[Haskell-cafe] Type level sets with GADTs, fundeps etc

[Jeff Polakow]

Hello,

> > > data LSet t where
> > >     Nil :: LSet Nil
> > >     --either add the new element or do nothing
> > >     Ins :: (Member a t b
> > >           , If b (LSet t) (LSet (a ::: t)) r)
> > >           => L a -> LSet t -> r
> > >
> > The constructor Ins needs to return an LSet. Maybe try replacing 
> occurrences of r with (LSet r).
> > 
> 
> I expected that r would be an LSet, as its the output of If which
> returns its 2nd or 3rd argument:
> 
> class If p x y z | p x y -> z
>     where if' :: p -> x -> y -> z
> instance If T x y x 
> instance If F x y y 
> 
Type classes are open so there is nothing to prevent you from adding 
another instance for If, perhaps in a different module, which returns some 
arbitrary type.

-Jeff



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