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haskellpointfree

Got confused with point free style code

So a function to replicate the elements of a list a given number of times looks like this

rep :: Int -> [a] -> [a]
rep = concatMap . replicate

by definition (f . g) x = f(g(x)) but

(concatMap (replicate 4 "abc")) is not working same as (concatMap . replicate) 4 "abc". In fact it doesn't work at all. And concatMap's first parameter must be function. I'm getting confused by this. How is that point free version even working? Can anyone explain it please.

Solution

  • You're right that (f . g) x = f (g x), but that doesn't mean that (concatMap . replicate) 4 "abc" = concatMap (replicate 4 "abc"). Rather, it means that (concatMap . replicate) 4 "abc" = concatMap (replicate 4) "abc", which is true.