[Haskell-beginners] State Transformer Confusion

[Chris Pettitt]

Hello Haskell-Beginners,

I'm having a fair amount of difficulty understanding the ST monad. In
my basic experimentation I'm finding that I'm getting more heap
allocated with STUArray than with a regular Array, which is not what I
would expect.

I've attached a sample program that attempts to sum a list of Ints
into an array of length 1, using STUArray and UArray. The idea is
based loosely on http://www.haskell.org/haskellwiki/Monad/ST, but
using an array instead of an STRef.

I compile using these arguments: ghc -fforce-recomp -O2 -prof
-auto-all -caf-all --make sttest.hs

When I run the ST-based function (sumInPlace1), I get the following output:

./sttest +RTS -sstderr
1784293664
     240,944,212 bytes allocated in the heap
          35,448 bytes copied during GC
           4,052 bytes maximum residency (1 sample(s))
          12,332 bytes maximum slop
               1 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:   459 collections,     0 parallel,  0.00s,  0.00s elapsed
  Generation 1:     1 collections,     0 parallel,  0.00s,  0.00s elapsed

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time    0.30s  (  0.31s elapsed)
  GC    time    0.00s  (  0.00s elapsed)
  RP    time    0.00s  (  0.00s elapsed)
  PROF  time    0.00s  (  0.00s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time    0.30s  (  0.31s elapsed)

  %GC time       0.8%  (1.2% elapsed)

  Alloc rate    804,910,126 bytes per MUT second

  Productivity  99.0% of total user, 96.5% of total elapsed



While with the Array based function (sumInPlace2), I get the following output:

./sttest +RTS -sstderr
1784293664
     173,627,572 bytes allocated in the heap
          22,596 bytes copied during GC
           3,920 bytes maximum residency (1 sample(s))
          12,464 bytes maximum slop
               1 MB total memory in use (0 MB lost due to fragmentation)

  Generation 0:   333 collections,     0 parallel,  0.00s,  0.00s elapsed
  Generation 1:     1 collections,     0 parallel,  0.00s,  0.00s elapsed

  INIT  time    0.00s  (  0.00s elapsed)
  MUT   time    0.14s  (  0.15s elapsed)
  GC    time    0.00s  (  0.00s elapsed)
  RP    time    0.00s  (  0.00s elapsed)
  PROF  time    0.00s  (  0.00s elapsed)
  EXIT  time    0.00s  (  0.00s elapsed)
  Total time    0.15s  (  0.15s elapsed)

  %GC time       1.3%  (1.9% elapsed)

  Alloc rate    1,200,569,571 bytes per MUT second

  Productivity  98.3% of total user, 94.7% of total elapsed




One additional point of confusion for me: when I run either function
with +RTS -hc and use hp2ps I get an empty graph. I've seen these
tools work quite well before, so I suspect I'm doing something wrong
now.

Thanks,
Chris
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