Question

While trying to compile the following code, which is enhanced version of read build on readMay from Safe package.

readI :: (Typeable a, Read a) => String -> a
readI str = case readMay str of
               Just x -> x 
               Nothing -> error ("Prelude.read failed, expected type: " ++ 
                                 (show (typeOf > (undefined :: a))) ++ 
                                 "String was: " ++ str)

I get an error from GHC:

WavefrontSimple.hs:54:81:
Ambiguous type variable `a’ in the constraint:
`Typeable a’
arising from a use of `typeOf’ at src/WavefrontSimple.hs:54:81-103
Probable fix: add a type signature that fixes these type variable(s)`

I don’t understand why. What should be fixed to get what I meant?

EDIT: Ok, so the solution to use ScopedTypeVariables and forall a in type signature works. But why the following produces very similar error to the one above? The compiler should infer the right type since there is asTypeOf :: a -> a -> a used.

readI :: (Typeable a, Read a) => String -> a
readI str = let xx = undefined in
            case readMay str of
              Just x -> x `asTypeOf` xx
              Nothing -> error ("Prelude.read failed, expected type: " 
                               ++ (show (typeOf xx)) ++ 
                                "String was: " ++ str)

Answer 1

The latter doesn’t work because the type of xx is the same as the type of undefined — i.e., “forall a. a.” The fact that you force xx to be used with one concrete type with the asTypeOf operator doesn’t mean that it is less polymorphic everywhere else.