I’ve been watching a number of videos about monads. They’re trendy right now. Essentially, monads are a system for safely chaining commands together. If you’ve used JQuery, you’ve used Monads. People talk a lot about the “actor model”: chaining commands together, where each command is run on a service provider, often on a different computer. In other words, distributed monads.

What people don’t seem to realize is the grandfather of all monadic systems outside the ivory tower is Unix. The pipeline that Unix users love is a system for safely chaining commands together, i.e. monads. And as the saying goes: “those who fail to learn from Unix are doomed to re-implement it poorly.”

So what does Unix have that actors don’t? A system for debugging. Namely an out-of-band logging channel known as “standard error”, or “stderr”.

Why is this important? At the end of this video, the presenter is asked for advice on debugging actors, since when something goes wrong across multiple computers it can be hideous to follow the chain of events that went wrong. I’ve seen this myself. The presenter’s response is simply that actors/monads are easier to debug than threads. That’s like saying that keeping pools of gasoline in your basement next to the furnace is safer than storing a nuclear bomb. Not helpful.

So here’s my plea: anyone who does functional programming should include a “stderr” in their monads.

Imagine if JQuery objects had a “history” function, which would point out where you wrote “foo” instead of “.foo” and suddenly got zero results. Imagine if Akka had a log which showed you the chain of events for a particular query, across computers, rather than logging unrelated events on each computer.

Programmers have gotten used to using a stack trace for debugging. But with actors, the stack tells you nothing, because the action is horizontal–across chains of messages– rather than vertical. The fix is really simple, and it’s something Unix got right.