#7391: "The 'impossible' happened" trying to derive Binary for a data instance
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Reporter: ydewit | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Resolution: invalid | Keywords:
Os: MacOS X | Architecture: x86
Failure: Compile-time crash | Difficulty: Unknown
Testcase: | Blockedby:
Blocking: | Related:
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Changes (by simonpj):
* cc: ndmitchell@… (added)
* difficulty: => Unknown
* status: new => closed
* resolution: => invalid
Comment:
This is a bug in the `derive` package, not in GHC. The full message looks
like this
{{{
T7391.hs:1:1:
Exception when trying to run compile-time code:
Could not convert Dec to Decl
TySynD T7391.ADataCmd [] (AppT (ConT T7391.Cmd) (AppT (ConT T7391.Id)
(ConT T7391.AData)))
Language\Haskell\Convert.hs:(37,14)-(40,8): Non-exhaustive patterns in
case
Code: derive makeBinary ''ADataCmd
}}}
And indeed the `Convert` module of the `derive` package has this
{{{
instance Convert TH.Dec HS.Decl where
conv x = case x of
DataD cxt n vs con ds -> f DataType cxt n vs con ds
NewtypeD cxt n vs con ds -> f NewType cxt n vs [con] ds
where
f t cxt n vs con ds = DataDecl sl t (c cxt) (c n) (c vs) (c
con) []
}}}
Notice taht there are only two cases in the `case` expression. Your
program uses a `data family` which is not dealt with by these two cases.
Perhaps you need to talk to Neil Mitchell, who is the mainainer of
`derive`.
I'll close as invalid for now; please reopen if you think it's GHC's
problem.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7391#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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