Why We Use OCaml
tech.esper.com
tech.esper.com
I say "somewhat unique" because there are other languages in this space: Rust in particular may be the language of choice for these feature needs in the future.
High performance code in Haskell tends to rely on a combination of stream fusion, rewrite rules firing and judicious use of unboxing and bang patterns. I've heard it said that the OCaml compilers simplicity makes it far easier to know what code it's going to produce.
Disclaimer: I'm a Haskell user, so it might just be that I'm more familiar with Haskell's warts.
Over F#, OCaml provides a module system. This helps programming in the very large.
Over Haskell, OCaml provides a module system, and strict evaluation. The latter is a contended point, but proponents of OCaml claim it leads to memory/execution predictability. In Haskell (GHC) you often have to turn on optimizations in order to understand how your code will perform in reality, whereas byte-code interpreted OCaml acts just like natively compiled OCaml space wise, but is roughly 10 times faster in execution speed.
Wouldn't it change the require programming style to make use of the strict setting?
More info on strictness and Bang Patterns: https://wiki.haskell.org/Performance/Strictness#Explicit_str...
This was posted to r/haskell today and helps explain things a bit: http://blog.johantibell.com/2015/11/the-design-of-strict-has...
I think it only makes sense to talk about naive, idiomatic implementations of an algorithm when you're talking about the performance of a language. If you start talking about highly optimized versions of algorithms, it's harder to make statements about which language is faster. People who use numpy adeptly get extraordinary performance in Python, but I don't think that makes the argument that Python is faster than OCaml; it's hard to say what an equivalent level of optimization in OCaml would look like.
Every few years I take another look. Last time I researched this, I came across a bunch of incipient attempts at distributed, concurrent programming frameworks, but they were all abandoned around 2009. I couldn't find any mature, maintained frameworks, any easy-to-use concurrency primitives (lwt has, to my untrained eyes, a terrible API and is no match for goroutines or Erlang processes), and apparently the global interpreter lock is still there.
That's why I'm not, at this moment, using Rust or Nim in production. I want a mature toolset, a large, cohesive standard library, a large, active ecosystem, etc.
Go only recently reached this stage. Node.js has been there for some time. Elixir is there thanks to leveraging Erlang.
There's no cost-benefit analysis here discussing the obvious pitfalls from a business perspective, such as a much reduced talent pool and probably higher salaries for the few programmers who are comfortable working with such an uncommon language. There's also no discussion about why they believe a business offering a calendar application needs to use such a technically demanding language. This would be more understandable for, say, Jane Street (high-frequency trading), but for this particular business my naive belief is that this probably will not be the "competitive advantage" they claim it is.
That might be true, but if they are competent in both, that's often because they have switched from OOP to FP and prefer the latter nowadays so if there is a selection between otherwise equivalent jobs, they would tend toward the FP job.
[1] https://github.com/libguestfs/libguestfs/tree/master/v2v
Ocaml was easier than even Java. Much less demanding than the minefield that is C++. And it certainly doesn't require more education. A different education, that's for sure, but not more.
As for requiring a stronger understanding of math… Languages like OCaml just don't pretend that programming is not math. So it tends to scare away those who fled math in the first place. C++ and Java are just as mathy (if not more, just see the sheer size of the specifications), but they don't really tell you.
<Grumpy Dijkstra> Programming should have been of the applied mathematics department, not the electronic engineering one. </Grumpy Dijkstra>
I have little experience in education, but my naive hypothesis is that it would be easier to teach a child, say, Python than any FP-oriented language. I suppose it goes without saying that if children were introduced to the lambda calculus beforehand this might not remain true.
See my comment beneath the root to see my clarification of the original post.
I don't think Dijkstra would disagree with me. After all, it was he who suggested the average American programmer would benefit "more from learning, say, Latin than yet another programming language."
It is also a form of resource management and incur resource management issues (do I need that value in this variable? Can I overwrite it and discard old value?).
