Ractor.receive_if

Receive only a specific message.

Instead of Ractor.receive, Ractor.receive_if can be given a pattern (or any filter) in a block and you can choose the messages to accept that are available in your ractor’s incoming queue.

r = Ractor.new do
  p Ractor.receive_if{|msg| msg.match?(/foo/)} #=> "foo3"
  p Ractor.receive_if{|msg| msg.match?(/bar/)} #=> "bar1"
  p Ractor.receive_if{|msg| msg.match?(/baz/)} #=> "baz2"
end
r << "bar1"
r << "baz2"
r << "foo3"
r.take

This will output:

foo3
bar1
baz2

If the block returns a truthy value, the message is removed from the incoming queue and returned. Otherwise, the message remains in the incoming queue and the next messages are checked by the given block.

If there are no messages left in the incoming queue, the method will block until new messages arrive.

If the block is escaped by break/return/exception/throw, the message is removed from the incoming queue as if a truthy value had been returned.

r = Ractor.new do
  val = Ractor.receive_if{|msg| msg.is_a?(Array)}
  puts "Received successfully: #{val}"
end

r.send(1)
r.send('test')
wait
puts "2 non-matching sent, nothing received"
r.send([1, 2, 3])
wait

Prints:

2 non-matching sent, nothing received
Received successfully: [1, 2, 3]

Note that you can not call receive/receive_if in the given block recursively. You should not do any tasks in the block other than message filtration.

Ractor.current << true
Ractor.receive_if{|msg| Ractor.receive}
#=> `receive': can not call receive/receive_if recursively (Ractor::Error)