Error Handling

Guard Clause

Guard clauses return early for invalid or missing input, leaving the main path less nested.

Program

Play the program to classify a score, then select a missing value to follow the first guard.

score
guard_clause.dart
Replay: real traced execution (multi-file project)
String classify(int? score) {
  if (score == null) return 'missing';
  if (score < 60) return 'retry';
  return 'pass';
}

void main() {
  int? score = 85;
  var label = classify(score);
  print(label);
}

String classify(int? score) {
  if (score == null) return 'missing';
  if (score < 60) return 'retry';
  return 'pass';
}

void main() {
  int? score = null;
  var label = classify(score);
  print(label);
}

  1. score ← 85

    7void main() {8  int? score = 85;9  var label = classify(score);
    values this step85score

  2. call ← classify(85)

    8int? score = 85;9var label = classify(score);10print(label);
    values this stepclassify(85)call85score

  3. score == null ← false

    1String classify(int? score) {2  if (score == null) return 'missing';3  if (score < 60) return 'retry';
    values this stepfalsescore == null85score

  4. score < 60 ← false

    2if (score == null) return 'missing';3if (score < 60) return 'retry';4return 'pass';
    values this stepfalsescore < 6085score

  5. return value ← pass

    3  if (score < 60) return 'retry';4  return 'pass';5}
    values this steppassreturn value

  6. label ← pass

    8int? score = 85;9var label = classify(score);10print(label);
    values this steppasslabel

  7. print(label);

    9  var label = classify(score);10  print(label);11}
    outputpass
    values this steppasslabel

  1. score ← null

    7void main() {8  int? score = null;9  var label = classify(score);
    values this stepnullscore

  2. call ← classify(null)

    8int? score = null;9var label = classify(score);10print(label);
    values this stepclassify(null)callnullscore

  3. return value ← missing

    1String classify(int? score) {2  if (score == null) return 'missing';3  if (score < 60) return 'retry';
    values this stepmissingreturn valuenullscore

  4. label ← missing

    8int? score = null;9var label = classify(score);10print(label);
    values this stepmissinglabel

  5. print(label);

    9  var label = classify(score);10  print(label);11}
    outputmissing
    values this stepmissinglabel
guard clause A guard handles an edge case and exits early.
nullable input `int? score` allows the missing-value case.
flat main path After guards, the remaining code is the normal case.