At Tue, 15 Jul 2008 10:02:23 -0400, Jeff Polakow wrote: [...]
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.
I see what you mean...so I tried to make Ins return an LSet of "something", like this: data LSet t where Nil :: LSet Nil Ins :: (Member a t b , If b t (a ::: t) r) => L a -> LSet t -> LSet r /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:9: No instance for (If b t (a ::: t) r) arising from a use of `Ins' at /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:9-11 Possible fix: add an instance declaration for (If b t (a ::: t) r) In the expression: Ins In the definition of `insert': insert = Ins /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:68:6: Inferred type is less polymorphic than expected Quantified type variable `a' is mentioned in the environment: insert :: L a -> LSet t -> LSet r (bound at /home/jim/sdf-bzr/dsel/TF/Set-July08.hs:44:0) When checking an existential match that binds t :: L a The pattern(s) have type(s): LBox The body has type: LSetBox In a case alternative: LBox t -> LSetBox (insert t s) In the expression: case fromChar c of LBox t -> LSetBox (insert t s) Failed, modules loaded: none.
-Jeff
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