python and macros (again) [Was: python3: 'where' keyword]

[Nick Coghlan]

Paul Rubin wrote:
> Come on, that is vacuous.  The claim was "expressions are not
> statements".  But it turns out that expressions ARE statements.  The
> explanation is "well, that's because they're expression statements".
> And there is no obvious case of an expression that can't be used as a
> statement.  So it's not inherently obvious that there needs to be any
> kind of statement that can't be used as an expression.  It's just an
> artifact.  Whether the artifact is a desirable one is a matter of
> discussion.

No, it's entirely to do with building a readable language that uses significant 
whitespace to delineate scope.

Simple statements are not allowed to contain other statements, but they are 
allowed to contain expressions. In their most degenerate form, ALL they contain 
is a *single* expression (e.g. a function call). That expression itself is still 
not a statement, though. Think of it as the difference between value and [value].

Suites are sequences of statements.

Compound statements are statements which incorporate a suite as part of their 
syntax.

Python allows statements inside suites and suites inside compound statements. It 
also allows expressions inside statements and expressions inside expressions. 
The one thing it never ever does is allow a suite or a statement inside an 
expression, because doing so would utterly destroy the handling of significant 
white space.

And that's why expressions are special in Python - they demarcate the boundary 
of the significance of whitespace. Within an expression, whitespace is 
insignificant. At the level of statements and suites, whitespace is extremely 
significant.

So, precisely how should one go about cleanly embedding something that cares 
about whitespace into a context which doesn't care in the slightest?

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at email.com   |   Brisbane, Australia
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