The implicit state also is an implicit communication channel between different parts of program. In FP this communication is much more explicit, especially if you use something like Software Transactional Memory.
I think I listed enough points for you to reconsider your stance about Python being easier to grasp.
I know CMU does, however.
However, I'm puzzled about your remark about industry-oriented programs. I'm not sure what "industry-oriented programs" even means, but assuming you actually meant "FP doesn't have widespread use in the industry", I'd say that while this is true, a- it's becoming less true every day, through the use of mixed-paradigm languages, and b- even the purer FP languages see industry use.
"Right or wrong" this current reality you mentioned is shifting. Hybrid languages are becoming more common. Traditional languages are adopting functional idioms (and idioms from other paradigms as well!). It's not true that there is a clear-cut "natural" kind of programs for which to use FP; the area for which FP is good is pretty much good old "let's write software that works and has few bugs". If you have this goal, you can use FP to your advantage.
Any success story of someone using OCaml for this is welcome.
I've said nothing of depth about the merits of OCaml or FP in itself. I'm simply pointing out that the blog post clearly identifies the use of OCaml as a competitive advantage without discussing any of its drawbacks, whether these arise from the nature of the language itself or for historical reasons. This makes the article significantly less useful for a person trying to select a language for their company's codebase, which I take to be the whole point of writing the post to begin with.
Perhaps the real goal of the post was to attract the attention of OCaml developers, rather than discussing the titular question "Why we use OCaml." If this is the case, I'm sorry for having wasted my time criticizing it.
But then I realised that explaining why the listed features are good is hard. Someone who haven't been exposed to FP is going to take a lot of teaching and persuading. Or a damn good motivating example, but I have yet to find one.
Like a sibling comment said, this is an advocacy piece. I don't know if anyone will pick OCaml (or any language) for their company immediately after reading a blog, but it might pique their interest and make them research the language and consider it for their own projects, an interest which might eventually develop into considering the language for their day job. I know it was the case for me with Scala!
Really, this "oh no a foreign language" thing is way off. Every good software engineer I know would not bat an eye to learn a new language.
Ocaml is not quantum physics, it's pretty understandable language.
I'd also like to suggest that, for certain applications, the most touted benefit of functional programming (the ease of proving algorithms behave as intended) is not especially relevant. I would even dare to suggest that, in some cases, it may be more rational to hire a "decent" programmer than a "great" one.
This is to say nothing about the specific language features of OCaml or the utility of functional programming in general, just that the benefits need to be weighed against the costs. In an article claiming that using OCaml provides a "competitive advantage" from a business perspective, I would have expected a discussion of both.
More precisely, OCaml allows me to refactor my code very quickly, finding almost all mistakes even before any testsuite runs.
This is possible due to a combination of language features which only few languages have (notably, Haskell, SML and F#).
Not sure how this goes into total productivity, but this acceleration in development speed is something that no number of frameworks or libraries can match up.
I see this argument all the time. I don't know if the type of software development I do is special (I doubt it, it's fairly standard business software), but the amount of time I spent on business logic dominates total development time.
Past a certain fairly low threshold of core libraries (HTTP, JSON, CSV, database bindings, standard stuff like that), the amount of time libraries can save you is dwarfed by the cost of refactoring all that business logic.
On the other hand, if the language is so niche as to lack the basics, you can find your entire development budget spent on building functionality other languages offer out the box. I don't think OCaml is that niche...
It heavily depends on what you're doing. You might be surprised, but there is a lot of projects which do not rely on any 3rd party code and are built almost entirely from scratch.
$ npm install install_ocaml -g
$ install_ocaml
Yes, the OCaml tooling has really improved recently, and the OCaml workflow has been more and more smoother. Still. I wont recommend it for a business.
So, if the original author read this, could you answer these questions about how do you ship products with Ocaml:
- How do you do Quality Assurance ? (anything from unit-testing, integration testing, functional testing, etc. I guess you have to do it a lot to check that Gmail didn't break its integration). Testing in isolation has its share of challenges in Ocaml.
