null-coalescing and input checking in dart

coalesce.dart

library coalesce;

// To turn the situation where we have:
// var foo;
// if (mightBeNullA != null) {
//   foo = mightBeNullA;
// } else if (mightBeNullB != null) {
//   foo = mightBeNullB;
// } else {
//   foo = 2;
// }
//
// into:
// var foo = coalesce([mightBeNullA, mightBeNullB, 2]); 
coalesce(Iterable values) => values.firstWhere((value) => value != null);

// For the simple case where we don't want an array every time we call it
coalesce2(value1, value2) => value1 == null ? value2 : value1;

presence.dart

library presence;

// To turn the situation where we have:
// var arg = "rawvalue";
// var accepted = ["foo", "bar", "baz"];
// var default = "bar";
//
// var foo;
// if(accepted.contains(arg)) {
//   foo = arg;
// } else {
//   foo = default;
// }
//
// into:
// var foo = presence(arg, accepted, orElse: default);
presence(value, Iterable values, {orElse: null}) => values.contains(value) ? value : orElse;

This'd be a good addition to taco_helpers. Here's a slightly different API that follows List.firstWhere's syntax.

var value = coalesce([val1, val2], orElse: () => "val");