Parameter invalid error in Step Function while creating AWS Forecast Resources

Question

I am creating AWS Forecast Resources via Step Function and lambda, for which
I first used CreateDataset module in Step Function and then CreateDatasetGroup
The output of CreateDataset which is arn is passed to CreateDatasetGroup Input in this way-

{"DatasetArns.$": "$.CreateDatasetOutput.DatasetArn"}

but I am getting error-  
could not be used to start the Task: [Cannot construct instance of `java.util.ArrayList` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value

in the error I can see that the Arn is correctly read-
"DatasetArns":"arn:aws:forecast:ap-****-*:**********:dataset/*******"
but it is not running successfully

I am unable to pass the DatasetArn correctly to step function CreateDatasetGroup module.
Kindly help

Answer

It looks like the error you're seeing here is because you're trying to pass a single string to the DatasetArns field which (per the CreateDatasetGroup API) expects an array.

I believe you should be able to use the States.Array() intrinsic function to wrap your single dataset ARN into an array - something like:

{"DatasetArns.$": "States.Array($.CreateDatasetOutput.DatasetArn)"}

I believe you should be able to use the [States.Array() intrinsic function](https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-intrinsic-functions.html#asl-intrsc-func-arrays) to wrap your single dataset ARN into an array - something like:

```json
{"DatasetArns.$": "States.Array($.CreateDatasetOutput.DatasetArn)"}
```

Parameter invalid error in Step Function while creating AWS Forecast Resources

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