Main.roc ──────────────────────────────────
+
+    This function is effectful, but its name does not indicate so:
+
+    8│  printHello = \{} ->
+        ^^^^^^^^^^
+
+    Add an exclamation mark at the end of its name, like:
+
+        printHello!
+
+    This will help readers identify it as a source of effects.
+    "###
+    );
+
+    test_report!(
+        nested_function_def_fx_no_bang,
+        indoc!(
+            r#"
+            app [main!] { pf: platform "../../../../../examples/cli/effects-platform/main.roc" }
+
+            import pf.Effect
+
+            main! = \{} ->
+                printHello = \{} ->
+                    Effect.putLine! "hello"
+
+                printHello {}
+            "#
+        ),
+        @r###"
+    ── MISSING EXCLAMATION in /code/proj/Main.roc ──────────────────────────────────
+
+    This function is effectful, but its name does not indicate so:
+
+    6│      printHello = \{} ->
+            ^^^^^^^^^^
+
+    Add an exclamation mark at the end of its name, like:
+
+        printHello!
+
+    This will help readers identify it as a source of effects.
+    "###
+    );
 }
diff --git a/crates/compiler/lower_params/src/type_error.rs b/crates/compiler/lower_params/src/type_error.rs
index e0d2aea405..18ddc62be2 100644
--- a/crates/compiler/lower_params/src/type_error.rs
+++ b/crates/compiler/lower_params/src/type_error.rs
@@ -100,7 +100,8 @@ pub fn remove_module_param_arguments(
             | TypeError::UnexpectedModuleParams(_, _)
             | TypeError::MissingModuleParams(_, _, _)
             | TypeError::ModuleParamsMismatch(_, _, _, _)
-            | TypeError::PureStmt(_) => {}
+            | TypeError::PureStmt(_)
+            | TypeError::UnsuffixedEffectfulFunction(_, _) => {}
         }
     }
 }
diff --git a/crates/compiler/solve/src/solve.rs b/crates/compiler/solve/src/solve.rs
index 05152c5cf4..95cdc9fd1a 100644
--- a/crates/compiler/solve/src/solve.rs
+++ b/crates/compiler/solve/src/solve.rs
@@ -22,6 +22,7 @@ use roc_debug_flags::dbg_do;
 #[cfg(debug_assertions)]
 use roc_debug_flags::ROC_VERIFY_RIGID_LET_GENERALIZED;
 use roc_error_macros::internal_error;
+use roc_module::ident::IdentSuffix;
 use roc_module::symbol::{ModuleId, Symbol};
 use roc_problem::can::CycleEntry;
 use roc_region::all::Loc;
@@ -448,6 +449,8 @@ fn solve(
                     );
 
                     new_scope.insert_symbol_var_if_vacant(*symbol, loc_var.value);
+
+                    check_symbol_suffix(env, problems, *symbol, *loc_var);
                 }
 
                 // Note that this vars_by_symbol is the one returned by the
@@ -1520,6 +1523,31 @@ fn solve(
     state
 }
 
+fn check_symbol_suffix(
+    env: &mut InferenceEnv<'_>,
+    problems: &mut Vec<TypeError>,
+    symbol: Symbol,
+    loc_var: Loc<Variable>,
+) {
+    match symbol.suffix() {
+        IdentSuffix::None => {
+            if let Content::Structure(FlatType::Func(_, _, _, fx)) =
+                env.subs.get_content_without_compacting(loc_var.value)
+            {
+                if let Content::Effectful = env.subs.get_content_without_compacting(*fx) {
+                    problems.push(TypeError::UnsuffixedEffectfulFunction(
+                        loc_var.region,
+                        symbol,
+                    ));
+                }
+            }
+        }
+        IdentSuffix::Bang => {
+            // [purity-inference] TODO
+        }
+    }
+}
+
 fn chase_alias_content(subs: &Subs, mut var: Variable) -> (Variable, &Content) {
     loop {
         match subs.get_content_without_compacting(var) {
diff --git a/crates/compiler/solve_problem/src/lib.rs b/crates/compiler/solve_problem/src/lib.rs
index 4e49041a1d..49c6bcb389 100644
--- a/crates/compiler/solve_problem/src/lib.rs
+++ b/crates/compiler/solve_problem/src/lib.rs
@@ -40,6 +40,7 @@ pub enum TypeError {
     MissingModuleParams(Region, ModuleId, ErrorType),
     ModuleParamsMismatch(Region, ModuleId, ErrorType, ErrorType),
     PureStmt(Region),
+    UnsuffixedEffectfulFunction(Region, Symbol),
 }
 
 impl TypeError {
@@ -65,6 +66,7 @@ impl TypeError {
             TypeError::IngestedFileBadUtf8(..) => Fatal,
             TypeError::IngestedFileUnsupportedType(..) => Fatal,
             TypeError::PureStmt(..) => Warning,
+            TypeError::UnsuffixedEffectfulFunction(_, _) => Warning,
         }
     }
 
@@ -81,7 +83,8 @@ impl TypeError {
             | TypeError::UnexpectedModuleParams(region, ..)
             | TypeError::MissingModuleParams(region, ..)
             | TypeError::ModuleParamsMismatch(region, ..)
-            | TypeError::PureStmt(region) => Some(*region),
+            | TypeError::PureStmt(region)
+            | TypeError::UnsuffixedEffectfulFunction(region, _) => Some(*region),
             TypeError::UnfulfilledAbility(ab, ..) => ab.region(),
             TypeError::Exhaustive(e) => Some(e.region()),
             TypeError::CircularDef(c) => c.first().map(|ce| ce.symbol_region),
diff --git a/crates/reporting/src/error/type.rs b/crates/reporting/src/error/type.rs
index 09bf62bc18..e1584e459b 100644
--- a/crates/reporting/src/error/type.rs
+++ b/crates/reporting/src/error/type.rs
@@ -330,6 +330,23 @@ pub fn type_problem<'b>(
                 severity,
             })
         }
+        UnsuffixedEffectfulFunction(region, symbol) => {
+            let stack = [
+                alloc.reflow("This function is effectful, but its name does not indicate so:"),
+                alloc.region(lines.convert_region(region), severity),
+                alloc.reflow("Add an exclamation mark at the end of its name, like:"),
+                alloc
+                    .string(format!("{}!", symbol.as_str(alloc.interns)))
+                    .indent(4),
+                alloc.reflow("This will help readers identify it as a source of effects."),
+            ];
+            Some(Report {
+                title: "MISSING EXCLAMATION".to_string(),
+                filename,
+                doc: alloc.stack(stack),
+                severity,
+            })
+        }
     }
 }