why I got Cannot invoke method view() on null object

Question

why I got Cannot invoke method view() on null object,,, my code is like:

workflow parsefq {
  take: samplesheet
  main:
  Channel.fromPath( samplesheet )
         .splitCsv ( header:true, sep:'\t' ) // dict: [sample:test1, fqdir:xxx, barcode: 10, 11]
         .map { create_fastq_channel(it) }   //[test1, [fq1, fq2]]
         .set {reads}
  emit: reads
}
def create_fastq_channel(LinkedHashMap row) {...return [row.sample, fq1s, fq2s]}
workflow {
  ch_input = file(params.input)
  parsefq (ch_input)
         .set {ch_fq}
         .view()      //got error on this
}

Answer 1

You get the error above because the set operator just returns null. It is intended only to assign a variable name to a channel. Usually we use the set operator at the end of a chain of channel operations. If we don't need to apply any transformations to the output, we can just assign a channel name using the = assignment operator:

workflow {
    ch_input = file(params.input)
    ch_fq = parsefq(ch_input)

    ch_fq.view()
}