Dart Migrate To Checks Package Agent Skill

Migrating Dart Tests to Package Checks

Use this skill when you need to migrate a Dart test suite from the legacy

package:matcher (which is exported by default from package:test/test.dart)

to the modern, type-safe, and literate package:checks assertion library.

Contents

• When to Use This Skill
• How to Use This Skill (The Workflow)
• Key Syntax Differences and Pitfalls
• Matcher-to-Checks Mapping Table
• Matchers with No Direct Replacements
• Strategies for Discovery
• Examples

When to Use This Skill

• When asked to "migrate tests to checks", "use package:checks", or

"modernize test assertions".

• When updating legacy test suites where static type safety, better

autocomplete in IDEs, and highly detailed failure diagnostics are desired.

How to Use This Skill (The Workflow)

Follow this structured workflow to safely and systematically migrate a test suite:

1. Dependency Setup

• Add package:checks as a dev_dependency in pubspec.yaml:

  dart pub add dev:checks

• Remove package:matcher if it is explicitly listed under dev_dependencies

(it is typically transitively included by package:test, which is fine).

2. Identify and Plan Target Files

• Use the grep patterns in Strategies for Discovery

to locate all test files containing legacy expect or expectLater calls.

• Decide whether to migrate files fully or incrementally.

3. Migrating a File (Incremental or Full)

For any target test file:

• Update Imports:
• Replace the generic import 'package:test/test.dart'; with:

     import 'package:test/scaffolding.dart';
     import 'package:checks/checks.dart';

• For Incremental Migration: If you only want to migrate some test cases

in the file, or want to migrate one step at a time, add:

     import 'package:test/expect.dart'; // Temporarily allows legacy expect()

• Translate Assertions: Rewrite legacy expect and expectLater calls

to check syntax following the [Key Syntax Differences and

Pitfalls](#key-syntax-differences-and-pitfalls) and the

Matcher-to-Checks Mapping Table.

• Verify via Compiler: If migrating fully, remove the `import

'package:test/expect.dart'; line. Any remaining un-migrated expect`

calls will immediately surface as compiler errors, making them easy to

find and fix.

4. Verification and Feedback Loops

• Static Analysis: Run static analysis on the target package:

  dart analyze

Pay close attention to generic type parameters on .isA<Type>() and

ensure asynchronous expectations are properly awaited (check for

unawaited_futures warnings).

• Run Tests: Execute the tests to verify both behavior and correct

assertion runtime logic:

  dart test

If a test fails, review the extremely detailed failure output of

package:checks to diagnose if the test is genuinely failing or if the

expectation was translated incorrectly.

Key Syntax Differences and Pitfalls

> [!IMPORTANT]

> A line-for-line translation can sometimes introduce subtle bugs or false

> passes. Always review these key differences carefully:

1. Collection Equality Pitfall (equals vs deepEquals)

• Legacy Matcher: expect(actual, expected) or `expect(actual,

equals(expected))` performed a deep equality check if the arguments

were collections (Lists, Maps, Sets).

• Package Checks: .equals(expected) corresponds strictly to

operator ==. Since Dart collections do not override operator == for

element-wise comparison, using .equals on a collection will check for

*identity* and almost certainly fail at runtime.

• Remediation: You must replace collection equality assertions with

.deepEquals(expected).

  // BEFORE (Matcher)
  expect(myList, [1, 2, 3]);

  // AFTER (Checks)
  check(myList).deepEquals([1, 2, 3]);

2. The reason Parameter is now because

• Legacy Matcher: The explanation was passed as a trailing named

argument reason to expect:

  expect(actual, expectation, reason: 'Explanation');

• Package Checks: The explanation is passed as the named argument

because to the check function *before* the actual subject:

  check(because: 'Explanation', actual).expectation();

3. Regular Expression Matching (matches vs matchesPattern)

• Legacy Matcher: The matches(pattern) matcher automatically converted

a String argument into a RegExp (e.g., matches(r'\d') matched '1').

• Package Checks: .matchesPattern(pattern) treats a String argument

as a literal string pattern.

