How to Return a List Variable from a Helm Helper Function

Discover how to simplify your Helm templates by creating a helper function that outputs a list variable using Go templates.
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How to Return a List Variable from a Helm Helper Function

In Helm chart development, repetitive template code can be a source of frustration. Many developers seek to optimize their code to minimize duplication and enhance maintainability. One common scenario involves dealing with similar data structures defined in a values.yaml file, such as singular and list representations of the same object.

If you're grappling with this issue and looking to streamline your Helm templates, you've come to the right place. This guide will guide you through the process of creating a helper function that returns a list, enabling you to handle data representation seamlessly across your templates.

The Problem: Dealing with Single vs. List Fields

Consider a scenario where you have the following fields in your values.yaml:

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You want to simplify your templates so that they handle both schema (singular) and schemas (list) without repeating the same logic in every template file. With the goal of using a single list variable, you devised the following preliminary solution:

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However, you soon realized you needed a reusable helper function to avoid copy-pasting this code across multiple templates.

The Solution: Helper Function for List Variables

Understanding the Functionality

In the Go text/template language, helper functions defined with define always produce strings. Helm's include function, which wraps these helpers, returns only a string. Therefore, directly using a helper function to output a list will not work as expected.

Implementing the Helper Function

To work around this limitation, you can create a helper function to return a JSON string that can be parsed back into a list within your template. Below is the structure of your helper function:

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Using the Helper Function in Your Template

You can now invoke this helper function in your template like this:

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This code will take the JSON string output by the helper function, convert it back to a data structure, and assign it to $schemas, which can then be used for iteration.

Alternative Approach for Conciseness

If your use case is straightforward, you could simplify the process with a one-liner that leverages Helm functions without needing to define a full helper:

[[See Video to Reveal this Text or Code Snippet]]

This line achieves the following:

Packages the schema into a list.

Removes any falsey values (like null).

Falls back to using schemas if the resultant list is empty.

Conclusion

By using these techniques, you can significantly clean up your Helm templates and avoid redundancy, making your charts more readable and maintainable. While using both schema and schemas fields is possible, it's generally advisable to pick one distinctly typed field (preferably a list) for defining your structures to reduce complexity.

Take control of your Helm templates today, and enjoy the simplicity that a well-defined helper function can bring!

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