Re: [Haskell-cafe] ST.Lazy vs ST.Strict

5 May 2009

      On Tue, May 5, 2009 at 3:27 PM, Luke Palmer  wrote:
...
On Sun, May 3, 2009 at 11:27 AM, Tobias Olausson  wrote:
...
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)
As Ryan points out, this will not do what you want.  But that is
incidental, not essential:
foo y = do
     x <- something
     fmap (x:) $ foo (y+1)
Questioning my own reasoning, I must apologize.   I was wrong, these two
programs are identical and both do what you want.

Any references to the state *after* the infinite chain of foos will result
in _|_, but as long as "something" is the only place that state calls occur,
you will be fine.

I also suspect that ST.Lazy should be no less defined than ST.Strict in all
cases, modulo unsafe operations of course (you aren't doing those, are
you?), so you have encountered a bug.  Minimize the test case and submit a
bug report :-)

Luke