GCP Workflows: append to array

Question

Using GCP / Google Workflows I'd like to take an array and apply a transformation to each one calling http endpoints. To do this, I'm looking for a way to apply the transformation and then re-assemble the results into another array. One way to do this is experimental.executions.map, but it's an experimental feature that's subject to change, so I'm a little hesitant to use that. Another way to do this is to start with an empty array and append to that array as the transformation is applied. Is there a way to do this? Concatenating a string seems to work, but there doesn't seem to be a built-in away to append to an array. I tried something like this, which does execute successfully, but doesn't yield the expected result:

main:
  steps:
    - assignVariables:
        assign:
            - sourceArray: ["one", "two", "three"]
            - hashedArray: []
            - hashedString: ""
            - i: 0
    - checkCondition:
        switch:
            - condition: ${i < len(sourceArray)}
              next: iterate
        next: returnResult
    - iterate:
        assign:
            - hashedArray[i]: ${"#" + sourceArray[i]}
            - hashedString: ${hashedString + "#" + sourceArray[i] + " "}
            - i: ${i + 1}
        next: checkCondition
    - returnResult:
        return:
            - hashedString: ${hashedString}
            - hashedArray: ${hashedArray}

Actual result:

[
  {
    "hashedString": "#one #two #three "
  },
  {
    "hashedArray": []
  }
]

Expected result:

[
  {
    "hashedString": "#one #two #three "
  },
  {
    "hashedArray": ["#one", "#two", "#three"]
  }
]

Answer 1

Appending items to lists is now supported using list.concat:

assign:
    - listVar: ${list.concat(listVar, "new item")}

Answer 2

For those who are wondering how to solve this problem using the experimental experimental.executions.map feature, you can do it like this:

First create a callable workflow that will transform the array. I gave this workflow a name of hash-item. This name is important, as we'll refer to it as workflow_id later:

main:
    params: [item]
    steps:
    - transform:
        assign:
            - hashed: ${"#" + item}
    - returnResult:
        return: ${hashed}

Then create your main workflow, which calls hash-item using experimental.executions.map:

main:
  steps:
    - assignVariables:
        assign:
            - sourceArray: ["one", "two", "three"]
    - transform:
        call: experimental.executions.map
        args:
            workflow_id: hash-item
            arguments: ${sourceArray}
        result: result
    - returnResult:
        return: ${result}

The service account used to execute the main workflow will need the workflows.executions.create permission (usually via roles/workflows.invoker role). To assign this, you can do this with the Cloud SDK CLI:

gcloud projects add-iam-policy-binding <project-id> --member="serviceAccount:<service-account-email>" --role="roles/workflows.invoker"

Executing the main workflow will output the desired result:

[
  "#one",
  "#two",
  "#three"
]

Note: experimental.executions.map is experimental, so subject to change.