Q: Python 2.0 preliminary features?

[skaller]

Guido van Rossum wrote:
> 
> Regarding the use of 'global' in combination with nested scopes:
> I have always been in favor or *not* providing a way to assign to
> variables at the intermediate levels.  That is, you can *use*
> variables from intermediate levels, but you can't *set* them.  The
> 'global' statement will set the variable at the module-global level.

	Actually, I'd be in favour of getting rid of 'global'
altogether now. It is no longer necessary, now python
has classes ...

> Motivation: I agree that it is useful to reference variables in the
> surrounding scope.  But if you start modifying them, you're abusing
> the nested scope mechanism to hold mutable state; it's much clearer
> to define a class for this purpose.
> 
> In particular, I think that allowing
> 
>         def f():
>                 sum = 0
>                 def accumulate(x):
>                         global sum
>                         sum = sum+x
>                 ...code using accumulate()...
>                 return sum
> 
> tends to create obfuscated code.

	I would not dispute this.  I think the
problem when doing this to a module, however,
is WORSE because function stack frames are
transient, whereas a module lives forever.

	The resolution to the module level
has another problem in Viper: there is no single module level.
In particular, a normal module is NOT the bottom level,
the bottom level is actually 'builtins'. Module
environments get chained together like function
stack frames, so the lookup works. [There is a possibl
extension to nested modules, which I have resisted 
play with yet :-]

	The problem is that there is no way to detect, 
in a chain of linked environments, which one is the 'module'
level: they're ALL represented by a common abstraction.

	[Of course, each environment knows it
has a particular type, and so 'function environments'
know to check for the list of globals when doing
an assignment: they just delegate to the parent
environment .. which is why my implementation
requires multiple global statements if you want to
actually reach module level from a deeply nested
function definition]

-- 
John Skaller, mailto:skaller at maxtal.com.au
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