From: Eric Mahurin <eric_mahurin@...>
Date: 2005-10-03T01:18:15+09:00
Subject: Re: ideas for an RCR: variable locality



--- Bob Hutchison <hutch@recursive.ca> wrote:
> >> (a_1=x;b_2=(a_3=y;b_4=z;a_3<b_4 ? a_3 : b_4);a_1<b_2 ? a_1
> :
> >> b_2)
> >>
> > Yes, something like this would work (using your Gensym
> module):
> >
> > min2  = lambda { |a,b|
> >   v1 = Gensym.gensym
> >   v2 = Gensym.gensym
> >   "(#{v1}=#{a};#{v2}=#{b};#{v1}<#{v2} ? #{v1} : #{v2})"
> > }
> >
> > Here are the disadvantages of this compared to having ruby
> do
> > local variables:
> >
> > - not as natural/uglier
> 
> No argument there at all, I agree. However a little syntactic
> sugar  
> would make all the difference. I made a suggestion off the
> top of my  
> head as to what that might look like, but I've got no doubt
> that that  
> can be improved.
> 
> You might be able to make an argument that local variables
> are more  
> fundamental and that a macro system shouldn't be the way to
> solve the  
> problem. Though, the problem that motivated this discussion,
> which I  
> believe amounted to a problem with in-lining functions, is
> probably  
> better solved by the macro system.

For my exact situation (parser generator), I don't think the
lisp-style macros would help me.  My "macros" are more
object-oriented and dynamic in nature.  But, I would like to
think about it a little more to see if there would be a
general-purpose macro system that could encompass the usage I
have.

> > - clutters the namespace/variable table
> 
> Actually it doesn't in the current ruby. In the example I
> gave  
> earlier the gensymed names are not visible outside the eval.

Maybe you are thinking of blocks.  Looks like eval uses/makes
variables in the surrounding scope:

x,y,z = 1,-2,3
# => [1, -2, 3]
local_variables
# => ["_", "x", "y", "z"]
eval "(a_1=x;b_2=(a_3=y;b_4=z;a_3<b_4 ? a_3 : b_4);a_1<b_2 ?
a_1 : b_2)"
# -2
local_variables
# ["_", "x", "y", "z", "a_1", "b_2", "a_3", "b_4"]


> > - prevents GC from freeing objects that these variables
> > reference
> 
> Not if they don't exist (as I think they don't)

Looks like they do.



		
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