On 5 June 2012 18:46, Gábor Lehel
I must be missing something, but this seems a bit useless to me. You have a phantom type parameter on Proxy, and then you're hiding it. So when you pattern match on ProxyWrapper you recover the fact that there was a type which satisfies the constraint, but you don't know what type it was, and neither do you know about any values which are of the type. What are you trying to do?
I need a list of types that satisfy a certain constraint. I would like to have the static guarantee that types that don't satisfy the constraint can't be put in the list, as in: nums :: [ProxyWrapper Num] nums = [ ProxyWrapper (Proxy :: Proxy Int) , ProxyWrapper (Proxy :: Proxy Double) , ProxyWrapper (Proxy :: Proxy String) -- not allowed ] fracs :: [ProxyWrapper Fractional] fracs = [ ProxyWrapper (Proxy :: Proxy Double) , ProxyWrapper (Proxy :: Proxy Float) , ProxyWrapper (Proxy :: Proxy Int) -- not allowed ]
That said, if you want to be able to recover a Typeable constraint, I don't see any way except for using 'ProxyWrapper (Ext Typeable constraint)' as Andres says or putting 'forall a. (constraint a, Typeable a)' in the definition of ProxyWrapper.
Indeed, I'm now going for the latter option. Regards, Bas