check if future

maybeFuture.dart

import 'dart:async';

typedef _Future = Future Function();
typedef _Void = void Function();
typedef _FutureOrVoid = FutureOr<void> Function();

main () {
  _FutureOrVoid f;
  
  f = () async {};
  print(f is _Future); // true
  print(f is _Void); // TRUE
  print(f is _FutureOrVoid); // TRUE
  
  
  f = () {};
  print(f is _Future); // false
  print(f is _Void); // TRUE
  print(f is _FutureOrVoid); // TRUE
}



// ===============


import 'dart:async';

typedef FutureInt = Future<int> Function();
typedef Int = int Function();
typedef FutureOrInt = FutureOr<int> Function();

main () {
  Base either;
  either = InstanceFuture();
  print(either.isFuture); // true
  print(either.f is FutureInt); // true
  print(resolveOpaqueFutureInt() is FutureInt); // true
  
  either = InstanceInt();
  print(either.isFuture); // false
  print(either.f is FutureInt); // false
  print(resolveOpaqueInt() is FutureInt); // false
}

// always chooses first but is opaque
FutureOrInt resolveOpaqueFutureInt() => DateTime.now().isAfter(DateTime.fromMicrosecondsSinceEpoch(0))
  ? (InstanceFuture() as Base).f
  : (InstanceInt() as Base).f;

// always chooses second but is opaque
FutureOrInt resolveOpaqueInt() => DateTime.now().isBefore(DateTime.fromMicrosecondsSinceEpoch(0))
  ? (InstanceFuture() as Base).f
  : (InstanceInt() as Base).f;


abstract class Base {
  FutureOrInt get f;
  
  bool get isFuture => f is FutureInt;
}

class InstanceFuture extends Base {
  @override
  FutureInt get f => () async => 1;
}

class InstanceInt extends Base {
  @override
  Int get f => () => 1;
}