Standardized error handling in Go HTTP handlers using func receivers

April 1, 2020

One of the particularities of Go is that you can create “methods” using functions as receivers. We’ll take advantage of that fact to make the handling of errors in http.Handlers DRYer.

What I mean by DRYer? Look at this example:

func myHandler(w http.ResponseWriter, r *http.Request) {
	err := doSomething()
    if err != nil {
		logger.Print("doSomething() didn't work :(")
        http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
        return
    }

    err := doSomethingElse()
    if err != nil {
		logger.Print("doSomethingElse() didn't work :(")
        http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)
        return
    }
	// ...
}

This code:

Is not very DRY: logging, writing an error and returning are repeated with little variation.
Is hard to test: you’ll have to inspect the HTTP Response for errors.
Is open for error: you can forget to return at the end of the if statement.

How are we going to solve this? We’ll create a new HandlerWithError type that returns an error and abstracts error handling:

type HandlerWithError func(http.ResponseWriter, *http.Request) error

func (h HandlerWithError) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	err := h(w, r)
	if err != nil {
		// Logging, etc.
		w.WriteHeader(http.StatusInternalServerError)
		return
	}
}

In this way, we can place all of our logic inside the ServeHTTP function.

This is a very simple example and is not going to work in all but very simple cases. You should probably modify the HandlerWithError to return a custom struct instead of a primitive value to account for more cases. This is left as an exercise for the reader ;).

Here is the full working example:

package main

import (
	"errors"
	"net/http"
)

type HandlerWithError func(http.ResponseWriter, *http.Request) error

func main() {
	http.Handle("/", myHandler())
	http.ListenAndServe(":8080", nil)
}

func myHandler() HandlerWithError {
	return func(w http.ResponseWriter, r *http.Request) error {
		return errors.New("you, fools")
	}
}

func (h HandlerWithError) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	err := h(w, r)
	if err != nil {
		WriteHeader(http.StatusInternalServerError)
	}
}