I've been using F# professionally for the past seven years across different contexts. First in a small software shop and now while bootstrapping a SaaS company. Some observations:

* It’s easier to attract smart developers to an F# project than to a [mainstream language] project. This was one of my driving beliefs when I introduced F# seven years ago. https://www.paulgraham.com/pypar.html. This is probably just as true for languages like Elixir, Clojure, ... But F# is what we went with.

Small Software Shop Context

* We operated in a small market where customers eventually dictated our tech stack (.NET & React). In that market, F# was a major advantage—it allowed junior developers to build apps that "just worked" with minimal regressions. Even with mediocre code quality, I felt confident that we could refactor safely at any time.

* I constantly had to justify F# to clients, which was exhausting. We always delivered decent results, so it worked out, but my partners were never as confident in defending F#.

Bootstrapping a SaaS Company

* F# has been invaluable for shipping features quickly and taking shortcuts when needed.

* Three years in, our codebase is large and contains its fair share of messy parts. But we can still develop new features at high speed with minimal regressions. Refactoring is relatively safe and straightforward.

* Compilation speed is the Achilles’ heel. If you don’t monitor it, the compiler slows down to the point where it impacts productivity. Earlier this year, waiting over a minute for feedback after a small change became unbearable. A lot of our "clean-up" work focuses on optimizing compilation times. We're still learning, but we’re optimistic that we can restructure the project to significantly improve build performance.

EDIT: maybe one more point. I see a lot of C# vs F# popping up here. Yes, C# has all the features that F# has. But do not underestimate how well designed F# is. It is an extremely simple language to learn compared to C#. There is a very limited amount of keywords to learn. And they compose extremely well. If you learned F# 7 years ago, took a break, and came back today, you'd simply write the same boring code that you would have written 7 years ago. And along the way you'd find out that some things have gotten a bit nicer over time.

I’ll add that features isn’t really a constructive way to think about the differences between C# and F#. C# has more ‘features’ than most other programming languages. One of the core features of F# is less features and churn. Also, I really appreciate the way F# maintainers agonize over how to maintain the language and keep it coherent. You don’t get the feel they’re chasing features at all.

As a 20+ year C# dev...where do I learn how to structure apps in F#? In C# my ASP.NET Controller might use a service (or a mediator) to execute some domain logic and that in turn will use a repository pattern (or EF DbContext) to update/query a database. How are dependencies injected in? It seems like there are multiple ways of going about it, but I don't have enough knowledge of F# to know 'the proper way' to do it.