Blog / Go Language Spec

Constant errors

February 12, 2025

Last week I talked about how to create your own errors. Expanding on that idea, here’s a trick I like to use: Constant errors!

Constants in Go are limited to simple underlying types: bool, string, and the numeric types.

If we create our own type, which both satisfies the error interface, and has such an underlying type, we can then create constant error values:


type errString string

func (e errString) Error() string {
	return string(e)
}

const ErrTooManyFrogs = errString("too many frogs")

Okay. Neat trick. But why?

It’s impossible to change the value of a constant. That’s why they’re called constants, after all! But variables can be modified.

import "io"

func init() {
	io.EOF = errors.New("LOLWUT")
}

Now, I’ve never seen code that does that. I can’t imagine why anyone would want to do that, either, except maybe for nefarious purposes. But with a constant error, that’s impossible. So small win!

I rant a bit about exported error variables in a video I made a while back, too.

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