The Strange Ruby Splat

As of ruby 1.9, you can do some pretty odd things with array destructuring and splatting. Putting the star before an object invokes the splat operator, which has a variety of effects. First we’ll start with some very useful examples, then we will poke around the dark corners of ruby’s arrays and the splat operator.

Method Definitions

You can use a splat in a method definition to gather up any remaining arguments:

def say(what, *people)
  people.each{|person| puts "#{person}: #{what}"}

say "Hello!", "Alice", "Bob", "Carl"
# Alice: Hello!
# Bob: Hello!
# Carl: Hello!

In the example above, what will get the first argument, then *people will capture however many other arguments you pass into say. A real world example of this can be found in the definition of Delegator#method_missing. A common ruby idiom is to pass a hash in as the last argument to a method. Rails defines an array helper Array#extract_options! to make this idiom easier to handle with variable argument methods, but you can actually get a similar behavior using a splat at the beginning of the argument list:

def arguments_and_opts(*args, opts)
  puts "arguments: #{args} options: #{opts}"

arguments_and_opts 1,2,3, :a=>5
# arguments: [1, 2, 3] options: {:a=>5}

Now this example only works if you are guaranteed to pass in a hash at the end, but it illustrates that the splat does not need to always come at the end of a method’s parameters. There are also some other odd uses for the splat in method definitions, for instance this is valid:

def print_pair(a,b,*)
  puts "#{a} and #{b}"

print_pair 1,2,3,:cake,7
# 1 and 2

Outside of letting you mimic javascript calling conventions, I’m not sure what the practical use is:

function print_pair(a,b){ 
  console.log(a + " and " + b);

print_pair(1,2,3, "cake", 7);
//=> 1 and 2

Calling Methods

The splat can also be used when calling a method, not just when defining one. If you wanted to use the say method from above, but you have your list of people in an array, the splat can help you out:

people = ["Rudy", "Sarah", "Thomas"]
say "Howdy!", *people
# Rudy: Howdy!
# Sarah: Howdy!
# Thomas: Howdy!

In this case, the splat converted the array into method arguments. It doesn’t have to be used with methods that take a variable number of arguments though, you can use it in all kinds of other creative ways:

def add(a,b)
  a + b

pair = [3,7]
add *pair
# 10

Array Destructuring

First of all, lets quickly cover a few things you can do without splatting:

a,b = 1,2               # Assign 2 values at once
a,b = b,a               # Assign values in parallel
puts "#{a} and #{b}"
# 2 and 1

With the above samples in mind, let’s try some fancier stuff. You can use splats with multiple assignment to extract various elements from a list:

first, *list = [1,2,3,4]          # first= 1, list= [2,3,4]
*list, last  = [1,2,3,4]          # list= [1,2,3], last= 4
first, *center, last = [1,2,3,4]  # first= 1, center= [2,3], last=4

# Unquote a String (don't do this)
_, *unquoted, _ = '"quoted"'.split(//)
puts unquoted.join
# quoted

Array Coercion

If for some reason the previous examples seemed like great ideas to you, you’ll be thrilled to know that the splat can also be used to coerce values into arrays:

a = *"Hello"  #=> ["Hello"]
"Hello".to_a  #=> NoMethodError: undefined method `to_a' for "Hello":String
a = *(1..3)   #=> [1, 2, 3]
a = *[1,2,3]  #=> [1, 2, 3]

This can be a nice way to make sure that a value is always an array, especially since it will handle objects that do not implement to_a.

The splat is a wily beast popping up in odd corners of ruby. I rarely actually use it outside of in method definitions and method calls, but it’s interesting to know that it is available. Have you found any useful idioms that make use of the splat? I would love to hear about them.

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