So, I don't know what is causing your problem, but foo will not do
what you want even with lazy ST.
foo y = do
x <- something
xs <- foo (y+1)
return (x:xs)
Desugaring:
foo y = something >>= \x -> foo (y+1) >>= \xs -> return (x:xs)
= something >>= \x -> something >>= \x2 -> foo (y+2) >>= \xs2 ->
return (x2:xs2) >>= \xs -> return (x:xs)
= something >>= \x -> something >>= \x2 -> foo (y+2) >>= \xs2 ->
return (x:x2:xs2)
You see that there is an infinite chain of "foo" calls; the lazy ST
still needs to thread the state through that chain; so in the case of
foo 0 >>= something_else, the state is _|_ for something_else and you
will fail if you use read/write/newSTRef after that point. In fact,
I'm not sure that lazy ST is very useful :)
My guess is that you want one of
(1) mdo, when the effects in 'something' only matter once, or
(2) unsafeInterleaveST, if you just want to be able to traverse the
(x:xs) list lazily and the references it uses are dead after calling
foo.
-- ryan
On Sun, May 3, 2009 at 10:27 AM, Tobias Olausson
Hello! I have a program that is using ST.Strict, which works fine. However, the program needs to be extended, and to do that, lazy evaluation is needed. As a result of that, I have switched to ST.Lazy to be able to do stuff like
foo y = do x <- something xs <- foo (y+1) return (x:xs)
However, when running the program compiled with ST.Lazy, the following is outputted: [tobsi@wobsi]$ ./runnerLazy looper.hex runnerLazy: <<loop>>
The very same program compiled with ST.Strict outputs: [tobsi@wobsi]$ ./runner looper.hex 83298556
The code that is actually computing stuff is this: loopSys :: Int -> CPU s Int loopSys cc = do instr <- fetch if instr == 0xEA --NOP then return cc else do c <- execute instr loopSys $! (cc+c)
The CPU type looks as follows: type CPU s a = ReaderT (SysEnv s) (ST s) a
The program is run like runReaderT (loopSys 0) which in turn is being runST'd and then printed
Does anyone know why the program just outputs <<loop>> when compiled under ghc 6.10.2, and runs perfectly fine under ghc 6.8.2? The program is compiled with --make and -O2
Tobias Olausson tobsan@gmail.com _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe