TL;DR; F# is great for algorithms and analytics. It could work for libraries, interop and high-performance code as well, but the experience is far from great and I do not want to fight with it anymore. At least I tried…

I have been functional programming aficionado for a while. At some point, I took its advantages over mutable imperative code almost religiously. I fell in love with F#. Then I fell in love with mechanical sympathy, and F# - by being a multi-paradigm and .NET language - still allowed me to get almost all performance I needed, even if it required abusing it somewhat.

But lately, I started to dig even deeper and closer to the metal, to use unsafe code, to do many small experiments and microbenchmarks, to use .NET Core, and started to question my choice of F# as the primary language for .NET library development. Tooling, generated code performance, and .NET Core integration could be better. As a matter of fact, I almost haven’t touched my F# code over last several months while programming exclusively on my laptop.

F# is probably the best choice for end product development, but not the one for libraries that require performance, native interop, and (at this moment) .NET Core interop.


Recently I have bought the very top model of MacBook Air with 8 Gb of RAM for development. Specifically, for F# development, because IDE experience was always slower than C#. But even on this machine, the IDE is still not responsive and compilation is slow. While C# projects fly on this machine even with R#, F# ones crawl and creep even for such a small project as Spreads.Collections.

Refactoring and simple edits take a very noticeable pause for type inference to finish and for IDE to become responsive, and this pause grows non-linearly with a project size. I understand that type inference is expensive and it is almost a recompilation step, but when refactoring is done frequently it takes too much time.

Project recompilation happens on every build, even if there are no changes in the code. And it is slow. I searched for a solution in the Internet, but it didn’t help. The recompilation happened on all my 4 machines over the past couple of years. And the funny thing is that F# projects do not see changes in referenced C# projects when compiled separately, not as a part of a solution.

Each problem is minor in isolation, but when this happens very often, then most of the development time is spent on tooling, not coding. In a workflow when I make many small refactorings, add tests and run microbenchmarks to see the results of the changes - the total time costs become too significant. It is similar to a program with garbage collection that takes most of the execution time. In the era when live editing and reload is a norm, slow recompilation significantly impedes productivity. On the same machine, strongly-typed Angular2 (TypeScript) with WebPack reloads in just several seconds, while VS Code also flies. I wonder why F# compilation couldn’t be slower than C#’s just by several percents, not times!?

They say that Russian programmers used to be very good because they lacked frequent access to machines in the 80s and they had to think a lot and to perfect their code before a chance to actually run it. That contrasts to modern days when people start with opening an editor/IDE and think over a blank screen like a writer thinks over a blank sheet of paper. On my laptop, F# nearly forces me to do such Hammock-driven development. This is not bad per se, and for an end product, I think this is good. For some complex algorithm development, when most of the time is spent on thinking about domain subject and not about generated IL, F# type inference in the background is invaluable for correctness. And by my own experience, I could confirm that when F# code compiles it is very often already correct.

Generated code performance

Generated code performance is also an issue: to get it right in F# one has to write really ugly verbose mutable code and inspect IL and run a profiler to check for implicit allocations, which are sometimes hidden somewhere in the compiled code. This really defeats the F# elegance and terseness. (Also there is an interesting discussion here). Additions such as struct records/DU and fixed are kind of overdue, especially when I have already invested in unsafe C# as heavily as IL rewriting hack taken from corefxlab.

Native interop and unsafe could be done in F#, but this is ugly and clumsy. The Recent addition of fixed keyword is nice, but there is still no support for it in VS2015, and the NativePtr API is painful to work with compared to C-like pointers in unsafe C#.

Task Parallel Library is the best thing since sliced bread in .NET universe. Async/await came from F#. Yet, there is still a big overhead when interacting with TPL from F#, there is no native and idiomatic ways to do so. I tried to use Task computation expressions from FSharpx project, but they explode like Galaxy S7 in for/while loops and other non-trivial constructs. Simple Bind and Return work well with recursion, but in the end manual usage of GetAwaiter, OnComplete callbacks and TaskCompletionSource is more reliable. I believe TPL integration at the core language level should be a priority to perceive of F# as a sibling on .NET platform, and not as an adoptee.

