SO #67577995 let implementation

main.dart

O let<I extends Object, O>(
    I? value, 
    O Function(I) cb, 
   {O Function()? or}) {
  if (value != null) {
    return cb(value);
  }
  if (or != null) {
    return or();
  }
  O? returnNull = null;
  if (returnNull is O) {
    return returnNull;
  }
  throw ArgumentError.value(null, "or", 
      "Please provide a default non-null value");
}

class A {
  final int a = 42;
}

void test(A a, String name) {
  print("$name=${a.a}");
}

void main() {
  A? x;
  let(x, (it) => test(it, "x"), or: () => print("x is null"));
    
  A? y = A();
  let(y, (it) => test(it, "y"), or: () => print("y is null"));
    
  A? z = A();
  let<A, void>(z, (it) => test(it, "z"), or: () => print("z is null"));
}