prefer_explicit_function_type

v0.3.0 Warning Fix Type Annotations

Flags uses of the bare Function type that do not specify a return type or parameter list. Using the unparameterized Function type effectively makes the declaration dynamic and disables type checking on calls, which can hide bugs.

Why use this rule

The bare Function type accepts any number and type of arguments and returns dynamic, bypassing Dart’s type system entirely. Specifying the return type and parameter list catches mismatched signatures at compile time rather than at runtime.

See also: Dart language - Function type

Don’t

class BadWidget {
  final Function onTap;
  final Function? onLongPress;

  const BadWidget(this.onTap, this.onLongPress);
}

void badFunction(Function callback) {}

Function badReturnType() => () {};

List<Function> callbacks = [];

Do

class GoodWidget {
  final void Function() onTap;
  final void Function()? onLongPress;

  const GoodWidget(this.onTap, this.onLongPress);
}

void goodFunction(void Function() callback) {}

void Function() goodReturnType() => () {};

List<void Function()> callbacks = [];

// Function types with parameters and return types
final void Function(int value) onValueChanged = (_) {};
final int Function(String input) processInput = (_) => 0;

Configuration

This rule is in the recommended preset, so it is on with preset: recommended or preset: opinionated. Add it to preset: core with prefer_explicit_function_type: true.

To turn it off:

many_lints.yaml

rules:
  prefer_explicit_function_type: false

To keep the rule on but skip certain paths, use per-rule exclude.

Related rules

• prefer_explicit_parameter_names — Name the parameters of a function type.
• prefer_typedefs_for_callbacks — Name a multi-parameter function type with a typedef.
• prefer_void_callback — Use ‘VoidCallback’ instead of ‘void Function()’.
• prefer_explicit_type_arguments — Pin the type arguments of the APIs where inference surprises.