Re: [Caml-list] let mutable (was OCaml Speed for Block Convolutions)

[Jacques Garrigue <garrigue@kurims.kyoto-u.ac.jp>]

From: Pierre Weis <Pierre.Weis@inria.fr>

> > > 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 !

You can of course explain the semantics this way, but most people
apparently consider that there are lvalues in ocaml, just that they
are very restricted. It is certainly much simpler to explain than
lvalues in C!

Also, there is one unique case currently which can only be explained
by the distinction lvalue/rvalue: instance variables in objects.

  class counter = object
    val mutable x = 0
    method get = x
    method bump = x <- x + 1
  end

How can you explain the method bump without the notion of lvalue?

Perfectly coherent answers would be, let's remove mutable instance
variables, or force the notation self.x to access variables.
Another answer is that people so fond of objects have already heared
of lvalues anyway.

the problem is not only with lvalues either. With for loops, you have
a case of rvalue, where something which is not syntactically a
function have a changing variable, which is accessed directly. The
fact you cannot change it yourself is not relevant.

Best regards,

     Jacques Garrigue
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