[Python-ideas] Modern language design survey for "assign and compare" statements

[Tim Peters]

[Tim, on Rebol/Red]
>> >> x: 12 y: 13
>> == 13
>> >> [x y]
>> == [x y]
>> >>
>> >> do [x y]
>> == 13

[Greg Ewing <greg.ewing at canterbury.ac.nz>]
> How does scoping work? If you pass a block to a function
> which evaluates it, how are names in the block resolved?

Too involved, but basically a form of lexical scoping.  Rebol
struggled with this, and details vary by version.  Here are the last
docs for version 2:

     http://www.rebol.com/r3/docs/concepts/funcs-scope.html

Details changed again for version 3.  Best I can tell, Red hasn't
written up its own rules yet.

The model did _not_ support closures naturally (in the sense of local
bindings persisting beyond a function's return).  Version 3 added an
alternative to `func`, named `closure`, which could be used when you
wanted a closure:

    http://www.rebol.com/r3/docs/datatypes/closure.html

But I noticed just now that Red doesn't have `closure` built in, and
its funcs don't support closures naturally either:

>> adder: func [x] [func [y] [x + y]]
== func [x][func [y] [x + y]]

>> add3: adder 3
== func [y][x + y]

>> add3 12
*** Script Error: x is not in the specified context
*** Where: add3
*** Stack: add3

That's not dynamic scoping, either - it's still unhappy if `x` is
defined at top level:

>> x: 18
== 18
>> add3 12
*** Script Error: x is not in the specified context
*** Where: add3
*** Stack: add3