#7931: Deriving Read of an empty datatype crashes
-----------------------------+----------------------------------------------
Reporter: monoidal | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.3 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Blockedby:
Blocking: | Related:
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Standalone deriving Read of an empty datatype crashes:
{{{
{-# LANGUAGE StandaloneDeriving #-}
data A
deriving instance Read A
}}}
{{{
ghc: panic! (the 'impossible' happened)
(GHC version 7.7.20130515 for x86_64-unknown-linux):
Prelude.foldr1: empty list
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
}}}
(Oddly, the "panic" message is given only by ghc, not ghci, but this is
minor.)
While the Read instance seems to be of little use, it can be used for
reading types such as `Either Void Int`.
Patch attached.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7931>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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