Leveraging Rexx Stem Variables for Dynamic Data Mapping

The Problem: Managing Unstructured Data without Boilerplate

When writing scripts to parse log files, handle API responses, or map configuration settings, developers often face a choice: spend time defining complex classes and data structures, or use a loose collection of variables that becomes impossible to track. In many languages, creating a dynamic map requires importing a specific library or initializing a hash table with a defined type.

The takeaway is that Rexx solves this through stem variables. By treating variables as dynamic strings and using a specific naming convention, you can create associative arrays (pseudo-arrays) on the fly without any prior declaration or memory allocation.

How Stem Variables Work

A stem variable is not a distinct data type in the traditional sense. Instead, it is a naming convention where a base name (the stem) is followed by a dot and a suffix. For example, if user. is your stem, then user.1, user.name, and user.email are all treated as related entries in a dynamic collection.

Because Rexx is string-centric, the suffix can be anything—an integer for a list or a string for a key-value pair. This allows a single script to pivot from a sequential list to a named map without changing the underlying logic.

Efficient String Decomposition with PARSE

Stem variables become powerful when paired with the PARSE instruction. While other languages rely on complex regular expressions for string splitting, Rexx uses pattern matching to assign values directly into stems.

The PARSE VALUE instruction allows you to break a delimited string into variables. When combined with a loop, you can populate a stem variable from a raw data stream with very few lines of code.

Worked Example: Parsing a CSV-style Configuration

The following example demonstrates how to take a raw string of configuration pairs and map them into a stem variable for easy retrieval. This script assumes a Rexx interpreter like ooRexx or Regina is installed.

/* Configuration Parser Example */
raw_data = "timeout=30,retry=3,env=production"
config = ""

/* Split the string by commas */
do while raw_data =
    parse var raw_data pair ',' raw_data
    /* Split the pair by the equals sign */
    parse var pair key '=' value
    /* Assign to the 'config.' stem */
    config.key = value
end

/* Accessing the data */
say "The environment is: " config.env
say "The timeout is: " config.timeout

Execution Details

• Where to run: Any standard Rexx interpreter (e.g., rexx or regina command).
• Permissions: Standard user execution permissions.
• Expected Result: The output should display "The environment is: production" and "The timeout is: 30".
• Risk: If the input string contains an equals sign within the value itself, the PARSE instruction will split at the first occurrence, potentially truncating the value.

Trade-offs and Limitations

The flexibility of stem variables comes with a specific cost: lack of intrinsic metadata. Unlike a Python dictionary or a Java HashMap, a Rexx stem does not have a built-in .length or .size() property.

To track how many elements are in a stem, you must manually maintain a counter variable (e.g., config.count) or iterate through the variables using the SYMBOLS('R') function to find all variables starting with the stem name. This increases the debugging burden in large codebases where variables might be overwritten accidentally due to the lack of strong typing.

Verifying Your Implementation

To verify that your stem is behaving as expected, use the SAY SYMBOLS('R') command. This returns a list of all currently defined variables in the environment. If your stem is working, you should see all your stem.suffix variables listed in the output. If the list is empty or the suffixes are missing, check that your PARSE logic is correctly identifying the delimiters.