Dart Checks Migration

When to use this skill

Use this skill when:

• Migrating existing test files from

  package:matcher

  to

  package:checks

  .
• A user specifically asks for "modern checks" or similar.

The Workflow

1. Analysis:
   • Use

     grep

     to identify files using

     expect

     or

     package:matcher

     .
   • Review custom matchers; these may require manual migration.
2. Tools & Dependencies:
   • Ensure

     dev_dependencies

     includes

     checks

     .
   • Run

     dart pub add --dev checks

     if missing.
3. Discovery:
   • Use the Strategies for Discovery below to find candidates.
4. Replacement:
   • Add

     import 'package:checks/checks.dart';

     .
   • Apply the Common Patterns below.
   • Final Step: Replace

     import 'package:test/test.dart';

     with

     import 'package:test/scaffolding.dart';

     ONLY after all

     expect

     calls are replaced. This ensures incremental progress.
5. Verification:
   • Ensure the code analyzes cleanly.
   • Ensure tests pass.

Common Patterns

expect

check

expect(a, equals(b))

check(a).equals(b)

expect(a, isTrue)

check(a).isTrue()

expect(a, isFalse)

check(a).isFalse()

expect(a, isNull)

check(a).isNull()

expect(a, isNotNull)

check(a).isNotNull()

expect(() => fn(), throwsA<T>())

check(() => fn()).throws<T>()

expect(list, hasLength(n))

check(list).length.equals(n)

expect(a, closeTo(b, delta))

check(a).isA<num>().isCloseTo(b, delta)

expect(a, greaterThan(b))

check(a).isGreaterThan(b)

expect(a, lessThan(b))

check(a).isLessThan(b)

expect(list, isEmpty)

check(list).isEmpty()

expect(list, isNotEmpty)

check(list).isNotEmpty()

expect(list, contains(item))

check(list).contains(item)

expect(map, equals(otherMap))

check(map).deepEquals(otherMap)

expect(list, equals(otherList))

check(list).deepEquals(otherList)

expect(future, completes)

await check(future).completes()

expect(stream, emitsInOrder(...))

await check(stream).withQueue.inOrder(...)

Async & Futures (CRITICAL)

• Checking async functions:

  check(() => asyncFunc()).throws<T>()

  causes FALSE POSITIVES because the closure returns a

  Future

  , which is a value, so it "completes normally" (as a Future). Correct Usage:

  await check(asyncFunc()).throws<T>();
  

• Chaining on void returns: Many async check methods (like

  throws

  ) return

  Future<void>

  . You cannot chain directly on them. Use cascades or callbacks. Wrong:

  await check(future).throws<Error>().has((e) => e.message, 'message').equals('foo');
  

  Correct:

  await check(future).throws<Error>((it) => it.has((e) => e.message, 'message').equals('foo'));
  

Complex Examples

Deep Verification with

isA

having

Legacy:

expect(() => foo(), throwsA(isA<ArgumentError>()
    .having((e) => e.message, 'message', contains('MSG'))));

Modern:

check(() => foo())
    .throws<ArgumentError>()
    .has((e) => e.message, 'message')
    .contains('MSG');

Property Extraction:

Legacy:

expect(obj.prop, equals(value)); // When checking multiple props

Modern:

check(obj)
  ..has((e) => e.prop, 'prop').equals(value)
  ..has((e) => e.other, 'other').equals(otherValue);

One-line Cascades: Since checks often return

void

check(it)..isGreaterThan(10)..isLessThan(20);

Constraints

• Scope: Only modify files in

  test/

  (and

  pubspec.yaml

  ).
• Correctness: One failing test is unacceptable. If a test fails after migration and you cannot fix it immediately, REVERT that specific change.
• Type Safety:

  package:checks

  is stricter about types than

  matcher

  . You may need to add explicit

  as T

  casts or

  isA<T>()

  checks in the chain.

Related Skills

• dart-test-fundamentals: Core concepts for structuring tests, lifecycles, and configuration.
• dart-matcher-best-practices: Best practices for the traditional

  package:matcher

  that is being migrated away from.