From: grady@... (Steven Grady)
Date: 2000-12-02T08:40:06+09:00
Subject: [ruby-talk:6726] Re: behavior of Proc#arity?

Matju responds:
>this was part of a thread on the same topic as this one.

(Thanks for the pointer to the previous thread -- unfortunately,
I have only been playing with Ruby for a few days, so I missed
that discussion.  Next time, I'll check the mail archive!)

>> of parallel assignment".  But there is no such thing as "parallel
>> assignment" for a single variable, so I think that passage does
>> not explain things.
>
>In message 6339, I gave an example of parallel assignment for 1 var:
>
>a = [1]   #=> [1]
>a         #=> [1]
>a,= [1]   #=> [1]
>a         #=>  1

Hm.  I agree that that's parallel assignment for 1 var, but it is
doesn't clarify the case Matz brought up.  From the book (p. 222):
    If the assignment contains multiple lvalues and one rvalue, ...
[Note that this does _not_ refer to multiple rvalues and one lvalue]
    If there are more rvalues than lvalues, the excess will be ignored.
[This explains your behavior above, but note that it requires the
extra "," after the a.  The point of my question was that the magic
behavior of |a| happened when there is _not_ a ",".]

In particular, I see nothing that explains:
    p1 = proc {|a| a}
    p1.call(1)		#=> 1
    p1.call(1, 2)	#=> [1, 2]
If we compare this to parallel assignment, we would have to say that
a is the lvalue, and the arguments to p1.call() are the rvalues.
Hence:
    a = 1	#=> 1
    a = 1, 2	#=> Syntax Error
Alternatively:
    p2 = proc {|a,| a}
    p2.call(1)		#=> 1
    p2.call(1, 2)	#=> Argument Error
    p2.call([1])	#=> [1]

Yield functions the same way as Proc#call, BTW:
    def x(*a)
	yield *a
    end
    x(1) {|a| a}	#=> 1
    x(1, 2) {|a| a}	#=> [1, 2]

"*args" functions differently:
    def y(*a)
	a
    end
    y(1)		#=> [1]

So, I have to ask again: Is the magic behavior of |a| documented anywhere?
--
	Steven

"So whaddya want?  Wicker?!?"