- How do you manage your builds and releases ? Private Opam repositories ? Directly shipped to Google ? Do you have beta/staging channels ?
- And last but not least, what have been the pain points so far and have you been able to fix them or do you just work with it ? (it happens with any tech stack, but it's good to know what the trade-off are)
I am very curious how this could work at "entreprise-scale" and I would be glad to have some real world examples of ocaml in production.
Apparently F# has support for neither style of functor--it doesn't have parametric modules and it also doesn't have typeclasses. So in F# `map` is defined independently for each type:
Set.map : ('a -> 'b) -> 'a Set -> 'b Set
Seq.map : ('a -> 'b) -> 'a seq -> 'b seq
List.map : ('a -> 'b) -> 'a list -> 'b list
Array.map : ('a -> 'b) -> 'a [] -> 'b []I saw some pieces of F# in the FinTech here and there, I think Microsoft could do a better job at marketing it because it really has great potential in the .Net ecosystem.
Like, testing is basically just checking a bunch of if statements. You can use fancy frameworks that color your tests red and green, but other than that kind of thing, what's the actual problem? Why do you say testing in isolation is particularly challenging with OCaml?
It's been years since I used OCaml and I don't know anything about OPAM so I'm also interested in the answers.
Did you try to use OCaml and run into a bunch of difficult problems?
Yeah, I am a long date contributor and user of ocaml for little projects (mostly compilers and AIs). But not professionally.
> It's been years since I used OCaml and I don't know anything about OPAM so I'm also interested in the answers.
Opam is Bundler done right. It manages ocaml toolchains and packages dependencies. It's easy to pin a specific version or publish your own.
A setup oasis + ocamlbuild + $editor could go quite far. After that, it will depend of how you works and what you are developing.
> Like, testing is basically just checking a bunch of if statements. You can use fancy frameworks that color your tests red and green, but other than that kind of thing, what's the actual problem? Why do you say testing in isolation is particularly challenging with OCaml?
That is a very interesting question that deserve a blog post on its own. But I don't have the time nor the patience to do so, so let it be the HN comment.
> Like, testing is basically just checking a bunch of if statements.
Wrong ! There's multiple kind of "if statements to test";
-1- I want to test that `my_inner_fibo(0, 1) == 1`. This is unit test. Basically a transcript of the technical specifications into assertions. This is the easiest test, the most verbose, but it's also the kind that test the least.
-2- I want to test that `UserModule.retrieve_orders(user, command)` makes the good calls. This is integration testing in white box, which test that the API contracts between you inner interfaces are respected (which is also part of the technical specifications). I don't find it very useful but it's better than nothing when you can't do more.
-3- I want to test that, with a given state `user`, and `command`, when I call `UserModule.retrieve_orders(user, command)` I get exactly this object. This is integration testing in black box. This is useful to find errors in inner logic a group of modules (but you don't know immediately what gone wrong). As the call stack could be very large (and so the scope of what you are testing), you want to reduce the moving parts and replace some of the modules with trivial mocks. Integration testing in a black box test a lot of things and is very useful to find bugs.
-4- I want to test that a call to `$./my-binary ctl command --flag=true` have a specific behavior. Could also be a network request to a server, a message to a deamon, a click on a GUI... Anything exterior to the program. Here we test the behavior. This is what the user will use, that's why it's functional testing. This could break often (in the case of a GUI) or not (in the case of a CLI binary). You should always do functional testing if you respect you users a little.
All these kinds of testing have different requirements, and some need a perfectly controlled state to be created. The issue is not in the conditional testing, but in the setup this perfect controlled state, which sometimes need to make the code believe it use the good module, but you gave him a stubbed one which does trivial work, or signal you when something happens.
I don't know any simple way to spy on functions without using the MirageOS design which heavily use Functor injunction. It's quite an academic way of doing it.
