falcon

Rules / Style / prefer-underscore-for-unused-callback-parameters

prefer-underscore-for-unused-callback-parameters

lint/style/prefer-underscore-for-unused-callback-parameters
recommended
Same as prefer_underscore_for_unused_callback_parameters · Pyramid Lint

Flags unused closure parameters that are not named with underscores.

For each closure (function expression), collects every identifier referenced in its body — including those inside nested closures, cascades, collection literals, and string interpolations — and flags the first parameter that is never used and is not already an all-underscore name (_, __). Naming a deliberately ignored parameter _ signals intent to the reader and avoids the impression that a real value was forgotten, which matters for the fixed-arity callbacks Flutter APIs demand (builder: (context, _) => ...). At most one diagnostic is reported per closure.

Invalid

example.dartdart
// Bad: unused callback parameter should be named _

void example1() {
  final items = [1, 2, 3];
  items.forEach((item) {
    print('hello');
  });
}

void example2() {
  final map = {'a': 1, 'b': 2};
  map.forEach((key, value) {
    print('hello');
  });
}

void example3() {
  final items = ['x', 'y'];
  items.map((element) => 'mapped').toList();
}

void example4() {
  final numbers = [1, 2, 3];
  numbers.where((n) => true).toList();
}

// Bad: unused parameter in reduce-like operation
void example5() {
  final values = [1, 2, 3, 4];
  values.fold(0, (previous, current) => previous + 1);
}

// Bad: unused index parameter in indexed map
void example6() {
  final items = ['a', 'b', 'c'];
  items.asMap().forEach((index, value) {
    print(value);
  });
}
Unused callback parameter should be named '_'.
4 final items = [1, 2, 3];
5 items.forEach((item) {
Unused callback parameter should be named '_'.
11 final map = {'a': 1, 'b': 2};
12 map.forEach((key, value) {
Unused callback parameter should be named '_'.
18 final items = ['x', 'y'];
19 items.map((element) => 'mapped').toList();
Unused callback parameter should be named '_'.
23 final numbers = [1, 2, 3];
24 numbers.where((n) => true).toList();
Unused callback parameter should be named '_'.
29 final values = [1, 2, 3, 4];
30 values.fold(0, (previous, current) => previous + 1);
Unused callback parameter should be named '_'.
35 final items = ['a', 'b', 'c'];
36 items.asMap().forEach((index, value) {

Valid

example.dartdart
// Good: unused callback parameters renamed to _

void example1() {
  final items = [1, 2, 3];
  items.forEach((_) {
    print('hello');
  });
}

void example2() {
  final map = {'a': 1, 'b': 2};
  map.forEach((_, __) {
    print('hello');
  });
}

void example3() {
  final items = ['x', 'y'];
  items.map((_) => 'mapped').toList();
}

void example4() {
  final numbers = [1, 2, 3];
  numbers.where((_) => true).toList();
}

void example5() {
  final items = [1, 2, 3];
  items.forEach((item) {
    print(item);
  });
}

void example6() {
  final map = {'a': 1, 'b': 2};
  map.forEach((key, value) {
    print('$key: $value');
  });
}

How to configure

Set the severity of prefer-underscore-for-unused-callback-parameters in your falcon.json:

falcon.jsonjson
{
  "linter": {
    "rules": {
      "style": {
        "prefer-underscore-for-unused-callback-parameters": "error"
      }
    }
  }
}

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