I don't know of any way to examine this for a running program.  You can get GHC to spit out core and STG using -ddump-core and -ddump-stg flags:

There's no -ddump-core flag. I was puzzled about the proper way to get "final" core, and have been using -ddump-simpl, but I don't know if that's correct.

On Fri, Oct 15, 2010 at 1:19 AM, Jason Dagit wrote:

On Thu, Oct 14, 2010 at 2:52 PM, Evan Laforge wrote:

...

...

I don't know of any way to examine this for a running program.  You can get GHC to spit out core and STG using -ddump-core and -ddump-stg flags:

There's no -ddump-core flag.  I was puzzled about the proper way to get "final" core, and have been using -ddump-simpl, but I don't know if that's correct.

Indeed, I meant -ddump-simpl.  That's why I tried to link to the GHC manual, I make typos and it has extra info and enumerates the flags available :)  Just search that page for -ddump. Sorry about the confusing typo, Jason

Thanks, will look into the debugging dump flags. I should have done this before asking :) -- Mihai

Bernie Pope writes:

You can use side effects to observe the order of evaluation, by wrapping observed expressions (thunks) with some IO computation inside unsafePerformIO.

Not what OP asks for, but I've used a variant of this as a rather hackish to provide progress reporting. I take a list that is lazily generated, and wrap the elements with an IO action that outputs the count every 'step' elements. When the list is evaluated, the IO actions are executed. Code below. -k -- | Output (to stderr) progress while evaluating a lazy list. -- Useful for generating output while (conceptually, at least) in pure code countIO :: String -> String -> Int -> [a] -> IO [a] countIO msg post step xs = sequence' $ map unsafeInterleaveIO ((blank >> outmsg (0::Int) >> c):cs) where (c:cs) = ct 0 xs output = hPutStr stderr blank = output ('\r':take 70 (repeat ' ')) outmsg x = output ('\r':msg++show x) >> hFlush stderr ct s ys = let (a,b) = splitAt (step-1) ys next = s+step in case b of [b1] -> map return a ++ [outmsg (s+step) >> hPutStr stderr post >> return b1] [] -> map return (init a) ++ [outmsg (s+length a) >> hPutStr stderr post >> return (last a)] _ -> map return a ++ [outmsg s >> return (head b)] ++ ct next (tail b) -- | A lazier version of 'Control.Monad.sequence' in "Control.Monad", needed by 'countIO' above. sequence' :: [IO a] -> IO [a] sequence' ms = foldr k (return []) ms where k m m' = do { x <- m; xs <- unsafeInterleaveIO m'; return (x:xs) } -k -- If I haven't seen further, it is by standing in the footprints of giants