As an example, I want to make sure that when I hit a REST API endpoint, I do all my logic and get a proper return value (black box testing), maybe that the state of some bit that I've already set up is correctly changed (say, the database, by then checking it as part of the test to ensure the thing I said should be written was written), but that I also send a server sent event (SSE) to connected clients indicating the change. I don't want to actually have to have opened up clients that conform to the SSE specification as part of my integ tests (because that's a pain), so instead, I'll just assert that the library call to send that event does indeed get called with the right thing. From there, I can test once, that the library call does indeed lead to an SSE being sent to the browser (and can even write a full environment integ test with Selenium or something), and from then on, my single system tests just assert that call is made when it's supposed to. I'm effectively integration testing without implementing/mocking a connected user to test SSEs.
Similar things can be useful when putting items onto external queues, making calls to foreign interfaces, etc. You should never make assertions about the path through your code a call takes in white box testing (because refactoring can change those, and you have increased your test burden for little reason), you should care about side effects you can't easily otherwise test.
The remaining pain point, as far as tooling goes, is the debugging situation, but steady progress has been made and it should receive very large improvements in the next OCaml version or so.
https://github.com/ocaml/opam/compare/master...dra27:windows-build
If you're fine with, eg, hacking Python in VIM, you'll be pleasantly surprised by the OCAML situation. If you live in an integrated IDE, there will be some adaptation.
- you create and manage the build of your project with Oasis which call ocamlbuild nicely
- Opam works great with ocamlfind to manage the dependencies and the toolchain you use
- OUnit has backgroud workers for parallel testing
- Merlin and ocamlc annotations do wonders in term of semantic completion
- utop is an excellent toplevel with colors, completion and integration with ocamlfind so that you can load dependencies inside
All these technologies have greatly improved in the past 2 years, which is a really short span compared to the age of Ocaml.
- QA: We have some unit tests, but mainly we use a huge test suite that does end to end testing of tools. It uses automake's test framework, so it works across all the languages in the project.
- Builds and releases: We use autotools and tarballs. It's automated using a thing called 'goaljobs' which is like a generalized make.
- The pain points for us all derive from autotools itself, which is both crap and better than all the other build systems[2]. It is at least well understood.
I'd also say the killer advantages of OCaml for me are: Easy calling into C, and compiles to a native binary.
[1] http://libguestfs.org https://github.com/libguestfs/libguestfs
[2] I use the term "build system" in a rather narrow sense of something that (a) runs on Linux (b) lets the end user download a tarball and (c) builds using ./configure && make
> I'd also say the killer advantages of OCaml for me are: Easy calling into C, and compiles to a native binary.
Very true. And Ocamlbuild makes wonders.
[1]: http://ocaml.org/learn/companies.html
To answer one of your question. Yes, we have testing frameworks, both for unit[2] (inline[3]) testing and property testing[4]. As for the rest, it's not OCaml specific at all. :)
[2]: http://ounit.forge.ocamlcore.org/ [3]: https://github.com/vincent-hugot/iTeML [4]: https://github.com/c-cube/qcheck/
Yeah, you will notice that quotes that show you shouldn't take it too literally. ;) (also I have no doubt that Ocaml can work in a business. I just saw no one speak about it appart Jane Street, so I am curious)
What I was meaning that "a team with a hierarchical organization, not-ony-geniuses, and more than 4 people". Few of the serious Ocaml projects (if any) fall in this category, which is common for all businesses.
About the testing tooling, I am well aware of the state of these technologies. I even contributed to some of them. Sorry but it's light. It doesn't cover all the spectrum of what you'll want to test in a product.
Let's take your (excellent) libs and see:
- OUnit: unit tests
- QCheck: unit tests
- iTeML: unit tests
Where is integration and functional testing ? Unit Testing only test a very strict subset of the "does this work as intended" question. I recommend you to see how some projects do their testing. For example any classic Rails project. You could be surprised.
Some open questions I struggle myself to answer correctly:
- Mocking modules without having a build mess with oasis
- Managing the model in a sane way
- Integration testing is an horror
And many more that I can't recall now.
> Mocking modules without having a build mess with oasis
OCaml has parametrized modules (aka "functors"), which provide a very clean solution, compared to mocking whole modules anyway.
