How do I send a process output to multiple channels in Nextflow?

Question

I'm implementing a Nextflow workflow where each process can give multiple outputs that may be needed downstream on different processes.

process multiple_outputs {
   input:
   tuple id, input from previous_process

   output:
   tuple id, input_for_a, input_for_b, input_for_a_b into downstream
}

Nextflow documentation states that operator into can use multiple channels and promotes channel duplication as a pattern.

However none of these options seems to work inside the process primitive and multiple channel output is not documented:

• The pattern found on this answer (syntax error):

  output:
  tuple id, input_for_a, input_for_b, input_for_a_b into { downstream_a; downstream_b }
  // nor these variants:
  //
  // `into { downstream_a, downstream_b }`
  // `into downstream_a, downstream_b`
  // `into tuple a, b`
  

• Repeating the output into duplicated channels (one channel is empty):

  output:
  tuple id, input_for_a, input_for_a_b into downstream_a
  tuple id, input_for_b, input_for_a_b into downstream_b
  // runs, but cannot find the file in one of the channels
  

Which is the right way to use the output in multiple channels?

Answer 1

The output declaration should be:

output:
tuple id, input_for_a, input_for_b, input_for_a_b into(downstream_a, downstream_b)

In alternative, you may want to consider using DLS2 which does not have anymore the single-channel usage requirement. Read more about it here.

Answer 2

Answer b seems to work:

output:
tuple id, input_for_a, input_for_a_b into downstream_a
tuple id, input_for_b, input_for_a_b into downstream_b

It seems that it failed for non related reasons.