# Form channel content using a function in the .map{} operator

**URL:** <https://community.seqera.io/t/form-channel-content-using-a-function-in-the-map-operator/1536>\
**Category:** Ask for help\
**Tags:** nextflow\
**Created:** [December 10, 2024, 6:18pm UTC](https://community.seqera.io/t/form-channel-content-using-a-function-in-the-map-operator/1536 "2024-12-10T18:18:45Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![brgew](https://dub1.discourse-cdn.com/flex013/user_avatar/community.seqera.io/brgew/32/647_2.png) [@brgew](https://community.seqera.io/u/brgew)\
**Post date:** [December 10, 2024, 6:18pm UTC](https://community.seqera.io/t/form-channel-content-using-a-function-in-the-map-operator/1536/1 "2024-12-10T18:18:45Z")

</div>

Hi,

I am working on a Nextflow DSL2 pipeline and I am unsuccessful in my efforts to use a function in a .map{} operator to form a new channel. I was able to do this in Nextflow DSL1 where I formed a list of tuples that I returned but I have been unable to get this to work in DSL2.

The current problem involves merging bam files. A python script forms a JSON file that has contents

```auto
{
  "bam_merge_list": [
    {
      "out_file": "133.019-001_001_000.merged.bam",
      "in_file_list": [
        "133.019-001_001_000.bam",
        "133.019-002_001_000.bam",
        "133.019-003_001_000.bam",
        "133.019-004_001_000.bam"
      ]
    },
    {
      "out_file": "133.020-001_001_000.merged.bam",
      "in_file_list": [
        "133.020-001_001_000.bam",
        "133.020-002_001_000.bam",
        "133.020-003_001_000.bam",
        "133.020-004_001_000.bam"
      ]
    }, ...

```

A channel with the input files is formed in the workflow and is passed to a function using the .map{} operator. The function reads the JSON file and makes a list of tuples where one of the tuple elements is a list of the input files and the second element is the out\_file name. (The function also prepends the path to the input bam files.) So the function returns a list that has the form

`[["133.019-001_001_000.merged.bam", ["/data/133.019-001_001_000.bam", "/data/133.019-002_001_000.bam", "/data/133.019-003_001_000.bam", "/data/133.019-004_001_000.bam"]], etc]`

I have tried various ways to use this as input to the downstream process without success. For example,

```auto
process xx {
  input:
  tuple val('out_file'), path('in_file_list')
...
}

```

Nothing that I’ve tried works for me. Incidentally, the number of input bam values will vary as well as the naming convention, which is the reason that I use the JSON file to set up the channel.

I am hoping that I am missing a magical incantation that’s required to make this work.

I appreciate your consideration and guidance.

Thank you.

Ever grateful,  
Brent

P.S. I stumbled on Nextflow training ‘Grouping and Splitting’. It looks like there may be a way to use the .splitJson() operator and, maybe, the .subMap() method although it’s not clear to me how I might use .subMap() in this case. I modified the python script to write absolute paths to the input bam files as a start.

P.P.S This closure appears to work

def closure01 = {  
item →   
meta = item.subMap(‘out\_file’, ‘in\_file\_list’)  
def out\_name = item[‘out\_file’]  
def in\_file\_list =   
for(in\_file in item[‘in\_file\_list’]) {  
in\_file\_list.add(file(in\_file))  
}  
[out\_name, in\_file\_list]  
}

so I believe that I can proceed.

Thank you.