Talk:Monad/ST

I think there is another caveat in that last example which is supposed to run in constant space.

The lines (in the last code example)

             x' <- readSTRef x
             y' <- readSTRef y
             writeSTRef x y'
             writeSTRef y (x'+y')

will build up a long unevaluated sum (1+1+2+3+5+8+..) in the STRef, which takes up stack space. When compiled I get a stack overflow when running fibST 1100000 (1.1 million) with a stack size of 8MB. There might be some hidden strictness in that "The ST monad provides support for strict state threads." but that isn't explained in this page.