gunfoldl

18 Feb 2005

      I wrote a little data structure with quantified constructors:

module MyModule where
   import Data.Generics
   import Data.HashTable

   data Item = forall a. (Data a) => Leaf Bool a
             | forall a. (Data a) => Branch Bool a Int Int

deriving (Typeable)

I want it to make an instance of Data:

   instance Data Item where
       gfoldl k z (Leaf b v) = z (Leaf b) `k` v
       gfoldl k z (Branch b v a1 a2) = z (\x -> Branch b x a1 a2) `k` v
       --gunfoldl k z c = case constrIndex c of
       --                                    1 -> k z (Leaf undefined 
undefined)
       toConstr (Leaf _ _) = leafConstr
       toConstr (Branch _ _ _ _) = branchConstr
       dataTypeOf _ = itemDataType

   itemDataType = mkDataType "Subliminal.Item" [leafConstr, branchConstr]
   leafConstr = mkConstr itemDataType "Leaf" [] Prefix
   branchConstr = mkConstr itemDataType "Branch" [] Prefix

But, when I try to compile it with ghc-6.4-20050217:

ghc -fglasgow-exts -i. -c kicsi.hs

kicsi.hs:13:4:
   Warning: No explicit method nor default method for `gunfold'
        In the instance declaration for `Data Item'
ghc-6.4.20050217: panic! (the `impossible' happened, GHC version 
6.4.20050217):
   cgPanic
   k{v a1vu}
   static binds for:
   local binds for:
   gunfold{v r22q}
   SRT labelghc-6.4.20050217: panic! (the `impossible' happened, GHC 
version 6.4.20050217):
   initC: srt

Please report it as a compiler bug to glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

If I uncomment the gunfoldl lines:

ghc -fglasgow-exts -i. -c kicsi.hs

kicsi.hs:12:8: `gunfoldl' is not a (visible) method of class `Data'

Compilation exited abnormally with code 1 at Fri Feb 18 20:55:32

Could you please help me?

Thanks
Akos
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