falconGitHub

prefer-first

Same as prefer-first in Dart Code Metrics

Group
Style
Path
lint/style/prefer-first
Default
Off, opt in
Origin
Dart Code Metrics
Rule
358 of 512

Flags [0] index access that should use the .first getter.

Reading the leading element as xs.first states the intent directly and is easier to read than the numeric xs[0], which forces the reader to recognize that zero means "first". The rule matches an index expression whose subscript is the integer literal 0; any other index, including a non-literal expression that evaluates to zero, is left alone. Matching is syntactic on the literal, so it does not confirm the receiver exposes a first getter.

Invalid

example.dartdart
// Bad: using [0] instead of .first
void example() {
  final items = [1, 2, 3];
  final first = items[0];
  print(first);
}

class Processor {
  String getFirstName(List<String> names) {
    return names[0];
  }

  void processHead(List<int> values) {
    final head = values[0];
    if (head > 0) {
      compute(head);
    }
  }
}

void multipleViolations(List<String> items) {
  final a = items[0];
  final b = items[0].length;
  print('$a $b');
}

// Regression: the violation must be found inside Dart 3 containers too.
void containers(List<int> xs, int count) {
  final (a, _) = (xs[0], 0);
  int b;
  int _u;
  (b, _u) = (xs[0], 0);
  lbl: {
    final c = xs[0];
    print(c);
  }
  final d = switch (count) {
    0 => xs[0],
    _ => 0,
  };
  final e = switch (xs[0]) {
    _ => 0,
  };
  final f = [if (count > 0) xs[0]];
  final g = [...[xs[0]]];
  final h = (p: xs[0], q: 0);
  assert(xs[0] > 0, "x");
  print([a, b, d, e, f, g, h]);
}
Prefer `.first` over `[0]` to access the first element.
3 final items = [1, 2, 3];
4 final first = items[0];
Prefer `.first` over `[0]` to access the first element.
9 String getFirstName(List<String> names) {
10 return names[0];
Prefer `.first` over `[0]` to access the first element.
13 void processHead(List<int> values) {
14 final head = values[0];
Prefer `.first` over `[0]` to access the first element.
21void multipleViolations(List<String> items) {
22 final a = items[0];
Prefer `.first` over `[0]` to access the first element.
22 final a = items[0];
23 final b = items[0].length;
Prefer `.first` over `[0]` to access the first element.
28void containers(List<int> xs, int count) {
29 final (a, _) = (xs[0], 0);
Prefer `.first` over `[0]` to access the first element.
31 int _u;
32 (b, _u) = (xs[0], 0);
Prefer `.first` over `[0]` to access the first element.
33 lbl: {
34 final c = xs[0];
Prefer `.first` over `[0]` to access the first element.
37 final d = switch (count) {
38 0 => xs[0],
Prefer `.first` over `[0]` to access the first element.
40 };
41 final e = switch (xs[0]) {
Prefer `.first` over `[0]` to access the first element.
43 };
44 final f = [if (count > 0) xs[0]];
Prefer `.first` over `[0]` to access the first element.
44 final f = [if (count > 0) xs[0]];
45 final g = [...[xs[0]]];
Prefer `.first` over `[0]` to access the first element.
45 final g = [...[xs[0]]];
46 final h = (p: xs[0], q: 0);
Prefer `.first` over `[0]` to access the first element.
46 final h = (p: xs[0], q: 0);
47 assert(xs[0] > 0, "x");

Valid

example.dartdart
// Good: using .first
void example() {
  final items = [1, 2, 3];
  final first = items.first;
  print(first);
}

class Processor {
  String getFirstName(List<String> names) {
    return names.first;
  }

  void processHead(List<int> values) {
    final head = values.first;
    if (head > 0) {
      compute(head);
    }
  }
}

void multipleAccess(List<String> items) {
  final a = items.first;
  final b = items.first.length;
  print('$a $b');
}

// OK: accessing non-zero indices
void accessOther(List<int> items) {
  final second = items[1];
  final third = items[2];
}

// OK: in loop context
void loopAccess(List<List<int>> matrix) {
  for (int i = 0; i < matrix.length; i++) {
    final row = matrix[i]; // accessing by variable index is OK
  }
}

Configure prefer-first

Set its severity in falcon.json. See the configuration guide.

falcon.jsonjson
{
  "linter": {
    "rules": {
      "style": {
        "prefer-first": "error"
      }
    }
  }
}

Or silence one occurrence with a suppression comment; the reason is required.

example.dartdart
// falcon-ignore lint/style/prefer-first: <reason>
  • prefer-for-in

    Flags zero-based, unit-step index loops over a syntactically known list.

  • prefer-for-loop-in-childrenflutter

    Prefers collection-for elements over eager map/toList chains in Flutter children.

  • prefer-function-declarations-over-variablesprefer_function_declarations_over_variablesrecommended

    Flags a local variable bound to a function literal that should be a local function declaration.

  • prefer-generic-function-type-aliasesprefer_generic_function_type_aliasesrecommended

    Flags old-style function typedefs written with inline parameter syntax.

  • prefer-getter-over-method

    Prefer getters for synchronous, zero-argument methods that only read a field.

  • prefer-initializing-formalsprefer_initializing_formalsrecommended

    Prefer initializing formals when a parameter is copied directly into its field.

  • prefer-inlined-addsprefer_inlined_addsrecommended

    Flags an add cascade on a list literal whose element could be written inline.

  • prefer-int-literalsprefer_int_literals

    Flags a whole-number double literal, such as 1.0, where the context type is double so the int literal 1 means the same value.