GHC Core question

[Tim Chevalier]

On 1/5/08, Neil Mitchell <ndmitchell at gmail.com> 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