Re: [Caml-list] Strange syntax used in queue.ml -- The "<-" operator

[bluestorm <bluestorm.dylc@gmail.com>]

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The semantics of mutable fields and references has changed a lot (for good)
between Caml Light and Objective Caml. Beware that this code is Caml Light
specific -- and if you want to learn Caml, you should rather be using
Objective Caml, which is the implementation that is still maintained.

In Objective Caml, only records fields are mutable. References are a derived
concept, the type 'a ref is defined as :
  type 'a ref = { mutable contents : 'a }

You change a mutable field with the syntax   foo.bar <- baz (where "bar" is
a record field, and foo and baz are any expression, foo being of a record
type)

In Caml Light, record fields are mutable, but sum type fields (variants) are
mutable as well; mutable variant fields are however not used here. See
http://caml.inria.fr/pub/docs/manual-caml-light/node4.6.html for
documentation.

In Caml Light, a record may return a mutable location, akin to a lvalue in
C-like languages. For example, with the mutable variant
  type foo = Foo of mutable int
you may write:
  let set_foo (f : foo) (n : int) =
    match f with
    | Foo loc ->
       loc <- n

"foo <- bar" is used here to assign a value "bar" to a lvalue "foo" bound in
a mutable pattern.

In your example, two mutable patterns are used :
     | { tail = Cons(_, ref newtail) as oldtail } ->

- oldtail is a mutable pattern denoting the mutable "tail" field of the
record
- (ref newtail) is a specific syntax, a pattern on references. It binds a
mutable pattern "newtail" corresponding to the location of the reference

In other words, in Caml Light you can write the ":=" operator as such:
  let prefix := r v =
    match r with
    | ref loc ->
      loc <- v

Hope that helps.

On Thu, Jan 20, 2011 at 5:53 PM, Andrew <newsgroups.fr@gmail.com> wrote:

> Hello everyone,
>
> While browsing the Caml Light library for programming examples, I stumbled
> across the following code, taken from the Caml Light queue.ml file:
>
>    type 'a queue_cell =
>        Nil
>      | Cons of 'a * 'a queue_cell ref
>    ;;
>
>    type 'a t =
>      { mutable head: 'a queue_cell;
>        mutable tail: 'a queue_cell }
>    ;;
>
>    let add x = function
>        { head = h; tail = Nil as t } ->    (* if tail = Nil then head = Nil
> *)
>          let c = Cons(x, ref Nil) in
>            h <- c; t <- c
>      | { tail = Cons(_, ref newtail) as oldtail } ->
>          let c = Cons(x, ref Nil) in
>            newtail <- c; oldtail <- c
>    ;;
>
> This implementation of FIFO data structures puzzles me. I get the general
> idea, to keep a pointer to the last entry in the structure, so that
> appending at the end is possible. This makes perfect sense to me. However,
> it's the syntax of how this is done that bugs me.
>
> Consider the following:
>
>      | { tail = Cons(_, ref newtail) as oldtail } ->
>          let c = Cons(x, ref Nil) in
>            newtail <- c; oldtail <- c
>
> I have a problem with types here. By the type definition, "newtail" should
> be of type "'a queue cell", since it's retrieved using "Cons(_, ref
> newtail)" in the pattern matching: if I understand correctly, this would
> mean that "newtail" binds the value pointed by the second member of the
> "tail" record field (which is a reference).
>
> So what does the "newtail <- c" means? If I try to replace this statement
> by
> "(fun x -> x <- c) newtail", I get a "The identifier x is not mutable.",
> whereas the code sounds perfectly similar to the original variant to me.
>
> Would rewriting these few lines to read as follows mean the same?
>
>      | { tail = Cons(_, newtail) as oldtail } ->
>          let c = Cons(x, ref Nil) in
>            newtail := c; oldtail <- c
>
> Taking the question one step further, what does the following code actually
> do?
>
>    type t = Nil | Node of (t ref);;
>    type box = {mutable field: t};;
>
>    let poke = function
>      | {field = Node(ref n)} -> n <- Nil
>      | {field = Nil} -> ()
>    ;;
>
>    let test = {field = Node(ref (Node(ref Nil)))};;
>    poke test;;
>    test;;
>
> Is it the same to write
>    {field = Node(n)} -> n := Nil
> and
>    {field = Node(ref n)} -> n <- Nil
> ?
>
> Even stranger: the following code returns "The value identifier a is
> unbound."
>    let a = Nil;;
>    a <- Nil;; (* The value identifier a is unbound. *)
>
>
> Could someone take the time to clarify the use of "<-" for me? The various
> examples here are pretty puzzling to me...
> Thanks!
>
>
> --
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>

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