[Haskell-cafe] Type error with Type families

[Marco Túlio Pimenta Gontijo]

On Mon, Sep 17, 2012 at 6:27 PM, Alexander Solla <alex.solla at gmail.com> wrote:
>
>
> On Mon, Sep 17, 2012 at 10:59 AM, Ryan Ingram <ryani.spam at gmail.com> wrote:
>>
>> The problem is that the function 'element' is ambiguous, for the reasons
>> MigMit pointed out.
>>
>> The standard solution to this problem is to add a dummy argument to fix
>> the type argument to the type function:
>>
>> data Proxy a = Proxy
>>
>> class ... => ReplaceOneOf full where
>>     type Item full ::  *
>>
>>     -- implementations can just ignore the first argument
>>     element :: Proxy full -> Item full -> [Item full] -> Bool
>>
>>     replaceOneOf :: ...
>>         ...
>>         | element (Proxy :: Proxy full) x from = ...
>>
>> Now the choice of which 'element' to use can be determined by the type of
>> the proxy.
>
>
> It might be best if we point users to a "standard" Proxy type so we don't
> end up lots of packages defining distinct/incompatible types for the same
> code.
>
> Edward Kmett's "tagged" package has a Proxy type.

Thank you for your answers.

Greetings.
(...)
-- 
marcot
http://marcot.eti.br/