Handling Environment Variables Gracefully in Go

When writing Go applications, it’s common to configure runtime behavior using environment variables. But what happens when an environment variable isn’t set? Falling back to a default value is a typical pattern. The cmp package’s Or function, introduced in Go 1.22 provides a clean way to express this.

The Problem

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Admittedly, the word “problem” is perhaps a bit of an exaggeration. Anyways, a common pattern to retrieve a value from the environment with a fallback looks like this:

package main

import (
	"fmt"
	"os"
)

func main() {
	port := os.Getenv("APP_PORT")
	if port == "" {
		port = "8080"
	}
	fmt.Println(port)
}

This works, but it’s a bit verbose. If you’re doing this repeatedly across many config values, the boilerplate adds up.

A Cleaner Alternative: cmp.Or

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The cmp.Or function provides a neater, more expressive way to write this fallback logic:

package main

import (
	"cmp"
	"fmt"
	"os"
)

func main() {
	port := cmp.Or(os.Getenv("APP_PORT"), "8080")
	fmt.Println(port)
}

cmp.Or receives an arbitrary amount of arguments and returns the first non-zero (i.e. non-empty for strings) value it encounters. If APP_PORT is set, it uses that. If not, it falls back to "8080".

This means it could be used for checking multiple sources:

	port := cmp.Or(os.Getenv("APP_PORT"), os.Getenv("PORT"), "8080")

This pattern is especially useful when initializing configuration from multiple potential sources, e.g. environment → config file → hardcoded default.

Apparently, people use cmp.Or for more than configuration. Multi-field sorting is also an interesting application.