As a side note, I usually see mocking being used to test badly structured code. Refactoring that is usually a benefit for testing as well as a benefit for the code itself.
I am amazed to see how the MirageOS team used this, but this method cannot be used for all the code.
> As a side note, I usually see mocking being used to test badly structured code.
You could want to mock just because you are inspecting particular stack frames in a specific call stack, trying to reproduce a bug. It happens.
- QA: unit tests written using oUnit, and integration tests by using linked Docker containers.
- builds/releases: opam + packages written for Debian and Fedora based distributions. The packages provided on our website are built inside Docker just like all the other packages. Internal beta packages are uploaded to a separate volume/bucket and served via LibreS3 itself.
- Pain points: building a package compliant with packaging policy is more complicated because I usually need newer versions, or OCaml libraries that are not yet packaged. I planned to use opam to generate templates for LibreS3+dependencies but was waiting for the C backend to get packaged upstream first. My slides from last year list a few more problems that I encountered during development but they aren't an issue currently [4].
The main advantages of OCaml for me are: - the availability of high-level libraries that makes implementing HTTP APIs simpler (Ocsigen, Cryptokit, atdgen, etc.)
- event-driven/non-blocking architecture to support large number of concurrent users (Lwt)
- native binaries and static type system
- not having to look for certain bugs in my code like uninitialized variables, NULL dereferences, memory leaks or memory corruption bugs; which (used to) take up a significant amount of time when developing applications in C (although tooling on C side have improved with valgrind, clang -fsanitize= and Coverity).
That doesn't mean that code written in OCaml or the libraries that I use doesn't have bugs, I track them down and provide patches just as I would for a C library. Although perhaps writing code in OCaml has made me somewhat overconfident in my code, and I find bugs in other people's code more easier than in mine even if I'm not looking for them.
[1] http://gitweb.skylable.com/gitweb/?p=libres3.git;a=summary http://www.skylable.com/products/libres3
[2] http://gitweb.skylable.com/gitweb/?p=libres3.git;a=tree;f=li...
[3] http://www.skylable.com/products/sx
[4] https://ocaml.org/meetings/ocaml/2014/ocaml2014_13_slides.pd...
> Although perhaps writing code in OCaml has made me somewhat overconfident in my code
Many Ocamlers could be caught of overconfidence. It's hard to understate how ML typing help seeing the data flow.
Using a not very popular programming language can actually make it harder for you to hire people.
Mind giving an example?
> Using a not very popular programming language can actually make it harder for you to hire people.
Do you really want to work with people who can't learn a tiny little language?
> Mind giving an example?
You can use, for example, elaborate if-else constructs that firstly try to determine type of the value, then check for values.
I've never seen in imperative languages anything similar to conditional that checks both type and content of an expression, and allows to capture parts of that expression into variables. Well, except for precisely the pattern matching construct, where it was implemented deliberately after functional and declarative languages (Rust).
>> Using a not very popular programming language can actually make it harder for you to hire people.
> Do you really want to work with people who can't learn a tiny little language?
I see things a little differently. It's extremely difficult to find an idiot who can write OCaml, while it's fairly easy for C++, Java, or C#.
Also, allowing people to learn OCaml and use it every day on the job is a big plus for a company. It's way better than free sandwitches for breakfast in the long run.
That's nowhere near pattern matching. Two concepts make pattern matching vastly superior to if-else constructs:
- Non-exhaustive checks by the compiler.
- Destructuring
It's being a combination of destructuring, branch selection and variable assignment that makes pattern matching superior.
As for the hiring, it is a silly idea in general to seek "programmers in XXX language" (and to position yourself as an XXX programmer). Hire programmers. Just programmers. Languages are irrelevant.
That's dynamic typing. Very different from pattern matching over sum types: with dynamic typing, you can basically forget about most compile time guarantees. Seriously, you sound like, "why bother with Ocaml when JavaScript does the same thing?". I can tell you from first hand experience that they do not feel the same at all.
