From: Eric Mahurin <eric_mahurin@...>
Date: 2005-10-02T05:55:15+09:00
Subject: Re: ideas for an RCR: variable locality

--- ES <ruby-ml@magical-cat.org> wrote:

> Eric Mahurin wrote:
> > I would like to start this thread with end goal being to
> create
> > an RCR to control variable locality in the language (or
> > determine that there is already a reasonable alternative).
> > 
> > Problem: Within a method you can't reuse a variable name
> for a
> > local variable.  Some changes in ruby 2 may help the issue,
> but
> > also hurt it in another way.
> > 
> > Example: For performance reasons, I'm having my package
> > (grammar - generic parser), generate flattened (reduce
> > calls/stack depth) code.  As long as each "method" (to be
> > flattened) is simple enough that it doesn't require local
> > variables I'm OK.  But as soon one of these need a local
> > variable, I'm in trouble - that variable could step on the
> toes
> > of another including one just like it that it
> calls/flattens.
> 
> Could you perhaps offer a reduced code example? Your problem
> description makes no sense (though probably due to fault of
> mine).

Here's one - a poor man's macro facility.  Let's say a macro is
just a lambda that returns a string.  You just eval it when you
need to execute the code for that macro.

plus  = lambda { |a,b| "(#{a}+#{b})" }
# will have to re-evaluate a or b if they are an expression
min1  = lambda { |a,b| "(#{a}<#{b} ? #{a} : #{b})" }
# use local variables to prevent re-evaluation
min2  = lambda { |a,b| "(a=#{a};b=#{b};a<b ? a : b)" }

# y+z may get evaluated twice
min1["x",plus["y","z"]]
# => "(x<(y+z) ? x : (y+z))"

# y+z may evaluated once
min2["x",plus["y","z"]]
# => "(a=x;b=(y+z);a<b ? a : b)"

# need to localize a/b for the inner min2
min2["x",min2["y","z"]]
# => "(a=x;b=(a=y;b=z;a<b ? a : b);a<b ? a : b)"


See the problem on this last example?  We really need to
localize a and b.  There isn't a good facility to do this.



		
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