Feature #5735: Extending the use of splat operator to when it is inside a hash

• Status: Closed
• Assignee: matz (Yukihiro Matsumoto)
• Target version: -

[sawa (Tsuyoshi Sawada)]

Ruby convention of allowing omittion of the curly brackets for the last argument is convenient:

foo(arg1, arg2, 1 => :a, 2 => :b)

Sometimes, I want to pass a hash with some modifications. For example, suppose h = {3 => :c, 4 => :d, 5 => :e} is a hash already defined and that I want to add some key-value pairs as well as overwrite some values of h, such as {3 => :c, 4 => :f, 5 => :e, 6 => :g}, and pass that. The current convention only allows:

foo(arg1, arg2, h.merge(4 => :f, 6 => :g))

but it would be more convenient if a hash preceded by the splat operator is placed in a hash, it is interpreted as part of the hash, allowing notations like:

foo(arg1, arg2, *h, 4 => :f, 6 => :g)

or, if I want to overwrite the hash {4 => :f, 6 => :g} with h, then:

foo(arg1, arg2, 4 => :f, 6 => :g, *h)

Or besides the argument position, in general, usages like the following:

{3 => :h, *h, 4 => :f, 6 => :g}

This is an analogy from the splat operator used within an array:

[1, 2, *[4, 5, 6], 7, 8]

[kernigh (George Koehler)]

=begin
Ruby trunk has a hash splat operator because of feature #5474, keyword argument. See ((URL:http://bugs.ruby-lang.org/issues/5474)).

With this version of Ruby,

ruby 2.0.0dev (2012-01-04 trunk 34211) [x86_64-openbsd5.0]

I can run this code,

def eat(i: 1, **hash)
printf "i is %p, hash is %p\n", i, hash
end
eat(h: 7, i: 8, j: 9)

=> "i is 8, hash is {:h=>7, :j=>9}"

So far, **hash only works when defining a method. We cannot (yet) use **hash in a method call or a hash literal.

args = {h: 2, i: 3, j: 4}
eat(**args) # syntax error!
args = {k: 5, **args} # syntax error!

There is much more discussion of hash splat at feature #5474.
=end

[mame (Yusuke Endoh)]

• Status changed from Open to Feedback
• Assignee set to matz (Yukihiro Matsumoto)

Marc-Andre proposed a similar idea at [ruby-core:41772], and Matz commented a little at [ruby-core:41822].
It would be good to investigate how many use cases occur in real world.

matz wrote:

I am still not sure if we need hash splat nor hash decomposition, it
might be useful in some cases, but also makes syntax more complex.
So we need more discussion before picking it.

But if we could made consensus I'd make small changes to proposed
syntax.

For hash splat, I'd rather use ** instead of *, because splat in array
and splat in hash are different. Using same operator could cause
confusion. Besides that, I would also restrict hash splat position to
the end, since hashes do not have order.

--
Yusuke Endoh [e-mail]

[mame (Yusuke Endoh)]

• Target version set to 2.6

[naruse (Yui NARUSE)]

• Target version deleted (2.6)

[sawa (Tsuyoshi Sawada)]

A similar feature has already been realized as the double splat operator. Please close this issue.

[matz (Yukihiro Matsumoto)]

• Status changed from Feedback to Closed