Using multiMap to output null does not reproduce the behavior of using map to output null

[maxulysse]

Bug report

I feel like when I try to output null from a multiMap it's treated as null, and not as an empty channel as it is when I output null from a map.

Expected behavior and actual behavior

I would expect any channel being null to be an empty channel however it was created

Steps to reproduce the problem

https://github.com/maxulysse/nf-multimap-mre

Environment

• Nextflow version: 24.10.2
• Java version:

openjdk version "21.0.5" 2024-10-15
OpenJDK Runtime Environment (build 21.0.5+11-Ubuntu-1ubuntu122.04)
OpenJDK 64-Bit Server VM (build 21.0.5+11-Ubuntu-1ubuntu122.04, mixed mode, sharing)

• Operating system: Linux Mint
• Bash version: zsh 5.8.1 (x86_64-ubuntu-linux-gnu)

Additional context

(Add any other context about the problem here)

[bentsherman]

whoa, I had no idea that map filters out null values:

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

Lines 60 to 61 in 5c5a810

	if (result != Channel.VOID)
	proc.bindOutput(result)

nextflow/modules/nextflow/src/main/groovy/nextflow/Channel.groovy

Line 66 in 5c5a810

static public NullObject VOID = NullObject.getNullObject()

In my opinion, multiMap does the right thing and map should not do this, but it's useful to know that it does. Changing either one could break so many things that I don't think we can do anything but document the current behavior

[maxulysse]

whoa, I had no idea that map filters out null values:

Happy to help discovering behavior

In my opinion, multiMap does the right thing and map should not do this, but it's useful to know that it does. Changing either one could break so many things that I don't think we can do anything but document the current behavior

ok, for me the same behavior should happen in both.