Add punning for object copying expressions.

[yallop]

This little patch extends the "punning" pattern in the OCaml syntax to object override expressions. The punning pattern conflates label names and variable names in various contexts; for example, you can define functions with labeled arguments either by binding variables for each argument or by punning, borrowing the label name as a variable:

let f ~a:x ~b:y = x + y
let f  ~a  ~b   = a + b

Similarly, you can pass labeled arguments either explicitly or by punning:

let x = 3 and y = 4 in f ~a:x ~b:y
let a = 3 and b = 4 in f  ~a  ~b

The same thing applies to record labels, both in patterns and expressions:

let conj { re = r ; im = i } = { re = r; im = -.i }
let conj { re; im } = { re; im = -.im }

This patch extends the pattern to object override expressions, which make copies of objects, changing the values of some fields. With the patch applied you can either use the existing syntax or the more concise version with punning:

object val a = 3 val b = 4 method m x y = {< a = x; b = y >} end
object val a = 3 val b = 4 method m a b = {< a; b >} end

[bobot]

In the last line of the example shouldn't you replace method m x y = {< a; b >} by method m a b = {< a; b >} ?

[yallop]

Indeed. Fixed!

[gasche]

This is minor but you could use opt_semi here, as is done in the meth_list rule. It would maybe make it easier to scan the grammar for optional terminators.

[yallop]

Thanks: implemented in e5a8e38.

[gasche]

I believe the code is correct and see no problem with the proposed change (it is not a new feature, rather a uniformization of existing uses of the label = expr syntax).

[Drup]

I wrote the punned version several time (which caused a nice syntax error, as you might guess) so I'm quite happy about this change.

[bobot]

This PR have been discussed during the last developers' meeting, where I had the pleasure to be. To sum-up:

• It seems not very useful
• But ok for integration because of consistency.

[gasche]

Merged in trunk, thanks!