Ruby 3 - Set Literal

Next in our series on new features in Ruby 3 we'll be looking at the new Set syntax recently discussed on the bug tracker

Quick Reference

What's it look like?

set = { 1, 2, 3 }
set.include?(3) # => true

Originally this might have looked like this:

set = Set[1, 2, 3]
set.include?(3) # => true


Now Set has some interesting usecases, so consider this a quick rundown of Set as well as an introduction of the new syntax.

It should be noted that Set inclusion is O(1) rather than O(n) for an Array. That means these examples can be pretty quick compared to searching an entire array.

What Can We Use It For?

Triple Equals - ===

=== is real fun, and for Set it's implemented as include? or member?. In the core docs it uses this example:

case :apple
when Set[:potato, :carrot] "vegetable"
when Set[:apple, :banana] "fruit"
# => "fruit"

With the new syntax we can change it to this:

case :apple
when { :potato, :carrot } "vegetable"
when { :apple, :banana } "fruit"
# => "fruit"

Remembering that some methods in Ruby now leverage === for matches:

text = "The rain in spain falls mainly on the plane"
words = text.split
fillers = Set['The', 'the', 'in', 'on']
fillers = { 'The', 'the', 'in', 'on' } new_words = words.grep_v(fillers)
# => ["rain", "spain", "falls", "mainly", "plane"] words.any?(fillers)
# => true words.slice_after(fillers).to_a
# => => [["The"], ["rain", "in"], ["spain", "falls", "mainly", "on"], ["the"], ["plane"]]

That means you get methods like any?, all?, none?, one?, grep, slice_before, slice_after, and grep_v to work with.

Granted I have opinions about find taking === patterns instead of an ifnone and select / reject / filter also using patterns, but that's a matter for another issue on the bug tracker like this one I submitted recently.

Duplicate Prevention

Set cannot have repeated elements, and that can be real useful for inline definition:

def example_method(a, b, c) { a, b, c }
end example_method(1, 1, 1)
# => { 1 }

I do wonder if one can splat inside these:

def example_method(*args) { *args }
end example_method(1, 1, 1, 1, 1)
# => { 1 }

...but that migth get confusing after a while, no? Especially with ambiguity with Hashes and keyword splatting:

a = { a: 1 }
b = { **a, b: 2 }

We'll get more into overloaded syntax issues in a bit though.

Compromises and Issues

New Syntax

I like expressive new syntax, but this does add another layer of concern as far as Hash vs Set by overloading some symbols. This could be the same as some of the class.[] confusion as well for cases like Hash[] and how those work.

That said, I like this syntax and think it in general makes sense. It's just a balancing act whenever you add new syntax to not break intuitiveness.

I do wonder what would happen if we had %s{ 1, 2, 3 } instead. While two more characters this does fit into the convention of %w, %i, and others.


Now the bad part about this syntax is that it overrides the potential for a very very powerful feature in Javascript called object punning:

const a = 1;
const b = { b: 1, c: 2 }; const c = { a, ...b }
// {a: 1, b: 1, c: 2}

I could see that being extremely useful for Ruby if it were to do something along the lines of this:

def move_north(x:, y:) { x, y + 1 }
end move_north(x: 1, y: 2)
# => { x: 1, y: 3 }

There's probably some way to shim that behavior in, sure, but with the new Set literal that appears to be unlikely. That said I don't disagree with the syntax either, and actually quite like it, so I'm a bit torn on this one.

Block Ambiguity

This could provide some fun with Block ambiguity as well. How so? Consider the trick of {} vs do ... end for blocks and what that does to parens:

describe 'something' do # This works
end describe 'something' { # This won't 

Most of these are already inherit issues with Hashes, sure, but will need to be watched out for in Set as well. I do wonder about one-arg blocks like any? and how they might respond:

There could be some ambiguity here, especially without parens, and exceptionally so like this:

Is that a block or a Set? I could see it being interpreted as a block very easily, which may create issues. It may be a fair compromise to just call this one an invalid case.

Final Thoughts?

I like it, I think it's succinct and it makes sense, I just have a few concerns over some of the above ambiguity issues as noted.

I can see some good uses for a few things I've used day-to-day as far as querying data and de-duping lists of items, but will probably need to write a Set tutorial later to refresh myself on what the rest of them might be and give it a bit more of a pragmatic edge.

Looking forward to what Ruby 3.0 brings, and I'll be writing on new features as I see them.

Notice one that I haven't? Feel free to DM me on Twitter @keystonelemur and I'll take a look into it.