Hi,

 

I'm making my way through Haskell which seems to me one of the languages with steepest learning curve around.

 

Now, please consider this snippet:

 

class FooOp a b where
  foo :: a -> b -> IO ()

instance FooOp Int Double where
  foo x y = putStrLn $ (show x) ++ " Double " ++ (show y)

partialFoo = foo (10::Int)

bar = partialFoo (5.0::Double)

I hope the indentation looks ok in your email client. I'm experimenting with currying and typeclasses at the moment.

 If I try to import this in ghci, it works flawlessy. Now, if I remove the type signature from 10 and 5.0, ghci complaints saying:

example.hs:12:6:
    Ambiguous type variable `t' in the constraint:
      `Num t' arising from use of `partialFoo' at example.hs:12:6-19
    Probable fix: add a type signature that fixes these type variable(s)

example.hs:12:6:
    Ambiguous type variables `t', `t1' in the constraint:
      `FooOp t t1' arising from use of `partialFoo' at example.hs:12:6-19
    Probable fix: add a type signature that fixes these type variable(s)

example.hs:12:17:
    Ambiguous type variable `t1' in the constraint:
      `Fractional t1'
        arising from the literal `5.0' at example.hs:12:17-19
    Probable fix: add a type signature that fixes these type variable(s)

I switched off the monomorphism restriction (btw, is this bad? No flame war please :D) otherwise it'd have complained louder.

Can you explain how to fix the code (if possible) and give some explanation?

Thanks,

Cristiano