Re: [Caml-list] destructive local opens

[John Whitington <john@coherentgraphics.co.uk>]

Hi,

Oleg wrote:
> [Since this discussion still continues, I think I will comment on
> Gabriel's comment after all.]
>
>   "It is remarkable that programming languages have avoided introducing
>    explicit weakening (the popping of a symbol out of scope) for now, and
>    I think it is a property that should be preserved. We're not yet ready
>    to go there."
>
> Actually there have been proposals to go into that direction, about 30
> years ago.
>
> 	http://research.microsoft.com/lampson/39-Pebble/39-PebbleAbstract.htm
>
> Please look at p 287 of the paper: ``Pebble also has an anti-LET,
> which impoverishes the environment instead of enriching it.'' This is
> exactly the issue under discussion in this thread.

I've sometimes thought it would be nice to be able to write something like:

let x = f a b c
and y = g d e in
   let x' for x y = x * 2 + y in
     (x', h x')

where "let x' for x y" introduces the binding of x' and takes x and y 
out of scope. Now, if we accidentally write...

let x = f a b c
and y = g d e in
   let x' for x, y = x * 2 + y in
     (x', h x)

...we would get an error, since x is out of scope on the last line. You 
could imagine also "let x' for _ = ... in ..." which removes all 
variables in the current scope used in the calculation of x' from scope, 
which could be determined syntactically. This seems draconian, though.

John

-- 
John Whitington
Director, Coherent Graphics Ltd
http://www.coherentpdf.com/