haskell, function composition

Question

I need some help to solve a task on function composition in Haskell. I need to write a function that given an Integer n and a list of inner lists of elements, returns the list of n-th element in each inner list. So it would be like: select 2 [[2,3,4],[5,6],[9,9,9]] = [3,6,9]. The thing is, I need to write it using function composition so it should look like select = .... In other words, I want to make this point-free.

For now, I have the following:

select::Int->[[Int]]->[Int] select a = map $ head. reverse. take a

I'm stuck with it, I don't know how to remove those a from the first and only clause. Can anybody help me with this?:)

Not sure I understand the question. You will have to give the integer n (guess that is a in your code?), to one of the combined functions. — Koebmand STO
– Koebmand STO, Commented Jan 24, 2016 at 15:57
So that's what is done by now, I need to remove a from the definition. — Paweł Poręba
– Paweł Poręba, Commented Jan 24, 2016 at 15:59
If you are learning, don't try to write a point-free function. Write a normal function with arguments, get it working, then transform it to point-free style. — crockeea
– crockeea, Commented Jan 24, 2016 at 16:21
Side note: You give the example: select 2 [[2,3,4],[5,6],[9,9,9]] = [3,6,9], which implies that the first element of the list is at index 1. This is non-standard in Haskell (and most programming languages)—typically lists are indexed starting at 0. To me, this would be the expected behavior: select 1 [[2,3,4],[5,6],[9,9,9]] = [3,6,9]. — MaxGabriel
– MaxGabriel, Commented Jan 24, 2016 at 19:00

Will Sewell · Accepted Answer · 2016-01-24 16:22:51Z

Based on what you currently have, you can use select = map . ((head . reverse) .) . take, you could also simplify this to select = map . (last .) . take. An alternative would be to use select = map . flip (!!) . subtract 1.

You can use the pointfree package to derive pointfree versions of functions automatically.

Generally I would advise against this though. Functions that have multiple parameters become quite obfuscated when they are defined in pointfree style.

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