Re: Haskell-Cafe digest, Vol 1 #293 - 4 msgs

Message: 4 From: Jorge Adriano To: haskell-cafe@haskell.org, glasgow-haskell-users@haskell.org Subject: State Transformer Date: Mon, 7 Jan 2002 15:41:57 +0000

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Till then I had just read what I needed to be able to use the IO Monad. Seems to me like having a State Transformer monad its the best way to do it. Now I've read a great deal of Richard Birds Book chap 10 (Monads), as well as the "Monads for the Haskell Working Programmer"[1] by Theodore Norvell.

I was going to try to make my own simple examples using a ST. A State Monad seemed something like would most probably be in some Standard Library, or at least in some GHC library. And it was (section 4.31.ST in the hslibs documentation) I wanted to use this ST, but then I noticed it was different from the one described in tutorial[1].

I was expecting the ST Monad ghc module to provide an apply function, analogue to the

...

applyST :: StateTrans s a -> s -> (s, a) applyST (ST p) s = p s

I believe you want runST. its type is not

in the tutorial. I also expected to have general functions to access and change State. I can't implement them myself since the ST constructor is (obviously) not exported. But this ST module seems to work in a completely diferent way.

...

From what I can tell it is not suposed to be applyed to an initial state, instead it starts with an 'empty' state... State is controled with Referencies (mutable variables).

yes, you can initialize any number of mutable variables from within the state monad.

Ok, now my problem, how do I use this?

---------------------------------------- stupidFunc runST ( do { initstate <- newSTRef ('x',0) -- and now what? }) -----------------------------------------

----- import ST main = print (runST (do { x <-newSTRef 3;add1 x; y<-newSTRef "Elvis"; saysomething y; a<-readSTRef x; b<-readSTRef y; return (a,b)})) add1 a = do l <-readSTRef a writeSTRef a (l+1) saysomething y = modifySTRef y leftthebuilding leftthebuilding::String->String leftthebuilding x = x ++ " has left the building." ----- compile the above code with -package lang perhaps that'll give you some idea how to deal with STRefs and all. Jay Cox