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