Where is splat implemented? / How does it work?

Ruby
1

Hi –

After a good amount of digging, I’m still not sure how [*[1,2]] ==
[1,2]. I understand what it does, but I want to know why.

I had originally assumed it was a normal method (like + or -), but it
appears to be implemented at a deeper level. It looks like rubinius does
something with cast_array. Ruby 1.8 does to_ary and ruby 1.9 does
to_splat. Where does this happen? to_ary says that it just returns self,
so how does self get exploded?

Short of reading eval.c, does anyone have any pointers?

Best,

Mischa

2

Hi –

Mischa F. wrote:

Short of reading eval.c, does anyone have any pointers?

It ties itself to to_a:

obj = Object.new
=> #Object:0xa43db54
def obj.to_a; [1,2,3]; end
=> nil
a = *obj
=> [1, 2, 3]

That code works the same in 1.8 and 1.9.1. There may be some classes
where it’s optimized away so that you can’t override it even by defining
to_a. I’m not sure.

David

–
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
Coming in 2009: The Well-Grounded Rubyist (The Well-Grounded Rubyist)

Ruby Training Atlanta! April 1-3, ruby training, atlanta, april 2009 - entp

3

David,

Thank you for your reply.

However, what I’m wondering is, where does it “tie” itself to
to_ary/to_a? Also, I think it is not the same thing as to_a, when used
like so:

[[1,2,3].to_a]
=> [[1, 2, 3]] # outer length = 1
vs
[*[1,2,3]]
=> [1, 2, 3] # length = 3

Or

str = “asdf”
def str.to_a
[“oops”]
end
=> nil

[*str]
=> [“oops”]

vs

[str.to_a]
=> [[“oops”]]

I’m really curious where / how this happens. If anyone can point me to
either rubinius or to mri, I would be very grateful.

M

David A. Black wrote:

Hi –

Mischa F. wrote:

Short of reading eval.c, does anyone have any pointers?

It ties itself to to_a:

obj = Object.new
=> #Object:0xa43db54
def obj.to_a; [1,2,3]; end
=> nil
a = *obj
=> [1, 2, 3]

That code works the same in 1.8 and 1.9.1. There may be some classes
where it’s optimized away so that you can’t override it even by defining
to_a. I’m not sure.

David

–
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
Coming in 2009: The Well-Grounded Rubyist (The Well-Grounded Rubyist)

Ruby Training Atlanta! April 1-3, ruby training, atlanta, april 2009 - entp

4

On Wed, Feb 25, 2009 at 9:37 AM, David A. Black [email protected]
wrote:

appears to be implemented at a deeper level. It looks like rubinius does
=> #Object:0xa43db54

def obj.to_a; [1,2,3]; end
=> nil
a = *obj
=> [1, 2, 3]

That code works the same in 1.8 and 1.9.1. There may be some classes where
it’s optimized away so that you can’t override it even by defining to_a. I’m
not sure.

I’m pretty sure that implicit conversions to array use to_ary, not to_a.
This is the way it works in Ruby 1.8, and after an experimental period
in
1.9 with to_splat, it appears to have gone back to using to_ary.
Grepping
the Ruby 1.9.1 source code for to_splat shows no hits.

To answer Mischa question about how it “ties itself,” this is done
internally by the Ruby interpreter/VM when it needs to implicitly
convert
the value of a *expression when it appears either as an actual parameter
to
a method call, or on the right hand side of an assignment.

So it can’t be explained completely at the ruby source code level, it’s
part
of the semantics of the language.

–
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

5

Hi –

Rick DeNatale wrote:

obj = Object.new

I’m pretty sure that implicit conversions to array use to_ary, not to_a.
This is the way it works in Ruby 1.8, and after an experimental period in
1.9 with to_splat, it appears to have gone back to using to_ary. Grepping
the Ruby 1.9.1 source code for to_splat shows no hits.

to_ary doesn’t seem to be involved in 1.9.1. If you swap to_a for to_ary
in my example it comes out the same in 1.8.6, but in 1.9.1 it doesn’t:

obj = Object.new
=> #Object:0x93413c8
def obj.to_ary; [1,2,3]; end
=> nil
a = *obj
=> [#Object:0x93413c8]

David

–
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
Coming in 2009: The Well-Grounded Rubyist (The Well-Grounded Rubyist)

Ruby Training Atlanta! April 1-3, ruby training, atlanta, april 2009 - entp

6

[*str]
=> [“oops”]

vs

[str.to_a]
=> [[“oops”]]

I’m really curious where / how this happens. If anyone can point me to
either rubinius or to mri, I would be very grateful.

If you do grep -i splat *.c, you’ll find hits in parse.c and compile.c
(for ruby 1.9.1).

I’d assume it’s a mostly parser thing. That to_a comes into play
probably it because it makes sure it’s an array it is unsplicing –
just an assumption.

7

I’m pretty sure that implicit conversions to array use to_ary, not to_a.
This is the way it works in Ruby 1.8, and after an experimental period in
1.9 with to_splat

Building on David Black’s examples, I tried the following to clarify
the use of #to_a and #to_ary for me.

RUBY_VERSION
=> “1.9.1”
o = Object.new
=> #Object:0x100af964
class << o
def to_a
[:a]
end
def to_ary
[:ary]
end
end
[*o]
=> [:a]
[1] + o
=> [1, :ary]

The #to_splat method seems to be gone.

