Functor and Monad in Swift

Post by bvanslyke » Fri, Jan 02, 2015, 6:09 PM UTC

hardwaresofton wrote: post #10

This is an excellent article, I think it explains the monad pretty simply, starting from an understanding of functors. For those that are struggling with understanding (or feeling like you understand) monads, here are some articles like this one that I found very useful (note that the links are for haskell, but obviously, monads are a generally applicable concept). A visual explanation of monads: http://adit.io/posts…

Post by _Robbie » Fri, Jan 02, 2015, 7:07 PM UTC

saosebastiao wrote: post #13

Can someone help me bridge the gap between this understanding of the definition of functor and the ocaml version? In ocaml, a functor is a module that takes another module as a parameter. In this, a functor is a type that implements the map method. Is there something I'm missing that explains how both of them are functors?

Post by JSoto » Fri, Jan 02, 2015, 8:45 PM UTC

DanWaterworth wrote: post #5

CookWithMe wrote: post #3

Nice article, well done! Minor question/comment regarding the implementation of Result.map: Why do you create a new Result in the error case? Result is immutable (or am I missing something specific to Swift here?), so the old and new Result are the same and stay that way. I'm not familiar with the internals of Swift, but e.g. in Scala, there is only one "None". If you call map or flatMap on it, the same None will alw…

I'm not familiar with swift, but in Haskell, you can't write the following: flatMap :: Either e a -> (a -> Either e b) -> Either e b flatMap x f = case x of Left _ -> x Right x' -> f x' In the case expression, you have to write `Left err -> Left err`, because `x` here has the type `Either e a`, but `Either e b` is expected.

    func map(f: T -> U) -> Result {
        switch self {
        case let .Value(value):
            return Result.Value(Box(f(value.unbox)))
        case let .Error(error):
            // self is Result, but the return type must be Result
            return self
        }
    }

Post by josephlord » Fri, Jan 02, 2015, 9:48 PM UTC

JSoto wrote: post #17

josephlord wrote: post #11

Earlier quoted context omitted.

enums and structs are value types (and Result is an enum) so I think that you could just return self for the error case (gaining cleanliness and a little efficiency). Objects (of classes) are reference types so if Result was a class there would be a difference between returning self and returning a new Result.

I agree that it would be a little cleaner, because it would express the intention better (the fact that you're simply giving out the same error). However, the type would be wrong: Yeah, this is correct. This would be wrong: func map (f: T -> U) -> Result { switch self { case let .Value(value): return Result .Value(Box(f(value.unbox))) case let .Error(error): // self is Result , but the return type must be Result retu…

Post by CookWithMe » Sat, Jan 03, 2015, 1:25 PM UTC

JSoto wrote: post #17

josephlord wrote: post #11

Earlier quoted context omitted.

enums and structs are value types (and Result is an enum) so I think that you could just return self for the error case (gaining cleanliness and a little efficiency). Objects (of classes) are reference types so if Result was a class there would be a difference between returning self and returning a new Result.

I agree that it would be a little cleaner, because it would express the intention better (the fact that you're simply giving out the same error). However, the type would be wrong: Yeah, this is correct. This would be wrong: func map (f: T -> U) -> Result { switch self { case let .Value(value): return Result .Value(Box(f(value.unbox))) case let .Error(error): // self is Result , but the return type must be Result retu…