I guess I see the intuitive confusion. It shouldn't matter what type Nothing is, it should always return False. But ghc doesn't like to give results of a determined type (Bool) based on operations put on values of undetermined type (Maybe a). On Sun, Nov 17, 2013 at 4:54 PM, Dan Krol wrote:

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 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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Thanks a lot for the thorough and quick explanations! On Sun, Nov 17, 2013 at 6:57 PM, Brandon Allbery wrote:

On Sun, Nov 17, 2013 at 7:47 PM, Robert Krahn wrote:

...

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'

Several things are happening here. First is that numbers undergo defaulting; with nothing else to specify the type of `3` or `0`, they default to Integer and all is well.

This cannot be said of `Nothing`; it needs to pick an instance, but `Nothing` doesn't give it anything to work from.

"But I only have an instance for Integer!" Typeclasses work under the "open world assumption": a typeclass could be added at any time, so it cannot commit to a given typeclass instance simply because it only happens to know of one valid instance. In particular, it cannot conclude that `Nothing` is of type `Maybe Integer` solely because it happens to only be aware of a `Tester Integer` instance.

(Instances are program global and cannot be hidden, so not making the open world assumption could easily cause programs to completely change their meaning with the addition of a single additional instance *anywhere* in that program, even if not imported to that module.)

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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