CODEWITHSUNDEEP
haskell
Jebathon
Jebathon

Reputation: 4581

Haskell - passing a function as a argument compilation error

I have a very simple function called 'fold' that takes a function f and executes it. Given a function 'add' and two numbers it calls add with those two numbers and displays the results. However I cannot compile it due to a compilation error below. What is the error specifically stating and what can I do to solve it?

  module Main where

    add x y = x + y

  --fold :: ((a,b)->c) -> a->b->c

    fold f n x = f n x


    main :: IO ()
    main = do 
        fold add 2 3

enter image description here

Upvotes: 0

Views: 101

Answers (1)

chepner
chepner

Reputation: 532333

The problem is that you've declared main (correctly) as having type IO (), but fold doesn't return that. The error message is a little more complicated because in theory, add (and thus fold) could return a value of type IO (), since add can return any type with a Num instance. IO (), however, is not an instance of Num. The solution is to return an IO action created from the return value of fold. One way to do that is to use the print function (which takes any value with a Show instance and converts it to a String before outputing it).

main = print $ fold add 2 3

Upvotes: 6

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