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

J.Burton at brighton.ac.uk J.Burton at brighton.ac.uk
Tue Jul 15 10:23:42 EDT 2008

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

    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

    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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