python3: 'where' keyword

[Carl Banks]

Nick Coghlan wrote:
> Andrey Tatarinov wrote:
> > Hi.
> >
> > It would be great to be able to reverse usage/definition parts in
> > haskell-way with "where" keyword. Since Python 3 would miss lambda,
that
> > would be extremly useful for creating readable sources.
> >
> > Usage could be something like:
> >
> >  >>> res = [ f(i) for i in objects ] where:
> >  >>>     def f(x):
> >  >>>         #do something
>
[snip]
> For compound statements, a where clause probably isn't appropriate,
as it would
> be rather unclear what the where clause applied to.

Right.  But you know that as soon as you add this to simple
expressions, a bunch of people are going to come here whining about how
they don't get to use where with if-expressions.

Frankly, they might have a point here.  Although we have replacing
lambda expressions on our minds, I have in mind a different problem
that a where-statement would solve perfectly.  But it would have to be
used with an if-expression.

However, I think it might not be so hard.  Let's take Paul Rubin's
advice and precede the if statement with where.  Let's also allow
"elif" clauses to be replaced with "else where ... if" clauses.  That
which is bound in the while-block would be visible in both the
if-expression and if-block.

Then we could do this:

. where:
.     m = someregexp.match(somestring)
. if m:
.     blah blah blah
. else where:
.     m = someotherregexp.match(somestring)
. if m:
.     blah blah blah

We might want to spell "else where" instead as "elwhere", to match
"elif", but that's not important now.  This would take away one of the
major minor annoyances of Python.  (In fact, I've suggested something
like this as a solution to the set-and-test idiom, which Python makes
difficult, only I used the keyword "suppose" instead of "where".)

Ok, but if you do that, now you have people whining that "where" comes
after some expressions, and before others.  (This would not bother me
one bit, BTW, but I'm pretty sure I'd lose the popular vote on this
one.)

So, let's go all out and say that while could precede any statement.
We now have consistency.  Well, that really wouldn't work for the
if-statement, though, because then how could we apply a different
while-block to an else clause?  We'd have to treat if-statements
specially anyways.  So we don't have consistency.

My solution would be to propose two different where statements: a
where...do statement, and a separate where...if statement.  The
where...do statement would look like this:

. where:
.     def whatever(): pass
. do:
.     blah blah use whatever blah

It has the advantage of being able to apply the where bindings to
several statements, and is, IMO, much cleaner looking than simply
applying where's bindings to the single following unindented statement.

I would recommend against where...while and where...for statements.
They can't accomplish anything you couldn't do with a break statement
inside the block, and it's not obvious whether the where clause gets
executed once or for each loop (since it's physically outside the loop
part).

One question: what do you do with a variable bound inside a while-block
that has the same name as a local variable?  (Or, horrors, a
surrounding while-block?)  I'm inclined to think it should be illegal,
but maybe it would be too restrictive.
Anyways, I like this idea a lot.

+1


-- 
CARL BANKS