State Transformer

Jorge Adriano
Fri, 11 Jan 2002 22:59:58 +0000

> I agree with you.  My work-around is then to define foo and bar locally
> to testfunc, in the scope of r:
> testfunc = do
>            r <- newSTRef ('x',0)
>            let foo = do
>                      (c,n) <- readSTRef r
>                      writeSTRef r ('a', n+1)
>                bar = do
>                      (c,n) <- readSTRef r
>                      writeSTRef r (c,n+2)
>            foo
>            bar
>            (c,n) <- readSTRef r
>            return n

Thought about that to... but it looks kind of... terribly ugly (sorry :)

> But if this looks like unsatisfactory (it does to me, too), perhaps
> you have to go back to DIY monads.
DIY? what does that means?

> DIY monads are good when: you fix the state variables, you don't want
> to mention them in subprogram parameters.

> The ST monad is good when: you create more state variables on the fly,
> you use mutable arrays, you don't want to write your own monad and
> put/get commands.

What if you want both and keep nice clean(*) programming style... :-)

(*) Clean as in "not dirty", not Clean the FL.