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[Caml-list] look operator
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Date: 2002-06-06 (11:01)
From: Pixel <pixel@m...>
Subject: Re: [Caml-list] look operator
Winfried Dreckmann <wd@lidingo.mail.telia.com> writes:


> It's about abstracting the ! operator by introducing a function
> val look : tref -> t
> which coerces a mutable object into a non-mutable one. Using this function
> is dangerous because, with this function, the non-mutable type t is not
> strictly non-mutable anymore. As the manual says, the result of "look r" is
> volatile, it is only guaranteed to be valid until the next in-place
> operation involving r. In my own experience, mistakes occur faster than
> expected. But this is a great and elegant trick.

=> "look" is only used for efficiency? couldn't the compiler achieve
the same result without using an unsafe construct?

> Using "look", every single
> function with arguments of type t, say
> val add_in : tref -> t -> t -> unit,
> replaces two or more functions which would otherwise be necessary, in this
> case
> val add_in1 : tref -> t -> t -> unit
> val add_in2 : tref -> tref -> t -> unit
> val add_in3 : tref -> tref -> tref -> unit

are you saying that i would be nice to have this? As far as i have
looked at numerix, it doesn't have this.

> at least. This would certainly blow up the library to impractical
> dimensions. Of course, overloading would help, and "look" might become
> obsolete in this way.
> However, I think the problem is not mainly about overloading,

agreed, row subtyping can already achieve this:

let add_in_wrapped r a b = r := !r + a + b

let deref = function
  | `Ref a -> !a
  | `Const a -> a

let add_in (`Ref a) b c = add_in_wrapped a (deref b) (deref c)

let x = `Ref (ref 1)
let y = `Ref (ref 2)
let c = `Const 3
add_in x y c ; x

> but about
> reintroducing imperative features in an abstract and controlled way. I
> could, for instance, also imagine an abstract assign operator
> val set : tref -> t -> unit
> where the contents of t is not copied but assigned to tref, and thus made
> mutable, which could be useful in certain restricted ways.
> My question to the caml list: Would you accept such constructions as decent
> Caml programming, if applied carefully and only in cases where it allows
> what is otherwise impossible (e. g. integrating mutable and non-mutable
> objects as it is done in "numerix"). Or is it all just a silent
> reintroduction of C pointers, and principally a bad thing?

I don't think this will never be in OCaml!
(but i may be prooved wrong :)

I've not found many information about this. AFAIK C++ is the only
language having constness subtyping (http://merd.net/inoutness.html)

The few links i've found: http://merd.net/inoutness.html#references
I someone knows better, please tell, i'm interested :)

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