#3866: Constr Eq instance should have better documentation or semantics
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: bug | Status: new
Priority: normal | Component: libraries (other)
Version: 6.12.1 | Keywords: syb
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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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-
[email protected]/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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3866>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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