8

2009/2/26 [email protected] [email protected]:

=> #Object:0x100af964

[1] + o
=> [1, :ary]

The #to_splat method seems to be gone.

Here’s another test set

09:31:50 OPSC_Gold_bas_dev_R2.0_foundation$ ruby /tmp/o.rb
“1.8.7”
1
2
3
“--------------”
#Object:0x1002dfcc
nil
nil
“--------------”
5
6
7
“--------------”
5
6
7
“==============”
1
2
3
“--------------”
error
error
5
6
7
“--------------”
09:31:55 OPSC_Gold_bas_dev_R2.0_foundation$ ruby19 /tmp/o.rb
“1.9.1”
1
2
3
“--------------”
#Object:0x1001a3dc
nil
nil
“--------------”
#Object:0x1001a378
nil
nil
“--------------”
5
6
7
“==============”
1
2
3
“--------------”
error
error
error
09:31:57 OPSC_Gold_bas_dev_R2.0_foundation$ cat /tmp/o.rb

p RUBY_VERSION

def t(a,b,c)
p a,b,c, ‘--------------’
end

o = Object.new

def o.to_a
[1,2,3]
end

q = Object.new

def q.to_ary
[5,6,7]
end

a, b, c = *o
p a,b,c, ‘--------------’

a, b, c = o
p a,b,c, ‘--------------’

a, b, c = *q
p a,b,c, ‘--------------’

a, b, c = q
p a,b,c, ‘==============’

t(*o) rescue puts “error”
t(o) rescue puts “error”

t(q) rescue puts “error”
t(*q) rescue puts “error”
09:32:04 OPSC_Gold_bas_dev_R2.0_foundation$

Cheers

robert

9

Thanks for all the replies.

I’ve taken a lot of the stuff here, plus all the examples I could find
for the splat, and written what I think is probably the most definitive
guide at this point. You can run it yourself, as well, which should be
useful.

Some of the examples are from a post David Black made a year or so ago,
others are from Rick and matz.

http://github.com/mischa/splat/tree/master

I’ve also written it so that you can try to guess what the return value
will be.

Please do let me know if anything is wrong with it, also feel free to
fork and add your own examples / tests.

Best,

M

Robert K. wrote:

2009/2/26 [email protected] [email protected]:

=> #Object:0x100af964

[1] + o
=> [1, :ary]

The #to_splat method seems to be gone.

Here’s another test set

09:31:50 OPSC_Gold_bas_dev_R2.0_foundation$ ruby /tmp/o.rb
“1.8.7”
1
2
3
“--------------”
#Object:0x1002dfcc
nil
nil
“--------------”
5
6
7
“--------------”
5
6
7
“==============”
1
2
3
“--------------”
error
error
5
6
7
“--------------”
09:31:55 OPSC_Gold_bas_dev_R2.0_foundation$ ruby19 /tmp/o.rb
“1.9.1”
1
2
3
“--------------”
#Object:0x1001a3dc
nil
nil
“--------------”
#Object:0x1001a378
nil
nil
“--------------”
5
6
7
“==============”
1
2
3
“--------------”
error
error
error
09:31:57 OPSC_Gold_bas_dev_R2.0_foundation$ cat /tmp/o.rb

p RUBY_VERSION

def t(a,b,c)
p a,b,c, ‘--------------’
end

o = Object.new

def o.to_a
[1,2,3]
end

q = Object.new

def q.to_ary
[5,6,7]
end

a, b, c = *o
p a,b,c, ‘--------------’

a, b, c = o
p a,b,c, ‘--------------’

a, b, c = *q
p a,b,c, ‘--------------’

a, b, c = q
p a,b,c, ‘==============’

t(*o) rescue puts “error”
t(o) rescue puts “error”

t(q) rescue puts “error”
t(*q) rescue puts “error”
09:32:04 OPSC_Gold_bas_dev_R2.0_foundation$

Cheers

robert

10

I think this means that my “that which, when wrapped in an array”
formula doesn’t work for this case any more – though that formula was
my best shot at a pithy way of describing the star, and may be outdated
and/or incomplete in other ways. (I can’t think of any, but someone else
might :slight_smile:

That’s why I’ve always called this the “unary unarray” operator.

Thanks, code updated accordingly.

11

Hi –

Mischa F. wrote:

http://github.com/mischa/splat/tree/master

I’ve also written it so that you can try to guess what the return value
will be.

Please do let me know if anything is wrong with it, also feel free to
fork and add your own examples / tests.

Note this change, between 1.8 and 1.9:

1.8:

*a = [1,2,3]
=> [[1, 2, 3]]
a
=> [[1, 2, 3]]

1.9:

*a = [1,2,3]
=> [1, 2, 3]
a
=> [1, 2, 3]

I think this means that my “that which, when wrapped in an array”
formula doesn’t work for this case any more – though that formula was
my best shot at a pithy way of describing the star, and may be outdated
and/or incomplete in other ways. (I can’t think of any, but someone else
might :slight_smile:

That’s why I’ve always called this the “unary unarray” operator.

David

–
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.rubypal.com
Coming in 2009: The Well-Grounded Rubyist (The Well-Grounded Rubyist)

Ruby Training Atlanta! April 1-3, ruby training, atlanta, april 2009 - entp