It is called "gunfold" rather than "gunfoldl" as you will see when you browse the Data.Generics.Basics. Also, your gunfold code looks like it will not work. Here is a simple example for Maybe: gunfold k z con = case constrIndex con of 1 -> z Nothing -- no children 2 -> k (z Just) -- one child, hence one k Bottom line: - apply z to the Constructor - apply k as many times as the number of children. No warranty that this is easy for your type Item. Good luck, Ralf Akos Korosmezey wrote:
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
-- Ralf Lammel ralfla@microsoft.com Microsoft Corp., Redmond, Webdata/XML http://www.cs.vu.nl/~ralf/