[Haskell-cafe] Flexible instances

[Anton Tayanovskyy]

Thanks for the portable solution.

I'd also like to know how is the following different from
-XFlexibleInstances with [Char]? Stronger, weaker, same thing?

{-# OPTIONS -XTypeSynonymInstances #-}

class Stringable a where
        toString :: a -> String

instance Stringable String where
         toString = id


--A

On Wed, Oct 15, 2008 at 10:16 AM, Ryan Ingram <ryani.spam at gmail.com> wrote:
> So, the Haskell98 solution to this is:
>
> class StringableList a where
>     listToString :: [a] -> String
>
> -- now [a] is of the proper form; T = [], a is a type variable
> instance StringableList a => Stringable [a] where
>    toString = listToString
>
> -- now to make an instance for Stringable [Char]
> -- we just make an instance for StringableList Char
> instance StringableList Char where
>    listToString = id
>
> I think "FlexibleInstances" just makes the compiler jump through these
> hoops instead of you.
>
>  -- ryan
>
> On Wed, Oct 15, 2008 at 3:20 AM, George Pollard <porges at porg.es> wrote:
>>
>> I'm a little confused. Why is this allowed:
>>
>>> data Blah = Blah
>>>
>>> instance Eq Blah where
>>>         x == y = True
>>
>> But not this:
>>
>>> class Stringable a where
>>>         toString :: a -> String
>>>
>>> instance Stringable [Char] where
>>>         toString = id
>>
>> (Resulting in:)
>>
>>> Illegal instance declaration for `Stringable [Char]'
>>>         (All instance types must be of the form (T a1 ... an)
>>>          where a1 ... an are distinct type *variables*
>>>          Use -XFlexibleInstances if you want to disable this.)
>>>     In the instance declaration for `Stringable [Char]'
>>
>> 'Blah' isn't a type variable, is it? Is my brain just not working right
>> today?
>>
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