• Remediation: To match using a regular expression, you must explicitly

pass a RegExp object:

  // BEFORE (Matcher)
  expect(someString, matches(r'\d+'));

  // AFTER (Checks)
  check(someString).matchesPattern(RegExp(r'\d+'));

4. Property Extraction (TypeMatcher.having vs .has)

• Legacy Matcher: Chained field/property expectations used

TypeMatcher.having(feature, description, matcher):

  expect(actual, isA<Person>().having((p) => p.name, 'name', startsWith('A')));

• Package Checks: The .has(feature, description) extension is

available on all Subjects, takes one fewer argument, and returns a new

Subject representing that property. You chain expectations directly off

it:

  check(actual).isA<Person>().has((p) => p.name, 'name').startsWith('A');

5. Synchronous vs. Asynchronous throws

• Legacy Matcher: In package:matcher, throwsA behaved similarly for both

synchronous closures and asynchronous futures when wrapped in expect or

expectLater.

• Package Checks: The .throws<E>() expectation behaves differently and

has different return types depending on whether the subject is synchronous or

asynchronous:

• Synchronous (Subject<T Function()>): .throws<E>() returns a

Subject<E> synchronously. This does not accept a callback argument!

You chain or cascade expectations directly off the returned Subject<E>:

    // YES (Synchronous chaining)
    check(() => triggerSyncError()).throws<ArgumentError>()
      ..has((e) => e.message, 'message').equals('invalid input');

    // NO (Passing a callback to sync throws will cause a compiler error!)
    check(() => triggerSync").throws<ArgumentError>((it) => ...); // ERROR!

• Asynchronous (Subject<Future<T>>): .throws<E>() returns

Future<void>. Because you cannot chain directly off a Future<void>, this

requires an inspection callback:

    // YES (Asynchronous callback)
    await check(triggerAsyncError()).throws<ArgumentError>((it) => it
      ..has((e) => e.message, 'message').equals('invalid input'));

• Crucial Pitfall: Trying to chain expectations directly after an awaited

asynchronous .throws<E>() (e.g.,

await check(future).throws<E>().equals(...)) will fail to compile

because it returns Future<void>.

6. RegExp / Pattern Equality

• Legacy Matcher: In package:matcher, expect(myPattern,

equals(RegExp('Hello'))) worked because the matcher comparison rules

handled RegExp instances.

• Package Checks: .equals() uses strict Dart == equality. Since separate

RegExp instances do not satisfy ==, using .equals() will fail at runtime.

• Remediation: Use .isA<RegExp>() type refinement along with cascades to

assert on the properties of the RegExp object explicitly:

  check(myPattern).isA<RegExp>()
    ..has((r) => r.pattern, 'pattern').equals('Hello')
    ..has((r) => r.isMultiLine, 'isMultiLine').isTrue();

7. Strict Nullable Boolean Safety (bool? fields)

• Legacy Matcher: Statically, isTrue and isFalse performed loose

dynamic checks at runtime, which silently accepted nullable booleans (bool?).

• Package Checks: .isTrue() and .isFalse() are defined strictly on

Subject<bool> (non-nullable). They are not available on Subject<bool?>.

• Remediation: For fields declared as bool?, you must either refine the

subject (e.g., .isNotNull().isTrue()) or simply use .equals(true) and

.equals(false) which are generic and work on all types:

  // If options.flagOutdated is a bool?
  check(options.flagOutdated).equals(true);
  check(options.flagOutdated).equals(false);

8. Map Key Containment (containsKey vs contains)

• Legacy Matcher: In package:matcher, contains(key) was used to assert

that a Map contained a specific key.

• Package Checks: Calling .contains(...) on a Subject<Map> is not

defined and will fail compilation.

• Remediation: Use the map-specific .containsKey(key) matcher instead:

  // BEFORE (Matcher)
  expect(myMap, contains('my_key'));

  // AFTER (Checks)
  check(myMap).containsKey('my_key');

9. Explicit Generic Parameters for Extension Types

• Legacy Matcher: expect(extensionTypeConst, 3) compiled because of loose

dynamic equality.

• Package Checks: If QrEciValue is an extension type representation of int

(e.g., extension type const QrEciValue(int value) implements int), calling

.equals(3) on a Subject<QrEciValue> fails because 3 (an int) is not

assignable to QrEciValue. Casting with as int will trigger an

"Unnecessary cast" static analysis warning because QrEciValue statically

implements int.

