From: daz <dooby@...10.karoo.co.uk>
Date: 2003-09-13T21:40:34+09:00
Subject: Re: order of evaluation for method arguments


"Michael Garriss" <mgarriss@earthlink.net> wrote:

> Michael Garriss wrote:
>
> > Is this code guaranteed to always work this way:
> >
> > a = [1,2,3,4,5]
> > --> [1, 2, 3, 4, 5]
> > def m( a, b, *c )
> >  puts a, b, c
> > end
> > --> nil
> > m( a.shift, a.shift, a )
> > 1
> > 2
> > 3
> > 4
> > 5
> > --> nil
> >
> > I'm wondering (hoping) that the arguments will always be evaluated
> > from left to right as is the case above.  Is this a 'ruby rule' or
> > something that could change?
> >

From Pickaxe:

The Ruby Language
  Method Definition
    Method Arguments

(I can't find this section on rubycentral/book to link to)


"The expressions are evaluated from left to right.
 An expression may reference a parameter that precedes it
 in the argument list."

#--------------------
a = [1,2,3,4,5]

def m( a, b, *c )
  p a, b, c
end

m( a.shift, b=a, a )
#--------------------


#-> 1
#-> [2, 3, 4, 5]
#-> [[2, 3, 4, 5]]


So that's as safe as Pickaxe.

>
> Let me extend the question to include arrays (which might
> be related to arguments, I don't know, I've never looked
> at the ruby source):
>
> a = [1,2,3,4,5]
> --> [1, 2, 3, 4, 5]
> b = [a.shift,a.shift,a.shift,a.shift,a.shift]
> --> [1, 2, 3, 4, 5]
>

Well, since:

   a, b = b, a

is 'defined' (in the sense that it's a ruby idiom),
your example works the way I'd expect, even though
Matz has guarded his options.  Perhaps he means that
this could look like a prime candidate for optimization
which would give  [1, 1, 1, 1, 1]  but I don't know if
anyone would like to see that happen.


>
> Michael
>


daz