AWS StepFunctions: sum() function returns error about not finding path

Question

I have a StepFunction with input:

```json
{
  "Jobs": [0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0]
}
```

and I want the sum of the values in this array. According to the [Json-Path docs](https://github.com/json-path/JsonPath) there should exist a `.sum()` function for this. When I try it [here](http://jsonpath.herokuapp.com/) it even works. So I defined the following Pass state:

```json
"Sum Jobs": {
  "Type": "Pass",
  "Parameters": {
   "Jobs.$": "$.Jobs.sum()"
  }
},
```

Nevertheless executions fail with:

```
"An error occurred while executing the state 'Sum Jobs' (entered at the event id #249). The JSONPath '$.Jobs.sum()' specified for the field 'Jobs.$' could not be found in the input '{\"Jobs\":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0]}'"
```

Answer

AWS Step Functions does not support functions in the path reference. It uses JSONPath expressions to identify a node, however has the following constraints:
* You can access object fields using only dot (.) and square bracket ([ ]) notation.
* The operators @ .. , : ? * aren't supported.
* Functions such as length() aren't supported.

This Step Functions documentation that covers this can be found [here](https://docs.aws.amazon.com/step-functions/latest/dg/amazon-states-language-paths.html#amazon-states-language-reference-paths).

AWS StepFunctions: sum() function returns error about not finding path

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