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-class However, 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 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? Cheers, Alexey