Gracjan Polak wrote:
Hi,
I stumbled at some interaction of Control.Monad.State.Strict, mdo and let I do not understand. The following program:
{-# OPTIONS_GHC -fglasgow-exts #-} module Main where import Control.Monad.State.Strict
thenumber :: Float thenumber = flip execState 1.3 $ mdo c <- donothing [] let donothing x = return x return ()
main = print thenumber
Says (in GHC 6.6.1):
Stack space overflow: current size 8388608 bytes. Use `+RTS -Ksize' to increase it.
Why is this so?
This mdo block goes into an infinite loop if the body is strict on donothing. In Control.Monad.State.Strict, m>>=k is strict on m. This m happens to be donothing[]. So the body is strict on donothing, and mdo goes into an infinite loop. (Control.Monad.State.Lazy postpones m until k really wants m's return value. If, like this example, k doesn't want it, everyone happily goes home.) It takes a lot more analysis and peaking into the implementation of Control.Monad.State.Strict to see why this infinite loop also consumes infinite stack space.