[Haskell-cafe] Type error with Type families

[Alexander Solla]

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.
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