[Haskell-cafe] SYB with class: Bug in Derive.hs module
Alexey Rodriguez Yakushev
mrchebas at gmail.com
Mon Mar 31 09:47:04 EDT 2008
Hi people (and Ralf and Alex),
I found a bug in the SYB with class library when trying to implement
generic equality. I am hoping that someone in the Cafe (maybe Ralf)
can confirm it is a bug, or maybe show me that I am doing something
I am using the "Scrap your boilerplate with class" library (http://
homepages.cwi.nl/~ralf/syb3/). More precisely, I am using the library
distributed by the HAppS project, because it works with GHC 6.8 . You
can get the repository as follows:
darcs get http://happs.org/HAppS/syb-with-class
However, the offending module (Derive.hs) produces broken instances
in both distributions.
Generic equality needs type safe casting when implemented in SYB3, I
have tried both the gzipwith variant and using Pack datatypes
(geq*.hs in the first distribution). However, both functions loop
when applied to a tree value.
This loop occurs when the functions try to cast one of the arguments.
I have managed to reduce the error to a smaller source file that I
send attached. It does the following:
> main = print typeReps
> tree = (Bin (Leaf 1) (Leaf 2))::BinTree Int
> data Pack = forall x. Typeable x => Pack x
> packedChildren = gmapQ geqCtx Pack tree
> typeOfPack (Pack x) = typeOf x
> typeReps = map typeOfPack packedChildren
Basically the tree is transformed into a list of Pack-ed values and
then to a list of type representations. This program loops at
"typeOf" when you call "main".
The Data instance that Derive generates is as follows:
> instance (Data ctx a,
> Data ctx (BinTree a),
> Sat (ctx (BinTree a))) =>
> Data ctx (BinTree a) where
Note the recursive |Data ctx (BinTree a)| in the context. If I get
rid of it (a correct manual instance is also included in the
attachment) the example works.
I thought of removing this from the context in the Derive source. But
maybe I might break some other use cases. So I am asking for help!
Should Derive be fixed? How?
-------------- next part --------------
A non-text attachment was scrubbed...
Size: 3490 bytes
Desc: not available
Url : http://www.haskell.org/pipermail/haskell-cafe/attachments/20080331/ff9d2d8f/BugBinTree.obj
More information about the Haskell-Cafe