#2457: Seq causes panic in GHCi
------------------------------+---------------------------------------------
Reporter: thoughtpolice | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.3 | Severity: normal
Keywords: | Testcase:
Architecture: x86 | Os: MacOS X
------------------------------+---------------------------------------------
While trying to understand the semantic differences between seq and pseq,
when looking at the Control.Parallel page, it says that the compiler may
transform "x `seq` y" into "y `seq` x `seq` y"
Trying this in GHCi lead me to this:
{{{
$ ghci
GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help
Loading package base ... linking ... done.
Prelude> let x = tail [1..]
Prelude> x `seq` ()
()
Prelude> () `seq` x `seq` ()
ghc-6.8.3: panic! (the 'impossible' happened)
(GHC version 6.8.3 for i386-apple-darwin):
nameModule (){v 71}
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
Prelude>
}}}
When testing this in a file however, things go smoothly:
{{{
$ cat seq.hs
x = tail [1..]
-- x `seq` y can apparently be transformed
-- into y `seq` x `seq y
v = () `seq` x `seq` ()
main = return v
$ ghc --make seq.hs
[1 of 1] Compiling Main ( seq.hs, seq.o )
Linking seq ...
$ ./seq; echo $?
0
$
}}}
I cannot be sure if this bug still occurs as of GHC 6.9.20080720, as ghci
is apparently broken in my build (this could have to do with build
settings but I am not sure, it has to do with the fact that the base
library can't find the _environ symbol; should I file another report?)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2457>
GHC <http://www.haskell.org/ghc/>
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