Question

I try to print functions in Haskell only for fun, like this example:

{-# LANGUAGE FlexibleInstances #-}

instance Show (Int -> Bool) where
    show _ = "function: Int -> Bool"

loading in GHCi and run and example:

λ> :l foo
[1 of 1] Compiling Main             ( foo.hs, interpreted )

foo.hs:2:1: Warning: Unrecognised pragma
Ok, modules loaded: Main.
λ> (==2) :: Int -> Bool
function: Int -> Bool

But, I wish to see that every function print yourself at invocation.

Answer 1

You can not have this for a general function as type information is present only at compile time, but you use Typeable class for writing something close enough if the type is an instance for Typeable class.

import Data.Typeable

instance (Typeable a, Typeable b) => Show (a -> b) where
    show f = "Function: " ++ (show $ typeOf f)

Testing this in ghci

*Main> (+)
Function: Integer -> Integer -> Integer
*Main> (+10)
Function: Integer -> Integer

But this will not work for general functions until the type is restricted to a type that has Typeable instance.

*Main> zip

<interactive>:3:1:
    Ambiguous type variable `a0' in the constraint:
      (Typeable a0) arising from a use of `print'
    Probable fix: add a type signature that fixes these type variable(s)
    In a stmt of an interactive GHCi command: print it

<interactive>:3:1:
    Ambiguous type variable `b0' in the constraint:
      (Typeable b0) arising from a use of `print'
    Probable fix: add a type signature that fixes these type variable(s)
    In a stmt of an interactive GHCi command: print it
*Main> zip :: [Int] -> [Bool] -> [(Int,Bool)]
Function: [Int] -> [Bool] -> [(Int,Bool)]