Assignments In-Style

The introduction of pattern matching in Ruby 2.7 brought us a new style of multi-assigning local variables: The pattern assignment, or, how you could also call it, the assignment in-style.

After you have deactivated the warnings for experimental features, try the following piece of code:

[1, 2, 3, 4] in [first, second, *other]

Think: Put [1, 2, 3, 4] into [first, second, *other]

first # => 1
second # => 2
other # => [3,4]

This is a great addition to the previous two and a half ways of assigning local variables. Let's see some more examples, starting with the most basic one:

0 in a # => nil
a # => 0

When assigning multiple values, be sure to supply exactly the number of expected values:

cities = %w[Berlin Potsdam Magdeburg]
cities in [a,b] # NoMatchingPatternError
cities in [x,y,z]
x #=> Berlin
y #=> Potsdam
z #=> Magdeburg

If your array is of unknown length, use an asterisk, like in the introductory example.

Pattern assignments also have type checks built in:

cities = %w[Berlin Potsdam Magdeburg]
cities in [Integer => c_1, *c_other] # NoMatchingPatternError
cities in [String => c_1, *c_other]
c_other # => ["Potsdam", "Magdeburg"]

Which, of course, also work for single assignments:

0 in Float => a # NoMatchingPatternError
0 in Integer => a

Since hashes are also supported, we can finally have JavaScript-like object destructuring┬╣:

  verb: "CREATE",
  endpoint: "/syntax",
  authed: true,
} in { verb:, endpoint: }

verb #=> "CREATE"
endpoint #=> "/syntax"

┬╣ Differently from arrays, there is no need to specify all hash keys of the given hash object

Also See

More Idiosyncratic Ruby