CastOr 🦫

CastOr provide a .castOr<T>(()), .castOrNull<T>() helpers and extension to safely transform/cast and provide defaults values when you deal with untyped data. The package provide also a .let<T>() implementation (similar to kotlin).

This package try to solve 2 typical issue when dealing with nullsafety on your dart projects.

as casting will fail if we not ensure the type before, and the usage of the ! postfix null promotion operator. Both usage should be avoided at all cost.

Consider the following code :

void main() {
  dynamic a = 3;
  final String? s;
  try {
    s = a as S;
  } catch (e) {
    s = null;
  }
}

which can be solved

import 'package:castor/castor.dart';

void main() {
  dynamic dyn = null;
  final obj = (dyn as Object?);
  // the basic [castOr] always expect to get an uwrapped value of the input type    
  int b = obj.castOr<String>(a, (_) => 'alternative string').length;
  int c = castOr<int>(a, (_) => 'alternative string').length;
  
  // the [castOrNull] equivalent just accept a nullable when casting is wrong
  bool? asHelper = castOrNull<bool>(a, (_) => 'alternative string').length;
  bool? asExt = obj?.castOrNull();
  
  int? s = castOrNull<String>(a)?.length;
  /// and with [a] an Object?
  int s2 = a?.castOr<String>((val) => 'alternative string').length;
  int? s = a?.castOrNull<String>(a)?.length;
}

It's ok for now but this will crash probably at some time if someone remove the if clause, because no static analysis will check on the code.

Usage

.castOr casting or provide a default value

You want ensure that the given value is of the correct type without having to mix with error prone as keyword. Consider the following code :

void main() {
  dynamic a = 3;
  final String? s;
  try {
    s = a as S;
  } catch (e) {
    s = null;
  }
}

import 'package:castor/castor.dart';


void main () {
  /// considering [a] a dynamic
  int s2 = castOr<String>(a, (_) => 'alternative string').length;
  int? s = castOrNull<String>(a)?.length;
  /// and with [a] an Object?
  int s2 = a?.castOr<String>((val) => 'alternative string').length;
  int? s = a?.castOrNull<String>(a)?.length;
}

promoting

promoting with the ! should be avoided at all cost. With the following code :

if (this.a != null) this.a!.length;

It's ok for now but this will crash probably at some time if someone remove the if clause, because no static analysis will check on the code.

Additional information

Before writing this package, I found many other alternative solutions, most of them trying to mimic the .let behavior of kotlin, and other guard keyword to ensure null safety everywhere in the code.

I choose to publish this little one because of the cute name but mostly about the small footprint or your codebase.

Regarding dart and null safety right now, there is 3 common pitfalls when you start believing in nullsafety in dart.

• First is the dynamic keyword, and the ! postfix null promotion operator. Both should be avoid at all cost and you should prefer the Object? which is not equivalent but can be simply switch
• Secondly, you'll have to check the as keyword, which without concealing it into a try/catch block or prefixing with if (a is b) control block.
• And finally, there is a lot of throwing methods out there that you can't avoid staticaly without tools like DCM.

Trivia

"Castor" is the french word for "Beaver". That's why you should find a beaver symbol 🦫 there and there.

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 credits : https://textart.sh/topic/beaver

More

• Discussion about the let keyword https://github.com/dart-lang/language/issues/2703
• Null safety patterns https://github.com/dart-lang/language/blob/main/accepted/3.0/patterns/feature-specification.md