{-# LANGUAGE MagicHash, BangPatterns #-}
import GHC.Exts
import GHC.HeapView
import Control.DeepSeq
import System.Environment
import System.Mem
l = [1,2,3]
main = do
args <- map length `fmap` getArgs
let l2 = 4:l
(l ++ l2 ++ args) `deepseq` return ()
let x = l ++ l2 ++ args
performGC
putStrLn "ghc-heap-view-demo"
putStrLn ""
putStrLn "Here are a few different lists."
putStrLn $ "The first one, l, found at " ++ show (asBox l) ++ " (where the /2 is the pointer tag information) is a module-level constant, and fully evaluated:"
getClosureData l >>= print
putStrLn $ "The second one, l2, found at " ++ show (asBox l2) ++ " is locally defined as l2 = 4:l. See how the cons-cell references l!"
getClosureData l2 >>= print
putStrLn $ "And here is the list args (" ++ show (asBox args) ++ ") that is static, but not known at compiletime, as it depends on the command line arguments:"
getClosureData args >>= print
putStrLn $ "And now we have, at " ++ show (asBox x) ++ ", the concatenation of them, but unevaluated. The thunk keeps a reference to l2 and args, but not l, as that is at a static address, unless you are running this in GHCi:"
getClosureData x >>= print
putStrLn ""
putStrLn "Now to some more closure types:"
let !(I# m) = length args + 42
let !(I# m') = length args + 23
let f = \x n -> take (I# m + I# x) n ++ args
t = f m' l2
putStrLn $ "The following value f (" ++ show (asBox f) ++ ") is a locally defined function that has args as a free variable, as well as an unboxed integer (42):"
getClosureData f >>= print
putStrLn "And the following is a thunk that applies f (also references here) to an unboxed value (23) and l2:"
getClosureData t >>= print
putStrLn ""
putStrLn "Here is the standard example for self reference:"
putStrLn "> let x = id (:) () x"
let x = id (:) () x
putStrLn $ "This is what x (" ++ show (asBox x) ++ ") looks like, at least without -O:"
getClosureData x >>= print
x `seq` return ()
putStrLn $ "So it is unevaluated. Let us evaluate it using seq. Now we have, still at " ++ show (asBox x) ++ ":"
getClosureData x >>= print
IndClosure {indirectee = target} <- getClosureData x
putStrLn $ "The thunk was replaced by an indirection. If we look at the target, " ++ show target ++ ", we see that it is a newly created cons-cell referencing the original location of x:"
getBoxedClosureData target >>= print
performGC
putStrLn $ "After running the garbage collector (performGC), we find that the address of x is now " ++ show (asBox x) ++ " and that the self-reference is without indirections:"
getClosureData x >>= print
recurse :: Int -> Box -> IO ()
recurse m = go 0
where go i b = if i >= m then return () else do
putStrLn $ ind ++ show b
c <- getBoxedClosureData b
putStrLn $ ind ++ show c
mapM_ (go (succ i)) (allPtrs c)
where
ind = concat $ replicate i " "