Re: [Haskell-beginners] A little explanation! follow up

I have a follow up question on this one. Is code like this: elementAt_w'pf = (last .) . take . (+ 1) considered good haskell? If so, will this ever become easy to read? What do I do to practice reading this? More specifically, how would I know in mind head, before asking the type checker in GHCi to write this pipeline with the section (last .) instead of simply making the (wrong) pipe: elementAt_w'pf = last . take . (+ 1) I am used to using pipes in the unix shell, but in the shell there is always only one input and one output. There is no currying going on and so it is very clear what we are dealing with. How do I learn this? Thanks, Dimitri Em 30/04/14 09:53, Kyle Murphy escreveu:

Near as I can tell, this is basically having to do with partial application and the order of precedence for the (.) operator. A great way to look at this stuff is using the :t command in GHCi and checking out the types involved.

Prelude> :t (.) (.) :: (b -> c) -> (a -> b) -> a -> c

Prelude> :t last last :: [a] -> a

Prelude> :t (last .) (last .) :: (a -> [c]) -> a -> c

Looking back at the type for (.) and plugging in "[a]" in place of "b" and "a" in place of "c" we get: ([b] -> b) -> (a -> [b]) -> a -> b

and since "last" takes the place of the first function we can reduce that to: (a -> [b]) -> a -> b

GHCi used "c" where we used "b" but you can clearly see the signatures are identical other than that detail.

What we're left with is, (last .) is used to take a function from some type "a" that returns a list of type "b", and a "a" value, and then returns the last value from that list of "b" types.

-R. Kyle Murphy -- Curiosity was framed, Ignorance killed the cat.

On Wed, Apr 30, 2014 at 11:29 AM, Gilberto Melfe mailto:gilbertomelfe@gmail.com> wrote:

Hello everybody !

Could someone explain me exactly how this function works?

elementAt_w'pf = (last .) . take . (+ 1)

It's a posted solution to: 99 Haskell problems, problem 3.

I'm having trouble with the "(last .) ." thing!

Hope someone can help!

Thank You!

Gilberto

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