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 wrong.
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-classHowever, the offending module (Derive.hs) produces broken instances in both distributions.
The bug: ----------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 packedChildrenBasically 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) whereNote 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?
Cheers, Alexey
BugBinTree.lhs
Description: Binary data
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