From: Dan Muller <dmuller@...>
Date: 2001-03-25T08:40:02+09:00
Subject: [ruby-talk:13181] Re: FEATURE REQUEST: 'my' local variables

<delurking on c.l.r for the first time...>

"Yukihiro Matsumoto" <matz@zetabits.com> wrote in message
news:984665134.285425.2254.nullmailer@ev.netlab.zetabits.com...
[text elided]
> |Introducing new variables at a beginning of the block is better for
> |me.  Otherwise I should be more careful of where a variable belonging
> |to, since variables can suddenly become block local in a sequence of
> |code.
>
> I can understand you.

I also prefer a declarative style. I find the ":=" to be too subtle, too
hard to see. Guess I'm getting old; my glasses get stronger every year. :-(

I'm not sure if I like the following proposal, but I spent some time
thinking about it, so I'll post it rather than toss it...

How about a variation on the let idea. In pseudo-BNF:

with-expr := 'with' with-body  var-init-list with-scope
with-scope := block | statement* 'end'
with-var-list :=
  '*'
 | with-var
 | '(' with-var ( ',' with-var )* ')' )*
with-var = simple-var-name ( = expr )?
var-list = simple-var-name ( ',' simple-var-name )*

The with-expr creates a scope consisting of the following block or list of
statements. The variables mentioned in with-var-list are local to that
scope.

If the with-expr ends with a block, it returns a block. Otherwise it returns
the value of the last statement.

Using an asterisk in place of a list makes all variables that are first
mentioned in the with-expr's scope be local. This is exactly the current
behavior of blocks, but this lets you get that behavior for an arbitrary
list of statements.

The block form of with-expr is important because it lets you make an
iterator variable be local, without introducing any syntax between the
beginning of the block and the iterator argument list. (I find that
introducing something in there is not aesthetically pleasing, and would
probably be hard to parse.)

a = 1
i = 1
c = 1
(1..3).each with (i,a,b) do |i|
  a = i
  b = i
  c = i
end
p a   # -> 1
p i    # -> 1
p b   # error
p c   # -> 3

To force something to have outer scope (e.g. the iterator), you would still
need to assign it before the block, but that doesn't seem like a hardship.

- Dan Muller