When you run "test Nothing", ghc doesn't know what type Nothing is. Just 0, ghc can deduce is a Maybe Integer. Nothing can be Maybe Integer, Maybe (Maybe Integer), Maybe Char, etc.

Try running:

test (Nothing :: Maybe Integer)

On Sun, Nov 17, 2013 at 4:47 PM, Robert Krahn <robert.krahn@gmail.com> wrote:

Hi everyone,

I'm currently enjoying to learn Haskell and came upon a problem I don't understand:

Motivated by the Learn you a Haskell / yes-no typeclass example I tried to create a simple "Tester" type class:

class Tester a where

    test :: a -> Bool

I wasn't to define the rules when test returns True/False for various types, e.g.

instance Tester Integer where

    test 0 = False

    test _ = True

For the Maybe instance I want to delegate to the value of Just and add a class constraint:

instance (Tester m) => Tester (Maybe m) where

    test Nothing = False

    test (Just x) = test x

It compiles nicely and works for Just values

test (Just 3) -- True

test (Just 0) -- False

But

test Nothing

gives me

    No instance for (Tester a0) arising from a use of `test'

    The type variable `a0' is ambiguous

    Possible fix: add a type signature that fixes these type variable(s)

    Note: there are several potential instances:

      instance Tester m => Tester (Maybe m)

      instance Tester Integer

    In the expression: test Nothing

Could you please enlighten me what's going on and how to fix my code?

Thank you!

Robert

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