Perhaps something here may be of use? http://okmij.org/ftp/Haskell/types.html#class-based-overloading http://okmij.org/ftp/Haskell/types.html#class-based-dispatch
-Brent On Mon, May 31, 2010 at 01:32:18PM +0200, Steffen Schuldenzucker wrote: > Dear Cafe, > > let: > > > data True > > data False > > > > class C a > > (arbitrary instances for C may follow) > > Now, how to obtain an "Indicator Type" for C, i.e. a type IndC that is defined > via a type family / fundep / ... , so that > > IndC a = True forall a which are instances of C > IndC a = False for all other a. > > I've collected some failed approaches here[1]. My key problem is that if I > define (in the 3rd try): > > > instance (C a) => IndC3 a True > > , it does *not* mean "Define this instance for all a which are an instance of > C", but "Define the instance IndC3 a True for all types a, but it's not gonna > work if a is not an instance of C". > > Does anyone have another idea? > > Background: > > After having implemented type-level lists[2] and a quicksort on them[3], I'd > like to have type-level sets. In their most simple implementation, sets are > just (unsorted) lists like this: > > > data Nil > > data Cons a b > > class Elem x l > (instances for Elem so that Elem x l iff x is an element of the list l) > > Now I want: > > > type family Insert x s :: * > > Insert x s = s forall (x, s) with (Elem x s) > Insert x s = Cons x s for all other (x, s). > > > Thanks a lot! > > Steffen > > > [1] http://hpaste.org/fastcgi/hpaste.fcgi/view?id=25832#a25832 > [2] Kiselyov, Peyton-Jones, Shan: "Fun with type functions" > > http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fun-with-type-funs/typefun.pdf > [3] I rewrote this algorithm using type families instead of fundeps: > > http://www.haskell.org/haskellwiki/Type_arithmetic#An_Advanced_Example_:_Type-Level_Quicksort > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
