Cryptic Issue with params assignment in maps keys

[VasLem]

Bug report

This bug took me so long to identify, and it caused my pipeline to infinitely freeze. Here is a minimum example:

params.reference = 'hg38'
params.flank = ""
params.test = 'test'

workflow{
    def x = "ensembl.annot.genes.flanked${params.flank}"
    def test = params.test
    def ids = ['ensembl.annot.genes',x, test]
    reference = channel.value(params.reference)
    files = reference.map { ref -> ids.collect { id -> [id, "${ref}.${id}.bed"] }.collectEntries() }
    files.map { it-> {
        println "trying to get ${x} from ${it}: ${it[x]}"}}
    files.map { it-> {
        println "trying to get ${test} from ${it}: ${it[test]}"}}
        
}

Expected behavior and actual behavior

Expected:

trying to get ensembl.annot.genes.flanked from [ensembl.annot.genes:hg38.ensembl.annot.genes.bed, ensembl.annot.genes.flanked:hg38.ensembl.annot.genes.flanked.bed, test:hg38.test.bed]: hg38.ensembl.annot.genes.flanked.bed
trying to get test from [ensembl.annot.genes:hg38.ensembl.annot.genes.bed, ensembl.annot.genes.flanked:hg38.ensembl.annot.genes.flanked.bed, test:hg38.test.bed]: hg38.test.bed

Actual:

trying to get ensembl.annot.genes.flanked from [ensembl.annot.genes:hg38.ensembl.annot.genes.bed, ensembl.annot.genes.flanked:hg38.ensembl.annot.genes.flanked.bed, test:hg38.test.bed]: null
trying to get test from [ensembl.annot.genes:hg38.ensembl.annot.genes.bed, ensembl.annot.genes.flanked:hg38.ensembl.annot.genes.flanked.bed, test:hg38.test.bed]: hg38.test.bed

The reason why my pipeline froze is that there is another bug, when merging with a null channel, that makes it infinitely wait. It is enough to add this line after the workflow above to reproduce this infinite wait:

out = files.map{it->it[x]}.merge(channel.value('hi'))

Environment

• Nextflow version: 25.04.6

[bentsherman]

With some testing I found that explicitly converting this line to a string solved the issue:

    def x = "ensembl.annot.genes.flanked${params.flank}" as String

I think the problem is that dynamic strings are sometimes lazily evaluated, which causes them to not be "equal" to the equivalent string, e.g. when accessing a map key.

Weirdly, even the strict syntax (NXF_SYNTAX_PARSER=v2) did not solve this problem, I still had to cast to a string. I might be able to solve this in the strict syntax one day, by injecting the as String at compile-time.