[Haskell-cafe] Existentially-quantified constructors: Hugs is fine, GHC is not?

[Otakar Smrz]

Dear all,

while WinHugs (20051031) lets me match against an existentially 
quantified constructor

    data ... = ... | forall b . FMap (b -> a) (Mapper s b)

    ... where FMap qf qc = stripFMap f q

the GHC compiler as well as GHCi (6.4.2 and earlier) issue an error

     My brain just exploded.
     I can't handle pattern bindings for existentially-quantified
     constructors.


Let me give the whole (non-practical) code, which is well typed and 
compiles in Hugs, and then show the change I had to do to make it work 
in GHC, too.

The question is why there is a difference. Am I misusing something?

The point of the complexFun below is to explore the Mapper data 
structure, taking care of the :&: constructor and quickly 
(transitively) skipping the FMap constructors, only accumulating and 
composing the tranformation functions that these provide.

-----------------------------------------------------------

module Problem where

import Data.Map as Map hiding (map)

type Labels a = [a]

data Mapper s a = Labels a :&: Map.Map s (Mapper s a)
                 | forall b . FMap (b -> a) (Mapper s b)


stripFMap :: Ord s => (a -> c) -> Mapper s a -> Mapper s c

stripFMap k (FMap f p)  = stripFMap (k . f) p
stripFMap k x           = FMap k x


complexFun :: Ord s => (b -> a) -> Mapper s b -> s -> [a]

complexFun f c y = case c of

     FMap t q -> complexFun qf qc y

                     where FMap qf qc = stripFMap (f . t) q   -- !!!

     r :&: k  -> case Map.lookup y k of

         Just q  ->  let FMap qf qc = stripFMap f q	     -- !!!
                     in case qc of

             ([] :&: _) -> complexFun qf qc y
             (xs :&: _) -> map qf xs
             _          -> error "Never matching"

         Nothing -> error "Irrelevant"

-----------------------------------------------------------

Even though GHC does not let me pattern-match on FMap, I can use a 
case expression in complexFun instead -- then it compiles alright:

------

complexFun f c y = case c of

     FMap t q -> case stripFMap (f . t) q of		-- !!!

         FMap qf qc -> complexFun qf qc y		-- !!!
         _          -> error "No option"			-- !!!

     r :&: k  -> case Map.lookup y k of

         Just q  ->  case stripFMap f q of		-- !!!

           FMap qf qc -> case qc of			-- !!!

             ([] :&: _) -> complexFun qf qc y
             (xs :&: _) -> map qf xs
             _          -> error "Never matching"

           _          -> error "No option"		-- !!!

         Nothing -> error "Irrelevant"

------

If I wanted to make this auxiliary case on stripFMap local, there 
would be a problem for both Hugs and GHC:

  Hugs: Existentially quantified variable in inferred type!
        *** Variable     : _48
        *** From pattern : FMap xf xc
        *** Result type  : (_48 -> _32,Mapper _30 _48)

  GHC: Inferred type is less polymorphic than expected
         Quantified type variable `b' is mentioned in the environment:
           qc :: Mapper s1 b (bound at Problem.hs:65:27)
           qf :: b -> a1 (bound at Problem.hs:65:23)
       When checking an existential match that binds
           xf :: b -> a
           xc :: Mapper s b
       The pattern(s) have type(s): Mapper s1 a1
       The body has type: (b -> a1, Mapper s1 b)
       In a case alternative: FMap xf xc -> (xf, xc)

------

complexFun f c y = case c of

     FMap t q -> complexFun qf qc y

                 where (qf, qc) = case stripFMap (f . t) q of  -- !!!

                         FMap xf xc -> (xf, xc)
                         _          -> error "No option"

     r :&: k  -> case Map.lookup y k of

         Just q -> let (qf, qc) = case stripFMap f q of        -- !!!

                         FMap xf xc -> (xf, xc)
                         _          -> error "No option"

                     in case qc of

             ([] :&: _) -> complexFun qf qc y
             (xs :&: _) -> map qf xs
             _          -> error "Never matching"

         Nothing -> error "Irrelevant"

------


Many thanks for your comments or advice!

Best,

Otakar Smrz