Question

I have a function (in a module) that returns IO (Maybe a) where a is an instance of the Serialize.

From my main Program I call this as follows:

 msg <- fun token
 print msg 

and get the error

Ambiguous type variable `a0' in the constraints:
      (Data.Serialize.Serialize a0) arising from a use of `foo'
                                    at test_00.hs:13:15-19
      (Show a0) arising from a use of `print' at test_00.hs:17:9-13
    Probable fix: add a type signature that fixes these type variable(s)

I know exactly what the problem is and I can fix it using -XScopedTypeVariables and some changes in how I call my library function as follows:

(msg :: Maybe String) <- cwPop token
print msg

However, I would rather like to avoid the ScopedTypeVariables and wonder where it is any way that I can test if msg is member of the show class and then print it. If not do something else .

Answer 1

You can give a type signature to the expression on the right hand side of the <- without extensions,

msg <- fun token :: IO (Maybe String)
print msg

(I have made the indentation so that print and msg aren’t arguments to fun anymore, your indentation seemed to be broken).