SE-0235 - Add Result to the Standard Library

[mbrandonw (Brandon Williams)]

@beccadax

The fold(onSuccess:onFailure:) method

This is...kind of weird? It's like a general form of all of the other operations, but I'm not sure when you'd use it in practice other than for implementing those. Keep it internal .

I was the one to originally suggest this function:

Adding Result II: Unconstrained Boogaloo

There is an operation on Result that isn't expressible by chaining maps and mapErrors, and it should probably be included with a stdlib Result: extension Result { func fold<C>(ifSuccess: (Value) -> C, ifFailure: (Error) -> C) -> C { switch self { case let .success(value): return ifSuccess(value) case let .failure(error): return ifFailure(error) } } } This allows you to fold either case of a result into a single value. If you happen to use C = Result<NewValue, NewError> then…

I mentioned it specifically because some people were having trouble understanding how you can perform certain operations that are not expressible by maps and flatMaps alone. It's basically a function version of a switch statement over the result.

And yeah you can implement map, mapError, flatMap and flatMapError in terms of fold, but many times it's more concise to use the simpler, less general function.

The same is true of reduce on arrays. You can reimplement all of array's API in terms of reduce (including map, suffix, prefix, count, filter, etc.) in terms of reduce, but of course you want probably the more specific functions most of the time. However that doesn't mean we shouldn't also have the more powerful reduce.