On Tue, Sep 18, 2007 at 04:41:33PM +0100, Barney Hilken wrote:
Hi Simon, thanks for the response.
In fact I really only need NameCmp to be defined for datatypes of the form data T = T but it's still a lot, so I was expecting to need an extension. Lexical comparison of fully qualified names is what I had in mind, but I wanted some confirmation that such things exist!
I could post a GHC feature request, but unless I get someone else interested in this, it would probably just sit in Trac indefinitely. Where should I look in the ghc source if I want to add it myself?
As usual, Oleg solved this problem long ago. I don't remember a citation, but the gist of the procedure is: data HCons a b data HNil type family TTypeOf a :: * type instance TTypeOf Int = ...ascii code for 'I' 'n' 't' represented using HCons/HNil... ... type family Combine a b :: * type instance Combine LT a = LT type instance Combine EQ a = a type instance Combine GT a = GT type family NumCmp a b :: * type instance NumCmp HNil HNil = EQ type instance NumCmp HNil (HCons a b) = LT type instance NumCmp (HCons a b) HNil = GT type instance NumCmp (HCons a b) (HCons c d) = Combine (NumCmp a c) (NumCmp b d) type family TypeCmp a b :: * type instance TypeCmp a b = NumCmp (TTypeOf a) (TTypeOf b) The O(n) remaining instances can be automated with my Data.Derive.TTypable, if you're willing to switch back to functional dependancies. (Simon, can a TF 'wrapper' for a fundep be meaningfully done?) Stefan