How to: Global Constant Maps and Slices in Go

For the most part, Go developers are pretty good about using constants for global configuration, rather than global variables. A problem arises however when we want a global constant slice or map. The go compiler doesn’t allow these more complex types to be set as constant. Many developers, upon making this realization, decide to then use a dangerous global variable. In this article we will explore a better option.

A Brief Refresher on Globals and Constants

package foo

// this is a global constant
const safeRateLimit = 10

// this is a global variable
var dangerousRateLimit = 10

When setting configuration globals, which should be read-only, there is no reason to use a global variable. By using a variable instead of a constant you:

• Open up the potential for bugs when you or someone else accidentally mutates the value
• Confuse future developers who assume the value is supposed to change

Most people already know this about global variables thankfully, and switching global variables to global constants is a fairly straightforward task.

What If I Want A Global Slice or Map?

Let’s assume the following situation:

We have a program that needs two sets of configurations. The configurations are:

• A list of supported social media networks
• A rate limit for making API calls to the networks (we assume they all have the same rate limit)

Now that we know a bit about the configs we make the following decisions:

• Because these configurations will not change based on the environment the program is running in, we elect to set the values in code rather than use environment variables
• Because they are needed in many places in the app, we choose to scope them globally , instead of passing them into 20+ functions
• Because they should not change during the execution of the program, we decide to make them constant

We then write the following code:

package main

const rateLimit = 10

const supportedNetworks = []string{"facebook", "twitter", "instagram"}

Much to our surprise, when we try to compile this code we get the following error:

const initializer []string literal is not a constant

Go doesn’t allow complex types like slices and maps to be constant! Our first instinct may be to lazily switch it to a variable, and add a comment:

package main

const rateLimit = 10

// this is meant to be constant! Please don't mutate it!
var supportedNetworks = []string{"facebook", "twitter", "instagram"}

Whenever we find ourselves leaving comments like this, we should be aware we are doing something wrong.

The Better Solution

It’s much better to use an initializer function (not to be confused with Go’s conventional init() function). An initializer function is a function that simply declares something and returns it. A good solution to our problem would be as follows:

package main

const rateLimit = 10

func getSupportedNetworks() []string {
    return []string{"facebook", "twitter", "instagram"}
}

Now, anywhere in the program we can use the result of getSupportedNetworks() and we know that there is no way we can get a mutated value.

Thanks for reading!

By Lane Wagner @wagslane

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