Strange behavior when using stable names inside ghci?

Simon Marlow marlowsd at
Fri Jun 29 17:32:56 CEST 2012

On 27/06/12 22:41, Facundo Domínguez wrote:
> Hi,
>    The program below when loaded in ghci prints always False, and when
> compiled with ghc it prints True. I'm using ghc-7.4.1 and I cannot
> quite explain such behavior. Any hints?
> Thanks in advance,
> Facundo
> {-# LANGUAGE GADTs #-}
> import System.Mem.StableName
> import Unsafe.Coerce
> import GHC.Conc
> data D where
>     D :: a ->  b ->  D
> main = do
>    putStr "type enter"
>    s<- getLine
>    let i = fromEnum$ head$ s++"0"
>        d = D i i
>    case d of
>      D a b ->  do
>          let a' = a
>          sn0<- pseq a'$ makeStableName a'
>          sn1<- pseq b$ makeStableName b
>          print (sn0==unsafeCoerce sn1)

GHCi adds some extra annotations around certain subexpressions to 
support the debugger.  This will make some things that would have equal 
StableNames when compiled have unequal StableNames in GHCi.  You would 
see the same problem if you compile with -fhpc, which adds annotations 
around every subexpression.

For your intended use of StableNames I imagine you can probably just 
live with this limitation - others are doing the same (e.g. Accelerate 
and Kansas Lava).


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