• Remediation: Explicitly specify the generic type parameter on the check

function to force checks to treat it as the primitive type:

  // YES (Type-safe and warning-free)
  check<int>(QrEciValue.iso8859_1).equals(3);

10. Dynamic Map / JSON Lookup Casting

• Legacy Matcher: Loose dynamic typing allowed comparing nested json lookups

statically typed as dynamic directly against lists or maps.

• Package Checks: Strict type safety rejects the implicit assignment of

dynamic to Iterable<Object?> in .deepEquals(...).

• Remediation: Statically cast the dynamic lookup result to a List or Map:

  // YES (Explicit cast to List)
  check(myIterable).deepEquals(json['data']['items'] as List);

Matcher-to-Checks Mapping Table

Use this table as a quick reference for direct matcher replacements:

| Legacy Matcher | Package Checks Equivalent | Notes |

| :--- | :--- | :--- |

| expect(actual, expected) | check(actual).equals(expected) | Use .deepEquals for collections! |

| expect(actual, equals(expected)) | check(actual).equals(expected) | Use .deepEquals for collections! |

| isA<T>() | check(actual).isA<T>() | Chaining is supported directly |

| same(expected) | check(actual).identicalTo(expected) | Verifies identity |

| anyElement(matcher) | check(iterable).any(conditionCallback) | E.g. check(list).any((e) => e.equals(1)) |

| everyElement(matcher) | check(iterable).every(conditionCallback) | E.g. check(list).every((e) => e.isGreaterThan(0)) |

| hasLength(expected) | check(actual).length.equals(expected) | Works on String, Map, Iterable, etc. |

| isNot(matcher) | check(actual).not(conditionCallback) | E.g. check(val).not((it) => it.equals(5)) |

| contains(element) | check(actual).contains(element) | Works on String, Iterable (use containsKey for Map!) |

| contains(key) (on a Map) | check(map).containsKey(key) | Map key containment |

| startsWith(prefix) | check(string).startsWith(prefix) | String only |

| endsWith(suffix) | check(string).endsWith(suffix) | String only |

| isEmpty | check(actual).isEmpty() | Works on String, Map, Iterable |

| isNotEmpty | check(actual).isNotEmpty() | Works on String, Map, Iterable |

| isNull | check(actual).isNull() | |

| isNotNull | check(actual).isNotNull() | |

| isTrue / true | check(actual).isTrue() | Works on non-nullable bool only |

| isFalse / false | check(actual).isFalse() | Works on non-nullable bool only |

| completion(matcher) | await check(future).completes(conditionCallback) | Must be awaited! |

| throwsA(matcher) | await check(future).throws<Type>() | Must be awaited! |

| emits(value) | await check(streamQueue).emits(conditionCallback) | Must be awaited! |

| emitsThrough(value) | await check(streamQueue).emitsThrough(conditionCallback) | Must be awaited! |

| stringContainsInOrder(list) | check(string).containsInOrder(list) | String only |

| pairwiseCompare(...) | check(actual).pairwiseMatches(...) | |

Matchers with No Direct Replacements

Some legacy matchers do not have a one-to-one equivalent in package:checks

due to API cleanup. Use these standard workarounds:

1. Specific Error Matchers

• Legacy: throwsArgumentError, throwsStateError,

throwsUnsupportedError, etc.

• Checks: Use .throws<T>() with the specific error type:

  await check(triggerError()).throws<ArgumentError>();

2. The anything Matcher

• Legacy: expect(actual, anything)
• Checks: Pass an empty condition callback (_) {} when a condition is

syntactically required:

  await check(someFuture).completes((_) {});

3. Specific Numeric Toggles

• Legacy: isPositive, isNegative, isZero, isNonPositive,

isNonNegative, isNonZero

• Checks: Use explicit comparative expectations:
• isPositive $\rightarrow$ isGreaterThan(0)
• isNegative $\rightarrow$ isLessThan(0)
• isZero $\rightarrow$ equals(0)
• isNonNegative $\rightarrow$ isGreaterOrEqual(0)

4. Numeric Ranges

• Legacy: inClosedOpenRange(min, max), inInclusiveRange(min, max),

etc.

• Checks: Chain the boundaries using the cascade operator (..):

  check(actualValue)
    ..isGreaterOrEqual(min)
    ..isLessThan(max);

Writing Custom Expectations (Replacing Custom Matchers)

When migrating from a legacy codebase, you may encounter custom Matcher

subclasses. In package:checks, custom assertions are implemented as

extension methods on Subject<T>.

