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Local references in Ocaml vs. state in Haskell
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Date: 2007-11-23 (15:50)
From: Jeff Polakow <jeff.polakow@d...>
Subject: Re: [Caml-list] Local references in Ocaml vs. state in Haskell

  The ocaml code can be directly translatedto Haskell using the ST monad 
and STRef.


caml-list-bounces@yquem.inria.fr wrote on 11/23/2007 05:16:00 AM:

> I have some programs written in ocaml that use references. I wanted to 
> translate them to Haskell, but since I am not an active Haskell user, I 
> got stuck with a very simple problem, namely, that the state monad in 
> Haskell provides _global_ references while ocaml has _local_ references.
> Suppose in ocaml I have this:
> (* Does a functional [f] look at its argument [a]? *)
> let touched f a =
>    let flag = ref false in
>    let _ = f (fun n -> flag := true; a n) in
>      !flag
> "touched f a" evaluates "f a" and records the fact that f actually 
> evaluated a at some argument. Note that flag is a local reference, so f 
> does not have access to it.
> Using a global flag does not work:
> let flag = ref false
> let touched' f a =
>    flag := false ;
>    let _ = f (fun n -> flag := true; a n) in
>      !flag
> Now we can write an f which touches its argument but fools touched' by 
> reversing the value of flag.
> let f a =
>    let y = a 42 in
>      touched := false ; y
> If I try to use the State monad I will get a global reference threaded 
> thrugh the computation, and f will have access to it. I want a local 
> reference that is inaccessible outside its lexical scope. I am probably 
> missing something obvious about Haskell.
> I am hoping that even though this is a question about Haskell, you won't 

> mind, as it gives everyone on this list an opportunity to show how ocaml 

> is superior to Haskell. Just kidding :-)
> Andrej
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