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haskellio-monad

Haskell IO-Monad error upon explicitly stating parameter for 'show'

Although I feel like a have a good understanding of Haskel IO and Monads, I am having a hard time understanding the following error message.

Consider the following simple function in Haskell

testf :: Show a => a -> String
testf x = show x

I tried implementing a variant that prints to the console by using an IO Monad

printtoscreen :: Show a => a -> IO()
printtoscreen x = putStrLn . show x

However, this yields the following error:

Couldn't match type ‘[Char]’ with ‘a0 -> String’ Expected type: a0 -> String Actual type: String

The correct version should omit explicitly stating the x parameter

printtoscreen :: Show a => a -> IO()
printtoscreen = putStrLn . show

I understand why the last code snippet works, but I cannot make sense out of the error message of the second code snippet, considering that it will also return a string to putStrLn

So why should the xparameter be omitted in the IO() variant?

Solution

  • ., the function composition operator, expects a function. show x however is not a function; it's an evaluated value (of type [Char]) by the time it's given to ..

    You'd have to use the function application operator instead:

    printtoscreen x = putStrLn $ show x