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Ruby Pattern Guards

A pattern can check shape but not size or range. A guard adds an if or unless check to an in clause for exactly those tests.

An if guard adds a size check

The reading matches the pair shape on both clauses. The guard sends high readings to the first clause and the rest below.

reading = [25, "warm"]
case reading
in [Integer, String] if reading.first >= 20
  puts "high reading"
in [Integer, String]
  puts "low reading"
end

An unless guard flips the check

The unless guard passes when its condition is false. The small pair takes the first clause and the plain clause stays as backup.

pair = [3, "cat"]
case pair
in [Integer, String] unless pair.first > 10
  puts "small number with a word"
in [Integer, String]
  puts "big number with a word"
end

A failed guard falls through

Both pairs match the guarded shape, but only the second passes its guard. The first pair lands on the unguarded clause below.

pairs = [[4, "ant"], [30, "bear"]]
pairs.each do |pair|
  case pair
  in [Integer, String] if pair.first > 10
    puts "big pair"
  in [Integer, String]
    puts "small pair"
  end
end

A guard reads a bound name

The hash pattern binds count before the guard runs. The guard then compares that bound count against ten.

order = {item: "pen", count: 12}
case order
in {count:} if count > 10
  puts "bulk order"
in {count:}
  puts "small order"
end

Example

entries = [[15, "Ana"], [3, "Bo"], [15, "Cy"]]
entries.each do |entry|
  case entry
  in [score, name] if score > 10
    puts "#{name} passed well"
  in [score, name] unless score > 3
    puts "#{name} needs practice"
  in [score, name]
    puts "#{name} just passed"
  end
end

Key points

  • A guard adds an if or unless check after the pattern in the same clause.
  • The pattern must match before the guard is checked at all.
  • When a guard fails, matching falls through to the next in clause.
  • A guard can read names bound by its own pattern.

In this example, both guards read the score bound by their own pattern. The high scores pass the if guard, the low score passes the unless guard, and the plain clause waits for anything in between.

Note

Guards carry the checks a pattern shape cannot express, such as size comparisons.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

A pattern matches but its guard fails. What happens next?