From: RichardOnRails <RichardDummyMailbox58407@...>
Date: 2010-12-29T13:35:17+09:00
Subject: Re: What am I missing about splat operator?

On Dec 28, 10:54 pm, RichardOnRails
<RichardDummyMailbox58...@USComputerGurus.com> wrote:
> On Dec 28, 10:12 pm, Ryan Davis <ryand-r...@zenspider.com> wrote:
>
>
>
> > On Dec 28, 2010, at 18:50 , RichardOnRails wrote:
>
> > > In trying to understand the splat operator, I visited:
> > >http://theplana.wordpress.com/2007/03/03/ruby-idioms-the-splat-operator/
>
> > > The first example that site offers is:
> > >   The split mode :
> > >   pet1, pet2, pet3 = *["duck","dog","cat"]
>
> > > That resulted in pet1 == "duck", etc
>
> > > But so did:
> > >   pet1, pet2, pet3 = ["duck","dog","cat"] # no splat operator
> > > and#
> > >   pet1, pet2, pet3 = "duck","dog","cat" # no array-literal markers
>
> > > So this first example makes no sense, does it?  What am I missing?
>
> > The fact that you're splatting an array doesn't matter, just the fact that you're splatting.
>
> > # 1
> > pet1, pet2, pet3 = *["duck","dog","cat"]
>
> > # 2
> > pet1, pet2, pet3 = ["duck","dog","cat"] # no splat operator
>
> > # 3
> > pet1, pet2, pet3 = "duck","dog","cat" # no array-literal markers
>
> > # parses as:
> > #
> > # s(:block,
> > #   # 1
> > #   s(:masgn,
> > #     s(:array, s(:lasgn, :pet1), s(:lasgn, :pet2), s(:lasgn, :pet3)),
> > #     s(:splat, s(:array, s(:str, "duck"), s(:str, "dog"), s(:str, "cat")))),
> > #   # 2
> > #   s(:masgn,
> > #     s(:array, s(:lasgn, :pet1), s(:lasgn, :pet2), s(:lasgn, :pet3)),
> > #     s(:to_ary, s(:array, s(:str, "duck"), s(:str, "dog"), s(:str, "cat")))),
> > #   # 3
> > #   s(:masgn,
> > #     s(:array, s(:lasgn, :pet1), s(:lasgn, :pet2), s(:lasgn, :pet3)),
> > #     s(:array, s(:str, "duck"), s(:str, "dog"), s(:str, "cat"))))
>
> > # In all 3 cases, you're dealing with a multi-assign w/ an array on LHS.
> > #
> > # RHS:
> > # 1) splatted array
> > # 2) to_ary array (or any object, see below)
> > # 3) array (whether you use [] or not, it is still an array literal)
>
> > a, b, c = x
> > a, b, c = *x
>
> > # s(:block,
> > #   # 1
> > #   s(:masgn,
> > #     s(:array, s(:lasgn, :a), s(:lasgn, :b), s(:lasgn, :c)),
> > #     s(:to_ary, s(:call, nil, :x, s(:arglist)))),
> > #   # 2
> > #   s(:masgn,
> > #     s(:array, s(:lasgn, :a), s(:lasgn, :b), s(:lasgn, :c)),
> > #     s(:splat, s(:call, nil, :x, s(:arglist)))))
>
> Hi Ryan,
>
> It looks to me the you gave me Lisp expressions one could use, in
> part, to translate the three right-hand-sides I provided.  I knew that
> Ruby treats "x,y = 1,2" as equivalent to "x=1; y=2",
> but I didn't know that Ruby first view them as "LHS-array assigned
> values from a RHS-array" (which is how I interpret your expressions,
> which look like Lisp to me).
>
> Ultimately, then, the website's first lines show me that splat on
> arrays is an Identity prefix operator, without mentioning that
> explicitly.  That's not very helpful, IMHO.  Don't you agree?
>
> Best wishes,
> Richard

A little more info ... this time, incorrect IMHO:

Splat array of arrays:
	a = [[:planes, 21], [:cars, 36]]
	h = Hash[*a]  # => {[:planes, 21]=>[:cars, 36]}
	Allegedly, h = Hash[*a]  # => { :planes=>21, :cars=>36}
Do you concur in my finding that the site's in error here?

So, I hunt for a more informative (and more accurate) presentation of
the virtues of splat.

Again, Best Wishes,
Richard