Re: GHC Core question

6 Jan 2008


      On 1/5/08, Neil Mitchell  wrote:
...
Hi
I've compiled the Debug.Trace module to Core, but can't understand the
resulting output. The original code is:
trace string expr = unsafePerformIO $ do
    putTraceMsg string
    return expr
The core is:
Debug.Trace.trace =
   \ (@ a) ->
     __letrec {
       trace :: GHC.Base.String -> a -> a
       []
       trace =
         \ (string :: GHC.Base.String) (expr :: a) ->
           GHC.Base.$
             @ (GHC.IOBase.IO a)
             @ a
             (GHC.IOBase.unsafePerformIO @ a)
             (>> @ () @ a (Debug.Trace.putTraceMsg string) (return @ a expr));
       $dMonad :: GHC.Base.Monad GHC.IOBase.IO
       []
       $dMonad = $dMonad;
       return :: forall a. a -> GHC.IOBase.IO a
       []
       return = GHC.Base.return @ GHC.IOBase.IO $dMonad;
       $dMonad :: GHC.Base.Monad GHC.IOBase.IO
       []
       $dMonad = GHC.IOBase.$f16;
       >> :: forall a b.
             GHC.IOBase.IO a -> GHC.IOBase.IO b -> GHC.IOBase.IO b
       []
       >> = GHC.Base.>> @ GHC.IOBase.IO $dMonad;
     } in  trace;
And my Haskell reformatting of that is:
Debug.Trace.trace = let
       trace string expr = unsafePerformIO $ putTraceMsg string >> return expr
       $dMonad = $dMonad;
       return = GHC.Base.return $dMonad;
       $dMonad = GHC.IOBase.$f16;
       >> = GHC.Base.>> $dMonad;
    in  trace
However, that let expression has two bindings for $dMonad, one of
which looks like a black-hole. Are the semantics of __letrec different
from let in some way?
How are you printing out the Core? It looks like the unique ids are
missing (you can see them if you pass the -ppr-debug flag, which can
be set using the API) -- if the unique ids were being printed, you
would see that the bindings for $dMonad (likely) have different
uniques.

Cheers,
Tim

-- 
Tim Chevalier * http://cs.pdx.edu/~tjc * Often in error, never in doubt
"...Losing your mind, like losing your car keys, is a real hassle." --
Andrew Solomon