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 feedback@clojuredesign.club, 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

Ep 097: Application of Composition

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