The other classic example, class hierarchies, is also very different from pattern matching —and much more cumbersome. The only language I know of that kinda bridged the two is Scala, with case classes.
No, that's run-time branch selection. Exactly as with pattern matching (`match .. with ..'), except the latter is more convenient.
> Seriously, you sound like, "why bother with Ocaml when JavaScript does the same thing?".
And you really think I consider elaborate constructions a better thing than concise pattern matching?
With pattern matching you'll get static checking if your matching is exhaustive. Impossible with an if ladder. Therefore they're not semantically equivalent.
type 'a myOption = Some of 'a | None
let f (Some x) = x*x
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched:
NoneYour solution with type and value checks in an if ladder was called "dynamic typing" exactly for this reason - you select paths dynamically in the runtime without any static checks on soundness of this selection.
I wouldn't. Pattern matching is primarly a conditional, that's why I focused on if-else.
> We're talking about language features semantically equivalent to pattern matching, and, turns out, there are none.
Why would you think that pattern matching revolves around compile-time guarantees? It does not. It's primarly a conditional, everything else is an optional, additional effect. You can have pattern matching in dynamically typed languages (Lisp, Erlang).
This is a very popular opinion, but I never see anyone taking the time to bring actual arguments. Do you have any experience hiring people or are you just spreading this seemingly self-evident claim?
If a language is popular, wouldn't more companies want to hire for it, which would make recruiting harder? See http://blog.activelylazy.co.uk/2013/05/27/choosing-a-program.... Also, being popular means that it is being known by a wider audience of people: isn't it harder to filter through all the applicants who say they can write in the popular language?
We also do javascript and, despite its popularity, we also get quite a few applicants who have it on their resume and have never used the language for anything useful.
Personally, my opinion is just hiring is hard. It is a very valid point though that if you are not willing to hire remote (which my company is not) you are very limited when trying to hire for less popular languages. Alternatively just train people, but that can be expensive.
That's a non-sequitur! The "unpopular languages make it harder to hire programmers" myth has been debunked many times right here on HN. Just hire smart people, not blub programmers.
Disclosure: I'm a functional programmer myself so, what do I know.
Some evidence this is just not true: http://gazagnaire.org/pub/SSGM10.pdf
The type of people who go about learning OCaml are arguably on the whole more competent and passionate than your average run of the mill Java programmer. Using OCaml might be a good way to separate the wheat from the chaff.
First-class functions
C# has closures, and can pass function pointers with delegates.
--
immutability is the default
Mutability can be a useful tool in some situations, but it is often unnecessary and potentially harmful.
I think this point has probably been argued to death by now, but I still can't see this as a good idea. 'Side effects' are really just a reality of software development. For example if I want to fade in a GUI element on a page, I don't want to create a new element for every transition change. Theres nothing wrong with mutable state, but it is nice to have immutability for some use cases. In this case c# has valuetypes, and the const/readonly modifiers.
--
Strong static type checking
Yep C# has had this since the beginning. And you can even use 'dynamic' if you really want which moves all type checking to runtime.
--
Algebraic data types and pattern-matching
I feel like these are more buzz words than actual substance. The Html example given also doesn't seem exactly clear to me how the operations are being performed. Here's a pseudo c# way of doing it:
rootHtmlElement
.SelfAndDescendants()
.OfType<DivElement>()
.Where(d => d.Attrs["class"] == "foo" || d.Attrs["class"] == "bar")
.Select(d => d.ChildElements.OfType<TextElement>().FirstOfDefault())
I assume they mean that all element types would inherit from a base IHtmlElement? This seems like a good thing to me.
--
Type inference
Again C# has these abilities with 'var a = 0' for example. It won't infer method return types, not because it couldn't, but because it's just generally clearer to have the explicit return type written there in the code for the reader.
--
a replay-capable debugger
C# has intelliTrace for this. Though I expect both suffer from the same problems, slow execution while in use, and huge memory footprint.
