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Ruby Case/In Patterns

Case/in tries a value against a series of shape patterns. Each in clause checks the shape of the value and can pull parts of it out.

An array pattern checks types in order

The pair holds a number then a word. The first pattern checks both positions in order and matches.

pair = [3, "cat"]
case pair
in [Integer, String]
  puts "number then word"
in [String, Integer]
  puts "word then number"
else
  puts "another shape"
end

A hash pattern pulls values out

The hash pattern names item and count inside the hash. The clause body then prints both pulled values.

order = {item: "pen", count: 4}
case order
in {item:, count:}
  puts item
  puts count
else
  puts "missing fields"
end

A failed pattern falls to the next in

Three pairs run through the same case one at a time. Each pair lands on the first clause its shape satisfies.

shapes = [[1, 2], ["a", "b"], [true, false]]
shapes.each do |shape|
  case shape
  in [Integer, Integer]
    puts "two numbers"
  in [String, String]
    puts "two words"
  else
    puts "other pair"
  end
end

A bare value matches by ==

The level is a plain number, so the clauses hold bare values. The clause whose value equals the level is the one that runs.

level = 3
case level
in 1
  puts "easy"
in 3
  puts "hard"
else
  puts "unknown"
end

Example

events = [[1, "click"], {kind: "hover", count: 2}, "reset"]
events.each do |event|
  case event
  in [Integer, String]
    puts "numbered event"
  in {kind: String, count: Integer}
    puts "counted event"
  in "reset"
    puts "reset event"
  end
end

Key points

  • An array pattern checks each position in order, including the class of each part.
  • A hash pattern pulls named values out of a hash so the clause can use them.
  • When a pattern fails, matching falls through to the next in clause.
  • An in clause with a bare value matches by ==, like a plain comparison.
  • A value that matches no clause and has no else raises an error.

In this example, each event meets the clauses in order and stops at the first shape it fits. The array event matches the pair pattern, the hash event matches the hash pattern, and the word matches the bare value by ==.

Note

The first matching clause wins, so clause order matters.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

In a case/in statement, what happens when a pattern fails to match?