[Caml-list] Pattern-matching on Ocaml records

[Caml-list] Pattern-matching on Ocaml records

[Create Software]

Hi all,

I'm building an old Caml Light program with OCaml, and I've come across a reluctant piece of code. Here's a a mwe:

   type t = {mutable label: int};;
   let reset {label = lbl} = lbl <- 0;;

   let test = {label = -1};;
   reset test;;
   test;;

This compiles fine in Caml Light, yielding

   Type t defined.
   reset : t -> unit = <fun>
   test : t = {label = -1}
   - : unit = ()
   - : t = {label = 0}

On the other hand, Ocaml refuses this snippet, and prints the following message:

   Characters 26-34:
     let reset {label = lbl} = lbl <- 0;;
                               ^^^^^^^^
   Error: Unbound instance variable lbl

Was this construct deprecated? If so, why?

Thanks!
Clément.


PS: I also tried

   let reset' a =
     let lbl = a.label in
     lbl <- 0
   ;;

but "The identifier lbl is not mutable.", in both OCaml and Caml Light. Why so?

Re: [Caml-list] Pattern-matching on Ocaml records

[Andreas Rossberg]

On May 2, 2011, at 14:32, Create Software wrote:
> PS: I also tried
>
>  let reset' a =
>    let lbl = a.label in
>    lbl <- 0
>  ;;
>
> but "The identifier lbl is not mutable.", in both OCaml and Caml  
> Light. Why so?

Try:

   a.lbl <- 0

Only the record field itself is mutable. Extracting only gives you a  
value, not a reference.

/Andreas

Re: [Caml-list] Pattern-matching on Ocaml records

[Create Software]

> Only the record field itself is mutable. Extracting only gives you a value, not a reference.

Sure, but how come that
     let reset {label = lbl} = lbl <- 0;;
works fine in Caml Light and not in OCaml?

Clément.

Re: [Caml-list] Pattern-matching on Ocaml records

[Andreas Rossberg]

On May 2, 2011, at 15:16, Create Software wrote:
>> Only the record field itself is mutable. Extracting only gives you  
>> a value, not a reference.
>
> Sure, but how come that
>    let reset {label = lbl} = lbl <- 0;;
> works fine in Caml Light and not in OCaml?

Same reason: pattern matching extracts the value. I can't tell why it  
worked in Caml Light, though, since I'm not familiar enough with it.

/Andreas

Re: [Caml-list] Pattern-matching on Ocaml records

[Jacques Garrigue]

On 2011/05/02, at 22:16, Create Software wrote:

>> Only the record field itself is mutable. Extracting only gives you a value, not a reference.
> 
> Sure, but how come that
>    let reset {label = lbl} = lbl <- 0;;
> works fine in Caml Light and not in OCaml?

This has been deprecated.
Namely, this introduced a notion of lvalue, only for mutable record labels (and actually also for sum types IIRC).
The current approach of always requiring to specify the record is clearer, and it extends naturally the notations
   a.(i) <- e  for arrays
   s.[i] <- e  for strings

This said, later on object instance variables were added as lvalues, so the uniformity argument is not so strong...

Jacques Garrigue