#7391: "The 'impossible' happened" trying to derive Binary for a data instance
--------------------------------+-------------------------------------------
Reporter: ydewit | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.4.1 | Keywords:
Os: MacOS X | Architecture: x86
Failure: Compile-time crash | Testcase:
Blockedby: | Blocking:
Related: |
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I was trying to automatically derive Binary for a data instance (data
family) and was trying to work around the fact that "derive makeBinary"
expects a Name and not an expression by using a type alias. This is when I
got the GHC compilation error.
This is a simple test case:
{{{
import Data.Binary (Binary (..),
putWord8, getWord8)
import Data.DeriveTH
newtype Id a = Id String deriving Show
data family Cmd a
data instance Cmd (Id AData)
= A1
| A2
type ADataCmd = Cmd (Id AData)
$( derive makeBinary ''ADataCmd)
}}}
And here the compilation error:
{{{
ghc: panic! (the 'impossible' happened)
(GHC version 7.4.1 for i386-apple-darwin):
Could not convert Dec to Decl
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7391>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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