[Haskell-beginners] Here's why functions should return functions

[Ertugrul Söylemez]

"Carlos J. G. Duarte" <carlos.j.g.duarte at gmail.com> wrote:

> So, what is the usefulness of that "const" function anyway? (go easy
> on me, I'm just beginning on this)

There are many functions in Haskell, which you wouldn't define in most
other languages, including:

    const :: a -> b -> a
    flip  :: (a -> b -> c) -> (b -> a -> c)
    id    :: a -> a

They become useful when used as arguments to combinators.  Some
examples:

    maybe 8 (const 16)
    foldl' (flip (-)) 0
    id &&& length

In use:

    maybe 8 (const 16) (Just 3) = 16 = const 16 3
    maybe 8 (const 16) Nothing  = 8

    foldl' (flip (-)) 0 [1,2,3]
        = flip (-) (flip (-) (flip (-) 0 1) 2) 3
        = (-) 3 ((-) 2 ((-) 1 0))
        = 3 - (2 - (1 - 0))

    map (id &&& length) ["abc", "de", "f"]
        = [("abc", 3),
           ("de", 2),
           ("f", 1)]


Greets,
Ertugrul

-- 
Not to be or to be and (not to be or to be and (not to be or to be and
(not to be or to be and ... that is the list monad.
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