[ncd]

It's a function that takes a String and returns a new function which takes a String and returns a String. All functions in Haskell/Elm are arity 1.

So in order to construct functions that accept more than one argument, you actually return successive functions that apply successive arguments, known as currying.

[e12e]

This is really important when looking at it from "the outside" (or, one is too lazy to lookup the documentation ;-). Without knowing that, it could be a function that takes a string and compiles it to a function that takes two strings (ie: a macro, I guess). Or one string, and returns a string. Or...

Does this kind of 1/arity have performance implications for Haskell? Does the compiler lower (or is it rise? inline?) the functions to produce efficient machine code?

[Kiro]

Thanks! Is this the case in Elm as well? This is the example they give:

  connectWords : String -> String -> String
  connectWords firstWord secondWord =
    firstWord ++ secondWord

[elbenshira]

In this case, you think of `connectWords` as a function that takes two arguments. But since it is curried, you can also do this:

    let prefix = connectWords "Hello "
        world  = prefix "world"
        bob    = prefix "bob"
    in ...

`world` is "Hello world", and `bob` is "Hello bob". That is the power of currying. A maybe more useful example is specifying the mapping function in `List.map` without supplying the list to map over. This allows you to use the same map with multiple lists.

[taylorfausak]

Yes, all functions in Elm also have arity 1. That example desugars into this:

    connectWords = \ firstWord -> \ secondWord -> firstWord ++ secondWord