Question

I am a beginner of Haskell. What is wrong with this expression?:

Prelude> let { f op [] = [] ; f op (h:t) = op h : f op t }
Prelude> f (+) []
<interactive>:337:1:
    No instance for (Show (t0 -> t0))
      arising from a use of `print'
    Possible fix: add an instance declaration for (Show (t0 -> t0))
    In a stmt of an interactive GHCi command: print it

Many thanks for support.

Answer 1

Function (+) has type (+) :: Num a => a -> a -> a and function f has type f :: (t -> a) -> [t] -> [a].

f expects a function of one argument.

f (+1) [] will be correct