[Haskell-beginners] understanding type classes and class constraints

[Dan Krol]

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 <orblivion at gmail.com> 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 <robert.krahn at 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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>>
>
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