Channel.empty().first().count().view()==1 (instead of 0 ??)

[lindenb]

Bug report

Hi,
unless I'm missing something, the following statement:

Channel.empty().first().count().view()

prints 1 while it should print 0 (?)

Environment

• Nextflow version: 25.10.0
• Java version: 17.0.7
• Operating system: Linux
• Bash version:n 4.4.20

[bentsherman]

It is because of how the first operator is implemented:

nextflow/modules/nextflow/src/main/groovy/nextflow/extension/OperatorImpl.groovy

Lines 398 to 410 in 480c55e

	DataflowWriteChannel first( DataflowReadChannel source ) {
	if( source instanceof DataflowExpression ) {
	def msg = "The operator `first` is useless when applied to a value channel which returns a single value by definition"
	def name = NF.lookupVariable(source)
	if( name )
	msg += " -- check channel `$name`"
	log.warn msg
	}
	def target = new DataflowVariable()
	source.whenBound { target.bind(it) }
	return target
	}

It should really probably throw an error if the source channel is empty, since a value channel should always have exactly one value by definition

To be honest I would just avoid the first operator altogether