Gogledd-Orllewin

From October 2022, not sure what I was thinking but I tried to implement a Kotlin style when statement in Dart. Original readme below.

Repository: DartWhen-main.zip

Probably a bad idea but I miss Kotlin's when statement/expression and I'm not satisfied with only having Dart's if and switch. There's is a proposal in Dart to extend the Switch statement to become an expression (see Patterns Feature Specification) which will improve things massively but it's not here yet.

Warning: This was knocked together in an hour or two then refined a little, I can't vouch for its safety or correctness

Usage

You can add when to your project using this repo as a git package dependency:

• Add the repo as a dependency to pubspec.yaml:

  dependencies:
    dart_when:
      git: https://github.com/orllewin/DartWhen.git

• Then import:

  import 'package:dart_when/when.dart';
  

• and use, eg:

  Future _fetchStations() async {
    final response = await http.get(Uri.https('orllewin.uk', 'stations.json'));
  
    Stations? stations = when(response.statusCode, {
      200: Stations.fromJson(jsonDecode(response.body)), //
      Else: () => snack("Error loading stations feed")
    });
  
    return stations;
  }

  Note the double slash // - this tricks Dart Format and prevents it folding the when block into a single line

Dart's Switch

Dart's current Switch statement is identical to Java's:

switch (x) {
  case 1:
    print("is 1");
    break;
  case 2:
    print("is 2");
    break;
  case 3:
    print("is 3");
    break;
  default:
    print("default - no match");
}

The same code with DartWhen:

when(x, {
  1: () => print("is 1"),
  2: () => print("is 2"),
  3: () => print("is 3"),
  Else: () => print("default - no match")
});

Expression

Dart's switch can't currently be used as an expression, DartWhen can though:

var y = when(x, {
  1: 11,
  2: 22,
  3: 33,
  Else: -1
});

Boolean branch case

DartWhen also allows you to include a boolean expression as a branch case:

var y = when(x, {
  1: 11,
  2: 22,
  3: 33,
  x < 10: 44,
  Else: -1
});

Adding more than one boolean branch will result in unexpected behaviour, if you want to use multiple boolean branch cases you can use the alternative syntax which is less elegant:

when(x, [
  {x > 700: () => print("> 700")},
  {x > 600: () => print("> 600")},
  {x > 500: () => print("> 500")},
  {x > 400: () => print("> 400")}
]);

(when is overloaded, the default implementation uses a Map with the tidier syntax in the other examples; the supplied condition is matched against the key entries, obviously with true/false keys that would be a problem with multiple boolean branches. Having a second overload backed by a List allows the branches to be iterated sequentially instead).