HACKER Q&A
📣 dennisy

Functional Core, Imperative Shell


I was recently recommended an incredible talk on boundaries by Gary Bernhardt [1], and right away I fell in love with this concept. I then found a further video [2] on this topic and also a HN thread [3].

But I need more information on this, I would love to learn more and be able to apply this to my own software, so I was wondering:

- Are there example of non trivial apps written in this manner?

- Are there more resources which explain the data not code as boundaries concept?

- Is there possibly a relevant book?

[1] https://www.destroyallsoftware.com/talks/boundaries

[2] https://www.destroyallsoftware.com/screencasts/catalog/functional-core-imperative-shell

[3] https://news.ycombinator.com/item?id=18043058

  👤 usrbinbash Accepted Answer ✓
In this screencast we look at one method for crossing this divide. We review a Twitter client whose core is functional: managing tweets, syncing timelines to incoming Twitter API data, remembering cursor positions within the tweet list, and rendering tweets to text for display. This functional core is surrounded by a shell of imperative code: it manipulates stdin, stdout, the database, and the network, all based on values produced by the functional core.

To me, that sounds how basically every procedural codebase operates: You have an interface, eg. ORM layer & connection handlers to connect a core of functions between which data flows on its way in or out.

So yes to all 3 questions as far as I'm concerned. Almost every program before and after the OOP hype was and is written in this manner.

The reason why I think this seems like a new concept to many people, is because the world is still coming down from the aforementioned hype when everything and the kitchen sink was supposed to be an object, and handling things by passing data between freestanding functions was seen as a big nono for some reason.