id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
3866,Constr Eq instance should have better documentation or semantics,batterseapower,dreixel,"The following code doesn't do what you expect:

{{{
data WithData = forall a. Data a => WithData a

tupleDataCast :: Data a => ([WithData] -> c) -> a -> Maybe c
tupleDataCast f x | Just (s, _) <- find ((toConstr x ==) . snd) tuples
                  = trace (show (toConstr x, s)) $ Just (f [gmapQi i WithData x | i <- [0..s - 1]])
                  | otherwise = Nothing
  where tuples = [2..] `zip` [toConstr ((), ()), toConstr ((), (), ()), toConstr ((), (), (), ()), toConstr ((), (), (), (), ()), toConstr ((), (), (), (), (), ())]
}}}

The reason is that, for example:

{{{
Prelude Data.Data> toConstr False == toConstr ()
True
Prelude Data.Data> toConstr True == toConstr ()
False
}}}

The Constr data type is only compared on the index. The DataType information in the Constr is totally ignored. This is very surprising behaviour!

According to a mailing list thread (http://www.mail-archive.com/glasgow-haskell-bugs@haskell.org/msg08207.html) this is done for efficiency reasons.

Either the documentation should mention this behaviour, or the Eq instance should be fixed. My preference is for the latter -- though existing expectations of Constr == efficiency and semantics may be too entrenched to change the behaviour now.",bug,closed,normal,6.12.2,libraries (other),6.12.1,fixed,syb,,Unknown/Multiple,Unknown/Multiple,None/Unknown,,,,,