Question

Returning a datatype.
For instance, let’s say that I had made a datatype:

data Something = Something Int [Char]

And, then I did some manipulation with the following function (the exact function of which is irrelevant):

manipulativeFunc::Something->[Something]

I keep getting these strange error messages that

Top level:
    No instance for (Show (Int -> IO ()))
        arising from use of 'print' at Top level
    Probable fix: add an instance declaration for (Show (Int -> IO ()))
    In a 'do' expression: print it

Note that I don’t have any uses of print anywhere in my program, nor do I have any uses of IO. The data declaration and the manipulativeFunc are all I have on it.

What could I be doing wrong?

EDIT: From commenters, I get the message that I may need to declare a Show instance for this task. So, what if I had

data Something = Something Int Int

Then how would I write a Show instance function for it?

Answer 1

In order to use the function print, the compiler needs to be able to convert a value into a String, which is ensured by the Show class. You try to display a function, and there is no Show instance defined for it.

In order to be able to display your Something, use

data Something = Something Int [Char] deriving Show

manipulativeFunc can’t be displayed that way, but its result if you call it with an argument.