Um, both those concepts (algebraic data types and pattern matching) are very clearly defined, classic language features. Why would they be substanceless buzz words?
Here's a simple and oft-used example, which is option types. They correspond roughly to nulls so let's compare two blocks of pseudocode:
// traditional
if myValue == null:
print("nothing here!")
else:
print("value is " ++ myValue)
// with pattern-matching
switch myValue:
case None:
print("nothing here!")
case Some(thing):
print("value is " ++ thing)
It might seem weird on the surface but after a few times using it it's the simplest thing in the world and you'll wish you had it in other languages. In fact, Rust, which isn't (primarily anyway) a functional programming language, makes heavy use of pattern matching.
This is not a type inference. It is a mere type propagation.
http://stackoverflow.com/questions/479883/how-good-is-the-c-...
let f x = x + x
If you are interesting in studying and comparing programming languages, I suggest picking up one of the five big functional languages: F#, OCaml, Haskell, Scala, or Clojure. Scala and Clojure are great, they run on the JVM, they perform well, are practical and pragmatic, but are probably not the best way to learn "functional" programming because they include too many additional features or leave out some features that some consider important. I would recommend starting with Haskell and then comparing it to the others to see if one of these could be helpful in your own programming. There are good, free resources for learning functional programming, see [1].
Mutability is one side effect. OCaml is not Haskell and doesn't have a type system baked in. Sure, sometimes you need to cheat, but mutability as the general rule is crazyland. Immutability gives you a compile-time assurance that your data structures are consistent and valid, as long as the tests in your constructor function are correct.
> Yep C# has had this since the beginning. And you can even use 'dynamic' if you really want which moves all type checking to runtime.
Well, it has strong static type checking... within the bounds of what its type system supports. Which is fairly limited.
> I feel like these are more buzz words than actual substance.
Others have responded to that, but in short, your impression is wrong.
> Again C# has these abilities with 'var a = 0' for example. It won't infer method return types, not because it couldn't, but because it's just generally clearer to have the explicit return type written there in the code for the reader.
C#'s "type inference" is cute, but you can't seriously compare it with OCaml's.
First-class functions in C# are nice, nearly as good as those in OCaml. The main annoyance is the incompatibility between Action<> and Func<> (and, in general, the fact that C# uses void rather than unit).
--
Immutability: for every mutable class in our code base, there are three or four immutable classes, and `readonly` is likely the most frequent keyword. Among these, we have an entire hierarchy of dozens of classes that are entirely immutable (and make heavy use of immutable containers from Microsoft) because it's still the easiest way to implement a multi-reader, single-writer thread-safe data structure. You can survive without immutable-by-default, you can survive without OCaml's
{ record with a = b ; c = d }
{< self with a = b ; c = d >}
Algebraic data-types: I'm mostly writing compilers (yes, in C#, don't ask), and instead of writing a simple
| SetLineOp of param * param * index
--
Type inference: C# does manage to be fairly clever, although not as clever as OCaml. I have seldom found it to be lacking in general use, aside from the ability to define local functions (var f = a => { ... } does not work, you have to spell out the type of f). There have been a handful of times when I was trying to do something very clever and found myself limited by the type system, and had to write annotations myself.
And one time, I had to reimplement GADTs with generics and reflection. It was painful, but it works.
However, with a few minor tweaks (e.g. an option type), I would rather have the C# type inference than the OCaml one. The reason is that, if I want to do something very clever, I will not find myself limited to code that I can actually prove to the OCaml compiler as correct: I have, time and time again, resorted to reflection and code generation to work around such situations. In other words C#'s Obj.magic is a lot more powerful (and safe, and expressive) than OCaml's.
A fairly good example is Eliom's way of expressing the parameters of a service. In C# you would write in a PageController class
public Details Update(PageId id, UserId user, [PostBody] Details body)
In OCaml you have to understand the entire Eliom_parameter framework: https://ocsigen.org/eliom/4.2/api/server/Eliom_parameter Just think of the mental firepower needed to create that framework in the first place!