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
[EMAIL PROTECTED]
Microsoft Corp., Redmond, Webdata/XML
http://www.cs.vu.nl/~ralf/
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