[Haskell-cafe] Control.Monad.State.Strict, mdo and let

[Albert Y. C. Lai]

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.