Original post

One of the most essential things in software is a good interface for logging data to places. Logging is a surprisingly hard problem and there are many approaches to doing it. This time, we’re going to talk about my favorite logging library in that uses my favorite function I’ve ever written in Go.

Today we’re talking about ln, the natural log function. ln works with key value pairs and logs them to somewhere. By default it logs things to standard out. Here is how you use it:

package main

import (


func main() {
  ctx := context.Background()
  ln.Log(ctx, ln.Fmt("hello %s", "world"), ln.F{"demo": "usage"})

ln works with key value pairs called F. This type allows you to log just about anything you want, including custom data types with an Fer. This will let you annotate your data types so that you can automatically extract the important information into your logs while automatically filtering out passwords or other secret data. Here’s an example:

type User struct {
  ID       int
  Username string
  Password []byte

func (u User) F() ln.F {
        return ln.F{
                "user_id":       u.ID,
                "user_name": u.Username,

Then if you create that user somehow, you can log the ID and username without logging the password on accident:

var theDude User = abides()

ln.Log(ctx, ln.Info("created new user"), theDude)

This will create a log line that looks something like this:

level=info msg="created new user" user_name="The Dude" user_id=1337

Mara is hacker


You can also put values in contexts! See here for more detail on how this works.

The way this is all glued together is that F itself is an Fer, meaning that the Log/Error functions take a variadic set of Fers. This is where my favorite Go function comes into play, it is the implementation of the Fer interface for F. Here is that function verbatim:

// F makes F an Fer
func (f F) F() F {
        return f

I love how this function looks like some kind of abstract art. This function holds this library together.

If you end up using ln for your projects in the future, please let me know what your experience is like. I would love to make this library the best it can possibly be. It is not a nanosecond scale zero allocation library (I think those kind of things are a bit of a waste of time, because most of the time your logging library is NOT going to be your bottleneck), but it is designed to have very usable defaults and solve the problem good enough that you shouldn’t need to care. There are a few useful tools in the ex package nested in ln. The biggest thing is the HTTP middleware, which has saved me a lot of effort when writing web services in Go.

This article was posted on M10 17 2020. Facts and circumstances may have changed since publication. Please contact me before jumping to conclusions if something seems wrong or unclear.

Tags: golang go

The art for Mara was drawn by Selicre.