Could Matcher be changed to be typed as Matcher<T>?

[matanlurey]

Specifically, this allows strongly-typed assurances when creating specialized matchers:

abstract class StringMatcher extends Matcher<String> {}

I don't think this is a breaking change, unless we specifically want to allow something like:

expect(5, isNot(equalsIgnoringCase('5')))

---

Added after the fact by @nex3

Blocking issues (see comments for details):

• use min/max instead of LUB/GLB in strong mode inference? sdk#27917 - changes in strong mode inference. Closed as not planned - too breaking
• Sum/union types and type matching language#83 - add support for union types
• Inference should flow information between arguments in a generic function call language#731 - flow information between arguments during inference.

[jacehensley-wf]

I would also like to see something like this. In strong mode you cannot override the types of params in methods. Having something like this would be great.

To be able to use the item param, for instance, in a typed manner you have to cast it using as.

class StringMatcher<T extends String> extends Matcher<T> {
  @override
  bool matches(T item, Map matchState) {
    // Use a String method
    if (item.contains('whatever') return true;
  }
}

Vs.

class StringMatcher extends Matcher {
  @override
  bool matches(dynamic _item, Map matchState) {
    item = _item as String;

    // Use a String method
    if (item.contains('whatever') return true;
  }
}

[matanlurey]

@jacehensley-wf FWIW, I've been using this pattern for now:

@override
bool matches(item, _) {
  if (item is String) {
    return item.contains('whatever');
  }
  return false;
}

Still not preferable since you get runtime errors, not static errors.

[zoechi]

@jacehensley-wf you can now override parameters when you add the @checked annotation https://pub.dartlang.org/packages/meta (1.0.3)

[jacehensley-wf]

Hmm, I thought I tried that. I'll try it again.

Yeah I am still getting this error:

Invalid override. The type of ClassNameMatcher.matches ((String, Map<dynamic, dynamic>) → bool) is not a subtype of Matcher.matches ((dynamic, Map<dynamic, dynamic>) → bool).

When doing this:

bool matches(@checked String className, Map matchState)

I'm on Dart 1.19.1 using Atom if that matters.

[matanlurey]

You might need the latest Dart stable or even development branch.

[jacehensley-wf]

Thanks, I'll try it out on those versions.

Update: Works as expected on Dart 1.20.1!

[nex3]

I like this idea a lot, especially since strong mode provides a lot more benefits for having nice types everywhere. However, in order to make this useful, we'd need to integrate it into expect(), which would require a few features we don't have yet. In particular:

• use min/max instead of LUB/GLB in strong mode inference? sdk#27917. Without this, Dart will just infer expect<Object>() for any cases where the matcher doesn't match the value.
• dart-lang/sdk#4938. The second parameter to expect(), as well as most parameters that take matchers, allow either a matcher or a raw value. In order to have any inference at all, we'd need to be able to express T | Matcher<T>.

[matanlurey]

Cool. Stalking those issues :)

[jacehensley-wf]

@nex3 I think there is still value in adding a generic parameter to Matcher even with out those other issues. I do agree that those with those other issues it would make this much more meaningful.

[zoechi]

Just some update:

@jacehensley-wf you can now override parameters when you add the @checked annotation

@checked was kinda replaced by covariant

[kevmoo]

@nex3 any more thoughts on this?

[nex3]

Neither of the blocking issues are fixed, so... no?

[nex3]

I believe this is also blocked on dart-lang/sdk/issues/25490, since it requires that type information flow between arguments of expect(). I'm going to edit the original comment and add a list of blocking issues there.

[natebosch]

Closing this as not planned. This appears to rely on language features we are unlikely to get anytime soon, and we're instead delivering the benefit this would have brought in a new package with a different design. https://pub.dev/packages/checks