To write custom expectations, you must import the checks context API:

import 'package:checks/context.dart';

1. Simple Custom Expectations (using expect)

Use context.expect to check a property and return a Rejection on failure:

extension CustomPersonChecks on Subject<Person> {
  void isAdult() {
    context.expect(
      () => ['is an adult (age >= 18)'],
      (actual) {
        if (actual.age >= 18) return null; // Pass
        return Rejection(
          which: ['is only ${actual.age} years old'],
        );
      },
    );
  }
}

2. Nested Property Extraction (using nest or has)

To extract a property and allow further chained checks, use nest or the

simpler has helper:

• Using has (Recommended for simple, non-failing field access):

  extension CustomPersonChecks on Subject<Person> {
    Subject<Address> get address => has((p) => p.address, 'address');
  }

• Using nest (For property extraction that can fail or reject):

  extension CustomPersonChecks on Subject<Person> {
    Subject<String> get ssn => context.nest(
      'has a valid SSN',
      (actual) {
        final ssnValue = actual.ssn;
        if (ssnValue == null) {
          return Extracted.rejection(which: ['has no SSN']);
        }
        return Extracted.value(ssnValue);
      },
    );
  }

3. Asynchronous Custom Expectations

If the expectation is asynchronous (e.g. checking a Future or Stream), use

context.expectAsync or context.nestAsync and return the resulting Future:

extension CustomFutureChecks<T> on Subject<Future<T>> {
  Future<void> completesNormally() {
    return context.expectAsync(
      () => ['completes without throwing'],
      (actual) async {
        try {
          await actual;
          return null; // Pass
        } catch (e) {
          return Rejection(which: ['threw $e']);
        }
      },
    );
  }
}

Strategies for Discovery

Execute these commands in the terminal to identify legacy matchers and files

requiring migration:

# 1. Find all test files containing legacy expect() or expectLater()
grep -rn "expect(" test/
grep -rn "expectLater(" test/

# 2. Find potential collection equality pitfalls (literal lists or maps)
grep -rn "expect(.*, \[" test/
grep -rn "expect(.*, {" test/

# 3. Find matches() calls (need conversion to RegExp + matchesPattern)
grep -rn "matches(" test/

# 4. Find legacy TypeMatcher.having() calls (which need conversion to .has())
grep -rn "having(" test/

Examples

Basic Assertions

Before (Matcher):

expect(someValue, isNotNull);
expect(result, isTrue, reason: 'should be successful');
expect(myString, startsWith('hello'));

After (Checks):

check(someValue).isNotNull();
check(because: 'should be successful', result).isTrue();
check(myString).startsWith('hello');

Collection and Deep Equality

Before (Matcher):

expect(items, [1, 2, 3]);
expect(configMap, equals({'port': 8080}));

After (Checks):

check(items).deepEquals([1, 2, 3]);
check(configMap).deepEquals({'port': 8080});

Chaining and Cascades

Before (Matcher):

expect(someString, allOf([
  startsWith('a'),
  contains('b'),
  endsWith('c'),
]));

After (Checks):

check(someString)
  ..startsWith('a')
  ..contains('b')
  ..endsWith('c');

Complex Property Matching (has)

Before (Matcher):

expect(response, isA<Response>()
    .having((r) => r.statusCode, 'statusCode', 200)
    .having((r) => r.body, 'body', contains('success')));

After (Checks):

check(response).isA<Response>()
  ..has((r) => r.statusCode, 'statusCode').equals(200)
  ..has((r) => r.body, 'body').contains('success');

Asynchronous Futures

Before (Matcher):

expect(fetchData(), completes);
expect(fetchData(), completion(equals('data')));
expect(failingCall(), throwsA(isA<StateError>()));

After (Checks):

await check(fetchData()).completes();
await check(fetchData()).completes((it) => it.equals('data'));
await check(failingCall()).throws<StateError>();

Asynchronous Streams

Before (Matcher):

var queue = StreamQueue(Stream.fromIterable([1, 2, 3]));
await expectLater(queue, emitsInOrder([1, 2, 3]));

After (Checks):

var queue = StreamQueue(Stream.fromIterable([1, 2, 3]));
await check(queue).inOrder([
  (s) => s.emits((e) => e.equals(1)),
  (s) => s.emits((e) => e.equals(2)),
  (s) => s.emits((e) => e.equals(3)),
]);