File and type ordering is an F# limitation that magically turns into blessing most of the times. But when it doesn’t, workarounds are awkward. I spent some time to find a different architecture, but often recursive types across files are just the right thing. With the recent introduction of recursive modules and namespaces, I could put all my code in a single file, and sometimes I really thinking about this. #sarcasm

Implicit allocations and IL generation are almost fixed in latest versions and have proper attention from the core team, but sometimes a profiler shows unexpected results in the places where one would not expect. E.g. Array.average was allocating objects just recently. After a lot of work on eliminating allocations in my Spreads library, it was a surprise to find this. Generated IL code is not easy to reason about compared to C#, which is almost “what you write is what you get” and is to IL like C to assembly. F# sometimes surprises.

One could say I should fix F# itself since it is open source, but for the same reasons above just setting up and compiling VisualFSharp repo from a clean clone is a half-day exercise on a really powerful workstation machine; and editing is slow even if it eventually compiles with tests. By the way, I really tried to fix the array issue above and almost succeeded, however, the tooling was a great impediment in the process and I could hardly setup and compile the project after several attempts. I also reported the IL issue but had no idea how to fix it. I believe such contributions are optional and good will, not an obligation when I have my own work to do (even though I would love to contribute more if I could both from time and technical ability perspective). As they say in Russian, “Вам с шашечками или ехать?”/“Do you need limo or lift?”, and C# gives a good ride.

Despite the issues above, there is nothing wrong with F# itself and there is still a way to write efficient code. But to do so, I have to do many small experiments and changes very often, run tests and benchmarks - and repeat… In such workflow tooling again becomes the main obstacle to overcome the issues.

.NET Core interop

This is both tooling and language issue. I often feel that Microsoft almost said: “F^&k you, come back in a year”. It is just not there yet. Doesn’t work with C#/F# project mix. #dontnetcore

When .NET Core was in alpha/beta, it was OK. Now it is 1.0 and there should be no excuses. It feels again that F# is a side project for MSFT.

If you think this is just F# critics and rant - don’t get me wrong. This is an open question and a call for action. I am watching the development of CoreFX and CoreFXLab projects, and really like the recent trend of making C# even more performant and even less safe than existing unsafe keyword. It feels like all the “fairy tales” about C# as a system programming language are slowly materializing. At the same time, C# aggressively takes the good parts from F# and is becoming more and more functional (in both senses of the word). With Roslyn, it added interactive execution. If only they add if as expression and a compiler option to disable implicit type conversions, similar to checked arithmetics… It looks like F# relevance is diminishing unless Microsoft invests in its tooling.

F# is very great and functional programming has many good parts. F# is still faster than Scala, which takes forever to compile “Hello, World”. F# is great for an end product, like trading algorithms or analytical code. Its absence of implicit conversions and presence of units of measure should not be taken lightly. (I had a real bug due to implicit ints conversions that took a long time and many sanity checks to find, “select was not broken”). In addition to trading and analytics, there is a kind of libraries where F# shines like a supernova

  • metaprogramming. I have a project that implements a very basic query language with FParsec, and it blows my mind how easy it is to implement a new language. (Small Basic blog posts are a great start).

This post started this morning from an issue on GitHub after once again I was fighting with .NET Core C#/F# interop and many issues from above. But instead of bullets on what to change in the library I could’d help but just started writing about the issues and experiences I have had recently. I invested into F# quite a lot and tried to make it work where it doesn’t fit. But now I am tired and emotional as if I drunk a lot of it and am experiencing a hangover. I am still addicted to it, though. It tastes great and has an elegant flavor, makes programming fun, protects from many classes of errors, and will remain my #1 choice for end products where correctness is paramount. Oh, and F# interactive is awesome for such scenarios, but I still could not find a way to productively use FSI for a solution with multiple C#/F# projects and my workflow. What I am doing wrong?

“Functional programming is a tool for thought, imperative programming is a tool for hacking.” (c) Erik Meijer