#2782: Data instance for ratio changed, now incompatible
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Reporter: NeilMitchell | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution:
Keywords: | Testcase:
Architecture: Unknown/Multiple | Os: Unknown/Multiple
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Comment (by claus):
I'm not sure I follow: neither strictness nor `fromConstr` have changed (I
think?). The only change I'm aware of is that the `Data` instance for
`Ratio` used to be inconsistent (treating the type as abstract for
`gfoldl` and as concrete for `gunfold`), while it now treats the type as
concrete for both directions.
Your example tries to map a query over the subterms of an undefined
`Ratio`, relying on the old half-abstract behaviour for not actually
looking for subterms. Whereas with the new instance `gmapQ` will look for
subterms and fail. The old behaviour can be restored by making the
abstract handling of `Ratio` explicit:
{{{
ctr2 t = length $ (gmapQ (const undefined) `extQ` ratio) $ asTypeOf
(fromConstr $ ctr t) t
test = ctr2 u
ratio :: Ratio Integer -> [a]
ratio _ = []
}}}
Now we get `0` from both versions of ghc.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2782#comment:2>
GHC <http://www.haskell.org/ghc/>
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