Nextflow tutorial getting error "no such variable" - nextflow

Question

I'm trying to learn nextflow but it's not going very well. I started with the tutorial of this site: https://www.nextflow.io/docs/latest/getstarted.html (I'm the one who installed nextflow).
I copied this script :
#!/usr/bin/env nextflow
params.str = 'Hello world!'
process splitLetters {
output:
file 'chunk_*' into letters
"""
printf '${params.str}' | split -b 6 - chunk_
"""
}
process convertToUpper {
input:
file x from letters.flatten()
output:
stdout result
"""
cat $x | tr '[a-z]' '[A-Z]'
"""
}
result.view { it.trim() }
But when I run it (nextflow run tutorial.nf), in the terminal I have this :
N E X T F L O W ~ version 22.03.1-edge
Launching `tutorial.nf` [intergalactic_waddington] DSL2 - revision: be42f295f4
No such variable: result
-- Check script 'tutorial.nf' at line: 29 or see '.nextflow.log' file for more details
And in the log file I have this :
avr.-20 14:14:12.319 [main] DEBUG nextflow.cli.Launcher - $> nextflow run tutorial.nf
avr.-20 14:14:12.375 [main] INFO nextflow.cli.CmdRun - N E X T F L O W ~ version 22.03.1-edge
avr.-20 14:14:12.466 [main] INFO nextflow.cli.CmdRun - Launching `tutorial.nf` [intergalactic_waddington] DSL2 - revision: be42f295f4
avr.-20 14:14:12.481 [main] DEBUG nextflow.plugin.PluginsFacade - Setting up plugin manager > mode=prod; plugins-dir=/home/user/.nextflow/plugins; core-plugins: nf-amazon#1.6.0,nf-azure#0.13.0,nf-console#1.0.3,nf-ga4gh#1.0.3,nf-google#1.1.4,nf-sqldb#0.3.0,nf-tower#1.4.0
avr.-20 14:14:12.483 [main] DEBUG nextflow.plugin.PluginsFacade - Plugins default=[]
avr.-20 14:14:12.494 [main] INFO org.pf4j.DefaultPluginStatusProvider - Enabled plugins: []
avr.-20 14:14:12.495 [main] INFO org.pf4j.DefaultPluginStatusProvider - Disabled plugins: []
avr.-20 14:14:12.501 [main] INFO org.pf4j.DefaultPluginManager - PF4J version 3.4.1 in 'deployment' mode
avr.-20 14:14:12.515 [main] INFO org.pf4j.AbstractPluginManager - No plugins
avr.-20 14:14:12.571 [main] DEBUG nextflow.Session - Session uuid: 67344021-bff5-4131-9c07-e101756fb5ea
avr.-20 14:14:12.571 [main] DEBUG nextflow.Session - Run name: intergalactic_waddington
avr.-20 14:14:12.573 [main] DEBUG nextflow.Session - Executor pool size: 8
avr.-20 14:14:12.604 [main] DEBUG nextflow.cli.CmdRun -
Version: 22.03.1-edge build 5695
avr.-20 14:14:12.629 [main] DEBUG nextflow.Session - Work-dir: /home/user/Documents/formations/nextflow/testScript/work [ext2/ext3]
avr.-20 14:14:12.629 [main] DEBUG nextflow.Session - Script base path does not exist or is not a directory: /home/user/Documents/formations/nextflow/testScript/bin
avr.-20 14:14:12.637 [main] DEBUG nextflow.executor.ExecutorFactory - Extension executors providers=[]
avr.-20 14:14:12.648 [main] DEBUG nextflow.Session - Observer factory: DefaultObserverFactory
avr.-20 14:14:12.669 [main] DEBUG nextflow.cache.CacheFactory - Using Nextflow cache factory: nextflow.cache.DefaultCacheFactory
avr.-20 14:14:12.678 [main] DEBUG nextflow.util.CustomThreadPool - Creating default thread pool > poolSize: 9; maxThreads: 1000
avr.-20 14:14:12.741 [main] DEBUG nextflow.Session - Session start invoked
avr.-20 14:14:13.423 [main] DEBUG nextflow.script.ScriptRunner - > Launching execution
avr.-20 14:14:13.446 [main] DEBUG nextflow.Session - Session aborted -- Cause: No such property: result for class: Script_6634cd79
avr.-20 14:14:13.463 [main] ERROR nextflow.cli.Launcher - #unknown
groovy.lang.MissingPropertyException: No such property: result for class: Script_6634cd79
at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.unwrap(ScriptBytecodeAdapter.java:65)
at org.codehaus.groovy.runtime.callsite.PogoGetPropertySite.getProperty(PogoGetPropertySite.java:51)
at org.codehaus.groovy.runtime.callsite.AbstractCallSite.callGroovyObjectGetProperty(AbstractCallSite.java:341)
at Script_6634cd79.runScript(Script_6634cd79:29)
at nextflow.script.BaseScript.runDsl2(BaseScript.groovy:170)
at nextflow.script.BaseScript.run(BaseScript.groovy:203)
at nextflow.script.ScriptParser.runScript(ScriptParser.groovy:220)
at nextflow.script.ScriptRunner.run(ScriptRunner.groovy:212)
at nextflow.script.ScriptRunner.execute(ScriptRunner.groovy:120)
at nextflow.cli.CmdRun.run(CmdRun.groovy:334)
at nextflow.cli.Launcher.run(Launcher.groovy:480)
at nextflow.cli.Launcher.main(Launcher.groovy:639)
What should I do ?
Thanks a lot for your help.

Answer 1

Nextflow includes a new language extension called DSL2, which became the default syntax in version 22.03.0-edge. However, it's possible to override this behavior by either adding nextflow.enable.dsl=1 to the top of your script, or by setting the -dsl1 option when you run your script:
nextflow run tutorial.nf -dsl1
Alternatively, roll back to the latest release (as opposed to an 'edge' pre-release). It's possible to specify the Nextflow version using the NXF_VER environment variable:
NXF_VER=21.10.6 nextflow run tutorial.nf
I find DSL2 drastically simplifies the writing of complex workflows and would highly recommend getting started with it. Unfortunately, the documentation is lagging a bit, but lifting it over is relatively straight forward once you get the hang of things:
params.str = 'Hello world!'
process splitLetters {
output:
path 'chunk_*'
"""
printf '${params.str}' | split -b 6 - chunk_
"""
}
process convertToUpper {
input:
path x
output:
stdout
"""
cat $x | tr '[a-z]' '[A-Z]'
"""
}
workflow {
splitLetters | flatten() | convertToUpper | view()
}
Results:
nextflow run tutorial.nf -dsl2
N E X T F L O W ~ version 21.10.6
Launching `tutorial.nf` [prickly_kilby] - revision: 0c6f835b9c
executor > local (3)
[b8/84a1de] process > splitLetters [100%] 1 of 1 ✔
[86/fd19ea] process > convertToUpper (2) [100%] 2 of 2 ✔
HELLO
WORLD!