Small Int and Char closures in GHCi

Joachim Breitner mail at
Thu Aug 30 13:29:23 CEST 2012


I am preparing a talk about the details of how data and programs look in
memory in Haskell (well, GHC). When explaining the memory consumption of
a large String, I wanted to show the effect of short-int-replacement
that happens in

I use my ghc-heap-view-package to observe the heap. This programs shows
the effect:

        import GHC.HeapView
        import System.Mem
        main = do
            let hallo = "hallo"
            mapM_ (\x -> putStrLn $ show x ++ ": " ++ show (asBox x))
            mapM_ (\x -> putStrLn $ show x ++ ": " ++ show (asBox x))

gives, as expected:

$ ./SmallChar 
'h': 0x00007f2811e042a8/1
'a': 0x00007f2811e08128/1
'l': 0x00007f2811e09ef0/1
'l': 0x00007f2811e0bcd8/1
'o': 0x00007f2811e0db10/1
'h': 0x00000000006d9bd0/1
'a': 0x00000000006d9b60/1
'l': 0x00000000006d9c10/1
'l': 0x00000000006d9c10/1
'o': 0x00000000006d9c40/1

but in GHCi, it does not work:

$ runhaskell SmallChar.hs 
'h': 0x00007f5334623d58/1
'a': 0x00007f5334626208/1
'l': 0x00007f5334627fc0/1
'l': 0x00007f5334629dc0/1
'o': 0x00007f533462bba8/1
'h': 0x00007f533381a1c8/1
'a': 0x00007f5333672e30/1
'l': 0x00007f533381a408/1
'l': 0x00007f533381a6b8/1
'o': 0x00007f533389c5d0/1

Note that the GC does evacuate the closures, as the pointers change. Why
are these not replaced by the static ones here?

Joachim "nomeata" Breitner
  mail at  |  nomeata at  |  GPG: 0x4743206C
  xmpp: nomeata at |

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