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Domain Modeling Made Functional - Scott Wlaschin - KanDDDinsky 2019

KanDDDinsky3 HN recommenders · 2020–2026
74K views · talk · evergreen · Uploaded 2019-12-06

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.

HN treats it as the canonical, approachable example of domain-driven design with types and 'make invalid states unrepresentable'.

Functional programmingSoftware architecture & designProgramming language design

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4 comments from Hacker News

@devmunchies2020 · in “Draft of OCaml Scientific Computing book”

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: [VIDEO]

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@Huggernaut2021 · in “Learn Functional Programming Visually”

For anyone looking for a smaller introduction to this book, the author Scott Wlaschin has a talk with the same name: [VIDEO] It doesn't cover a lot of good topics in the book, but it might whet your appetite.

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@rapnie2025 · in “'Make invalid states unrepresentable' considered harmful”

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. [VIDEO]

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@rapnie2026 · in “Parse, Don't Validate and Type-Driven Design in Rust”

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] [VIDEO]

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