guarantee_tuple_structure.md

Guarantee tuple structure in channel

It's reasonably common to have a multi-item element in your channel. You could for example have a channel where every element consists of a string id and a list of other strings, or paths, or anything like that. Let's work with strings to keep it simple. The code to create such a channel would be like:

Channel
  .of(['Marcel', ['apple', 'grapes']],
      ['Allen', ['strawberry', 'orange']],
      ['John', 'watermelon'])
  .set { my_channel }

Checking the contents of this channel with my_channel.view() would show you the following:

N E X T F L O W  ~  version 23.03.0-edge
Launching `example1.nf` [magical_lovelace] DSL2 - revision: a95111e547
[Marcel, [apple, grapes]]
[Allen, [strawberry, orange]]
[John, watermelon]

As you can see, the second item in the third element of our channel, watermelon is a string, not a list with a string or a list with strings. Let's say that you actually need all second item of your tuples to be a list, so that the code in your process will work. In this case, before passing this channel to the process we would use the map channel operator to guarantee the desired structure. The snippet below will do that:

my_channel
  .map { name, fruits ->
    if (fruits !instanceof ArrayList) {
      [name, [fruits]]
    } else {
      [name, fruits]
      }
  }
  .set { my_channel2 }

You can get the instance type with getClass(), similarly to what is shown in this snippet. In this snippet, we checked for array lists as a condition to change the structure, but you could check if it's a file, a string, and so on 😃

With that done, you can view the content of the final channel with my_channel2.view(). The output should be similar to the one below:

N E X T F L O W  ~  version 23.03.0-edge
Launching `example2.nf` [condescending_woese] DSL2 - revision: 28ec5dfd7e
[Marcel, [apple, grapes]]
[Allen, [strawberry, orange]]
[John, [watermelon]]