Re: [Haskell-cafe] A small (?) problem with type families

Am Freitag 13 November 2009 21:36:59 schrieb David Menendez:

On Fri, Nov 13, 2009 at 3:26 PM, Andy Gimblett wrote:

...

First a type family where the type Y is functionally dependent on the type X, and we have a function from Y to ().

...

class X a where   type Y a   enact :: Y a -> ()

This is ambiguous. Type families are not injective (that is, Y a ~ Y b does not imply a ~ b), so there's no way for the compiler to figure out which instance of X is being used when it encounters enact.

Given these instances,

instance X Int where type Y Int = Bool enact _ = ()

instance X Char where type Y Char = Bool enact _ = undefined

What is "enact False"?

I recall seeing a discussion of this in the GHC documentation, but I can't seem to locate it.

Perhaps http://www.haskell.org/haskellwiki/GHC/Type_families#Frequently_asked_questi... ?