Impossible to define field of parameterized type with function type

[rakudrama]

In the following example I would like to declare compareFn as having the type E x E -> int.
Dart offers no way to do this. An attempt to declare the type directly conflicts with the syntax of a local function definition. Local typedefs are not supported.

class Methods<E> {
  final compareFn; // This needs improving

  Methods(bool compare(E a, E b)) : compareFn = compare;

  bool lessThan(E a, E b) => compareFn(a, b) < 0;
}

main() {
  var m = new Methods<int>((int a, int b) => a.compareTo(b));

  print('1 < 2 = ${m.lessThan(1, 2)}');
  print('2 < 2 = ${m.lessThan(2, 2)}');
}

It seems that the tightest bound is a binary function returning int. The best of the poor alternatives is to have a top-level typedef for Dynamic x Dynamic -> int.

[DartBot]

This comment was originally written by [email protected]

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Added Triaged label.

[gbracha]

Good point.

PS: Bugs against the language are really bugs against the spec. If the spec is wrong, that is a bug. If the language is not too your liking, no matter how displeased you are, it is a feature request.

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Removed Type-Defect label.
Added Type-Enhancement label.

[anders-sandholm]

Added this to the Later milestone.

[rakudrama]

This enhancement is now implemented:

typedef bool CompareFunction<E>(E a, E b);

class Methods<E> {
  final CompareFunction<E> compareFn;

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Added Fixed label.