How to Access Source Code or Definitions of Inbuilt Functions in Mule 4 DataWeave?

I've primarily relied on custom modules and functions I've created for specific needs. However, I've also been utilizing a number of inbuilt functions provided by Mule in DataWeave, such as the upper function, which returns the provided string in uppercase characters, with the syntax upper(text: String): String.

how does Mule implement this? Or where could I find the source code of this function?

Solution

For see implementations of the DataWeave functions in Anypoint studio, use [CTRL] button and LMB click at the function name.

picture of Set Payload where to CTRL and LMB

For example, upper:

/**
* Returns the provided string in uppercase characters.
*
* === Parameters
*
* [%header, cols="1,3"]
* |===
* | Name   | Description
* | `text` | The string to convert to uppercase.
* |===
*
* === Example
*
* This example converts lowercase characters to uppercase.
*
* ==== Source
*
* [source,DataWeave,linenums]
* ----
* %dw 2.0
* output application/json
* ---
* { "name" : upper("mulesoft") }
* ----
*
* ==== Output
*
* [source,JSON,linenums]
* ----
* { "name": "MULESOFT" }
* ----
**/
fun upper(text: String): String = native("system::StringUpperFunctionValue")

You can see native("system::StringUpperFunctionValue"). The function native means that the implementation is in jar or class files. Specifically, system::StringUpperFunctionValue StringUpperFunctionValue.class is located in the maven package groupId org.mule.weave artifactId runtime.