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