On Wed, Jan 31, 2007 at 11:22:54AM +1100, Donald Bruce Stewart wrote:
igloo:
http://hackage.haskell.org/trac/ghc/ticket/1127
Deadline: 28 February 2007.
I propose we make StateT lazy, as I suspect was the intention all along and as has been discussed (without reaching a conclusion) before.
The patch is simply (in the definition of (>>=)):
hunk ./Control/Monad/State.hs 195 - (a, s') <- runStateT m s + ~(a, s') <- runStateT m s
Is it possible to provide a short example of the different behaviours you need or expect?
The attached without the ~ gives: $ time ./p 1000000 Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it. ./p 1000000 1.96s user 0.05s system 96% cpu 2.095 total whereas with the ~ you can pump up the argument as high as you like without any runtime or stack use worries: $ time ./p 1000000 'a' ./p 1000000 0.00s user 0.00s system 93% cpu 0.004 total $ time ./p 1000000000 'a' ./p 1000000000 0.00s user 0.00s system 101% cpu 0.004 total For a non-contrived example see the compression package in hackage. Thanks Ian