Scott Wlaschin shows how statically typed functional languages, using F# examples, can model a business domain so that invalid states cannot be represented. The type system acts almost like an instant unit test at compile time.
Why Hacker News points people here: HN treats it as the canonical, approachable example of domain-driven design with types and 'make invalid states unrepresentable'.
“this talk opened my eyes to the power of f# philosophy and type system”HN comment, 2020
“this talk opened my eyes to the power of f# philosophy and type system”
Opened commenter's eyes to F# philosophy and type system.
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I wouldn't say its unloved or on a side track, it's baked into .net as a first class citizen and has dedicated engineers at Microsoft working on it. When I started I was pleasantly surprised how easy it was to download .net core on a mac and not do anything else to use F#. Its less popular than C# so its there's not as much documentation from 3rd party sources, youtube videos, etc, but you can use any C# modules in F# so you'll get used to reading C# docs. I would highly recommend this overview of the .Net ecosystem: link I really like how it showed a preview of "modern C#" at 12:20 which is being influenced by F#. After that, this talk opened my eyes to the power of f# philosophy and type system: this video
RecommendsHuggernautFeb 2021
“For anyone looking for a smaller introduction to this book”
Short introduction to the book of the same name.
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For anyone looking for a smaller introduction to this book, the author Scott Wlaschin has a talk with the same name: this video It doesn't cover a lot of good topics in the book, but it might whet your appetite.
RecommendsrapnieSep 2025
“offers a very appealing example for making making invalid states unrepresentable”
Appealing example of making invalid states unrepresentable.
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I can recommend "Domain Modeling Made Functional" by Scott Wlaschin presented at KanDDDinsky 2019, that offers a very appealing example for making making invalid states unrepresentable. The example code is in F# but that is a perfect language to demonstrate the general idea. Best-practice I guess boils down to finding the right balance throughout the application design. this video
RecommendsrapnieFeb 2026
“See "domain modeling made functional" [0] as a nice example”
Nice example of making invalid states unrepresentable.
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You can also search for "make invalid states impossible/unrepresentable" [0] to find more info on related practices. See "domain modeling made functional" [0] as a nice example [0] link [1] this video
