Ep 097: Application of Composition

Each week, we discuss a different topic about Clojure and functional programming.

If you have a question or topic you'd like us to discuss, tweet @clojuredesign, send an email to [email protected], or join the #clojuredesign-podcast channel on the Clojurians Slack.

This week, the topic is: "composing your application". We get a handle on bringing I/O resources together in an application.

Our discussion includes:

• What is a "handle"? Why is it called that?
• More puns!
• How do you handle I/O handles?
• How do you keep the pure and side-effecting parts away from each other?
• Managing and organizing application components.
• What is a framework vs a library?
• "Injection style" vs "singleton style".
• How far down the call stack should handles travel?
• Figuring out a start-up order and a shut-down order.
• How obtaining a resource can negatively affect composition.
• Separating the knowledge of finding a resource vs using a resource.
• The concept of an "application index".
• Why all this matters for long-term maintenance.
• Using feature flags.
• The two developers on every software project.
• What is the "band-aid test"?

Selected quotes:

• "If a program does a bunch of computation, but it doesn't have any I/O, did it do anything?"
• "Handles inherently are going to hurt you. You don't even have a hold on the whole thing. You have a little bit that you can use to to access that external system."
• "I visualized an axe or a knife. You have a handle, but you have to watch out for what the handle is connected to!"
• "In Clojure, I think of frameworks as an integration between your code and the purpose of that framework as opposed to the foundational layer that glues your whole world together."
• "Even in OO, it's a smell if I/O handles travel too far and wide!"
• "Lots of assuming!"
• "Sometimes ad hoc is confused with flexibility. You didn't have to think about it, so it felt flexible, but it just happened."
• "Composition is about being able to use things in multiple ways."
• "If you are accessing a singleton state from a function, you are no longer side effect free."
• "Why would I ever save anything anywhere other than the place the application has configured as its database?"
• "Have you ever had a physical "dead tree" book?"
• "Where are all the components?"
• "With the index style, you have a clear delineation of all the parts of the software and how they're connected together."
• "Whenever you start using a library or framework, you are hopefully solving a problem, but you're also inviting complexity in. You have to be wary of what it allows you to do, and what it allows future developers to do."
• "If you pull that band-aid off, how many little hairs are going to come off with it? How badly is it going to hurt?"
• "One of the tests of composability is the surface area of contact. More is not better!"

Links:

• Injection style: Component + component.repl, Integrant
• Singleton style: redelay, mount
• Built on Component: system
• Others: donut.system, Components in Biff
• Ep 063: Web of Complexity
• Ep 067: Handling Handler's Handles
• Ep 093: Waffle Cakes
• Ep 094: Concrete Composition
• Ep 095: Composing Core
• Ep 096: Gaming Data