Re: [Caml-list] destructive local opens

[Hendrik Boom <hendrik@topoi.pooq.com>]

On Wed, Aug 19, 2015 at 03:13:34PM +0100, John Whitington wrote:
> 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.

This is very explicit about what it allows and shadows.  I like it.

> 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.
> 

This is possibly less useful.

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