ab-abc-tap.nf

/* Insert process B beween A and C or leave it out, depending on an on/off switch.
      A -> B -> C 
      A -> C
   This is a variant on http://nextflow-io.github.io/patterns/index.html#_problem_19
   using `until` instead of Channel.empty().

   My prefered solution is ab-abc-when.nf
*/

params.includeB = true

process processA {        // Create a bunch of files, each with its ow sample ID.
  output: file('*.txt') into ch_dummy
  script: 'for i in {1..7}; do echo "sample_$i" > f$i.txt; done'
}
ch_dummy.flatMap().map { f -> [f.text.trim(), f] }.view()
  .tap { ch_AC }
  .until { !params.includeB }
  .set { ch_AB }

process processB {
  input:  set val(sampleid), file(thefile) from ch_AB
  output: set val(sampleid), file('out.txt') into ch_BC
  script: "(echo 'B process'; cat $thefile; md5sum $thefile) > out.txt"
}

ch_AC.until { params.includeB }.mix(ch_BC).set{ ch_C }

process processC {
  publishDir "results"
  input: set val(sampleid), file(a) from ch_C.view()
  output: file('*.out')
  script: "(echo 'C process'; cat $a) > ${sampleid}.out"
}