{-# 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 four different lists, where the first three are already evaluated."
putStrLn "The first one, l, was defined as a top level constant as "
putStrLn "> l = [1,2,3]"
putStrLn $ "and is now found at " ++ show (asBox l) ++ " (where the /2 is the pointer tag information) and fully evaluated:"
getClosureData l >>= printInd
putStrLn $ "The second one, l2, is locally defined"
putStrLn "> let l2 = 4:l"
putStrLn $ "and now found at " ++ show (asBox l2) ++ ". See how the cons-cell references l!"
getClosureData l2 >>= printInd
putStrLn "And the binding"
putStrLn "> args <- map length `fmap` getArgs"
putStrLn $ "gives us at " ++ show (asBox args) ++ " a static, but at compile time unknown list:"
getClosureData args >>= printInd
getClosureData [] >>= printInd
putStrLn $ "And now we have, at " ++ show (asBox x) ++ ", the concatenation of them, but unevaluated:"
putStrLn "> let x = l ++ l2 ++ args"
putStrLn "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 >>= printInd
putStrLn ""
putStrLn "Now to some more closure types. m and m' locally bound of type the unboxed type Int#, with values 42 resp. 23."
putStrLn "> let f = \\x n -> take (I# m + I# x) n ++ args"
putStrLn " t = f m' l2"
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 $ "So here is (" ++ show (asBox f) ++ "), referencing its free variables args and 42:"
getClosureData f >>= printInd
putStrLn "And t is a thunk that applies f (also referenced here) to an unboxed value (23) and l2:"
getClosureData t >>= printInd
putStrLn ""
putStrLn "Lastly, 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 >>= printInd
x `seq` return ()
putStrLn $ "So it is unevaluated. Let us evaluate it using seq. Now we have, still at " ++ show (asBox x) ++ ":"
getClosureData x >>= printInd
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 >>= printInd
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 >>= printInd
printInd :: Show a => a -> IO ()
printInd x = putStrLn $ " " ++ show x
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 " "