Re: [Haskell-cafe] Type error with Type families

On Mon, Sep 17, 2012 at 6:27 PM, Alexander Solla wrote:

On Mon, Sep 17, 2012 at 10:59 AM, Ryan Ingram 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/