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+---
+title: "ln - The Natural Log Function"
+date: 2020-10-17
+tags:
+ - golang
+ - go
+---
+
+# ln - The Natural Log Function
+
+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 Go that uses my favorite function I've ever written in Go.
+
+Today we're talking about [ln](https://github.com/Xe/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:
+
+```go
+package main
+
+import (
+  "context"
+
+  "within.website/ln"
+)
+
+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](https://godoc.org/within.website/ln#F).
+This type allows you to log just about _anything_ you want, including custom
+data types with an [Fer](https://godoc.org/within.website/ln#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:
+
+```go
+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:
+
+```go
+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
+```
+
+[You can also put values in contexts! See <a
+href="https://github.com/Xe/ln/blob/master/ex/http.go#L21">here</a> for more
+detail on how this works.](conversation://Mara/hacker)
+
+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:
+
+```go
+// 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](https://godoc.org/within.website/ln/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.