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From: Pierre Weis <Pierre.Weis@inria.fr>
Message-Id: <200106111342.PAA12420@pauillac.inria.fr>
Subject: Re: [Caml-list] let mutable (was OCaml Speed for Block Convolutions)
In-Reply-To: <01060821302901.00670@haguenauer> from Michel Quercia at "Jun 8, 101 09:30:29 pm"
To: michel.quercia@prepas.org
Date: Mon, 11 Jun 2001 15:42:58 +0200 (MET DST)
Cc: caml-list@inria.fr
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> Le Vendredi  8 Juin 2001 19:30,  Pierre Weis a écrit :
> 
> > The introduction of a ``let mutable'', more concisely noted with the
> > var keyword, is not new: it has been discussed in the Caml groups 3 or
> > 4 years ago. We chose to abandon it for sake of semantics simplicity
> > of the language.
> 
> For beginners (f.e. students) things look a bit complicated :
> 
> (* summing up all elements of an integer array *)
> let adda a =
>   let res = ref 0 in
>   let i = ref 0 in
>   while !i < Array.length(a) do res := !res+a.(!i); i := !i+1 done;
>   !res
> ;;
> 
> A lot of boring exclam, but that's the price to pay for having 
> mutable values, and that's logical. Okay ...
> 
> (* same, but with a for loop *)
> let add_1 a =
>   let res = ref 0 in
>   for i=0 to Array.length(a)-1 do res := !res + a.(i) done;
>   !res
> ;;
> 
> No exclam and no ref for i ?  And its value is changing though ? Where is 
> gone the logic ?

The for loop is a short hand for a call to a local recursive function:
no reference and no problem here, unless you consider that you cannot
change the arguments of a recursive call to a function.

(For readers not familiar with the subject, let's recall that

for i = e1 to e2 do e3 done

 is equivalent to

let rec _loop i _lim = if i <= _lim then begin e3; _loop (i + 1) _lim end in
_loop e1 e2

(where _loop and _lim stand for new fresh identifiers, not free in e1, e2,
or e3)
)

> > This construction would have introduced the notion of
> > Lvalue in Caml, thus introducing some additional semantics complexity,
> > and a new notion to explain to beginners.
> 
> Lvalues already exist in Ocaml (and have to be explained to beginners), for 
> example : "a.(i) <- a.(i)+1".

I'm afraid this is wrong.

The syntactic construction e1.(e2) <- e3 is a short hand for a
function call: Array.set e1 e2 e3. Once more there is no Lvalue here,
just a regular function call (hence you can write arbitrary complex
expressions in place of e1, provided it returns an array value).

I'm a bit surprised that you feel it necessary to explain the notion
of Lvalue to beginners when there is no such notion in the language !

Best regards.

Pierre Weis

INRIA, Projet Cristal, Pierre.Weis@inria.fr, http://pauillac.inria.fr/~weis/


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