Dart let extension

main.dart

import 'package:flutter_test/flutter_test.dart';

extension Let<T extends Object> on T? {
  R let<R>(R Function(T) callback, {R Function()? or}) {
    var self = this;
    if (self != null) {
      return callback(self);
    }
    if (or != null) {
      return or();
    }
    R? nullReturn;
    if (nullReturn is R) {
      return nullReturn;
    }
    throw ArgumentError.notNull("or");
  }
}

class A {}

void main() {
  A? aNull;
  A? aNonNull = A();

  test('Test let(null) void', () {
    aNull.let((it) => "bogus");
    expect("OK", "OK");
  });
  test('Test let(null, or) void', () {
    int a = 42;
    aNull.let((it) => "bogus", or: () => a = 1000);
    expect(1000, a);
  });
  test('Test let(null, or)', () => expect(aNull.let((it) => "bogus", or: () => "OK"), "OK"));
  test('Test let(null, null or)', () => expect(aNull.let((it) => "bogus", or: () => null), null));

  test('Test let(non null)', () => expect(aNonNull.let((it) => "OK"), "OK"));
  test('Test let(non null, or)', () => expect(aNonNull.let((it) => "OK", or: () => "KO"), "OK"));
  test('Test let(non null, null or)', () => expect(aNonNull.let((it) => "OK", or: () => null), "OK"));

  test('Test letNonNull(null, or)', () => expect(aNull.let((it) => "bogus", or: () => "OK"), "OK"));

  test('Test letNonNull(non null, or)', () => expect(aNonNull.let((it) => "OK", or: () => "KO"), "OK"));
  test('Test letNonNull(non null, null or)', () => expect(aNonNull.let((it) => "OK", or: () => null), "OK"));
}