Question

I have been playing around with haskell and I found out that if I write the following function in a code file:

f :: Int -> [a] -> a
f idx str = last $ (take . succ) idx str

then this works totally fine. Naturally, I figured the code would look better without the arguments.

f :: Int -> [a] -> a
f = last $ (take . succ)

But this generates an error when I try to load it into gchi

Couldn't match expected type `[a]'
       against inferred type `Int -> [a1] -> [a1]'
In the second argument of `($)', namely `(take . succ)'
In the expression: last $ (take . succ)
In the definition of `f': f = last $ (take . succ)

Failed, modules loaded: none.

I’m kind of confused about how this could be happening…

Answer 1

This is what happens when you try to compose last with (take . succ)

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

last :: [t] -> t  ~ (b -> c)  
-- so b ~ [t] and c ~ t   

(take . succ) :: Int -> [t] -> [t]  
-- so a ~ int and b ~ [t] -> [t]  

Type of b is inferred to be [t] from last but it couldn’t match against the type of b in (take . succ) which is [t] -> [t]