OCaml as my primary language

No way OCaml could have stolen the Rust's thunder: we have a number of very decent and performant GC-based languages, from Go to Haskell; we only had one bare-metal-worthy expressive language in 2010, C++, and it was pretty terrible (still is, but before C++11 and C++17 it was even more terrible).

Arguably, that could have been Scala and for a while it seemed like it would be Scala but then it kind of just... didn't.

I suspect some of that was that the programming style of some high profile Scala packages really alienated people by pushing the type system and operator overloading much farther than necessary.

I think they have been optional for like 20 years, except in the top-level interactive environment to force execution.

That being said, I still don't get why people are so much upset with the syntax. You'll integrate it after a week writing OCaml code.

The great thing is we have choice. We have a huge number of ways to express ideas and ... do them!

I might draw a parallel with the number of spoken languages extent in the UK (only ~65M people). You are probably familiar with English. There are rather a lot more languages here. Irish, Scottish, Welsh - these are the thriving Brythonic languages (and they probably have some sub-types). Cornish formally died out in the sixties (the last two sisters that spoke it natively, passed away) but it has been revived by some locals and given that living people who could communicate with relos with first hand experience, I think we can count that a language that is largely saved. Cumbric ... counting still used by shepherds - something like: yan, tan, tithera toe.

I am looking at OCAML because I'm the next generation to worry about genealogy in my family and my uncle has picked Geneweb to store the data, taking over from TMG - a Windows app. His database contains roughly 140,000 individuals. Geneweb is programmed in OCAML.

If you think that programming languages are complicated ... have a go at genealogy. You will soon discover something called GEDCOM and then you will weep!

That's about the time-frame where I got into OCaml so I followed this up close.

The biggest hindrance in my opinion is/was boxed types.

Too much overhead for low level stuff, although there was a guy from oxbridge doing GL stuff with it.

No, that wouldn't have made the difference. No-one didn't pick up OCaml because it didn't have multicore or they were annoyed by the semicolons.

People don't switch languages because the new language is "old language but better". They switch languages because a) new language does some unique thing that the old language didn't do, even if the unique thing is useless and irrelevant, b) new language has a bigger number than old language on benchmarks, even if the benchmark is irrelevant to your use case, or c) new language claims to have great interop with old language, even if this is a lie.

There is no way OCaml could have succeeded in the pop culture that is programming language popularity. Yes, all of the actual reasons to use Rust apply just as much to OCaml and if our industry operated on technical merit we would have migrated to OCaml decades ago. But it doesn't so we didn't.

Haskell of course has some of this, but immutability means that Haskell doesn't have to have answers for lots of things. And you want pattern matching as your basic building block, but at the end of the day most of your code won't have pattern matching and will instead rely on higher level patterns (that can build off of ADTs providing some degree of certainty on totality etc)

I am no strage to ML type systems, my first one was Caml Light, OCaml was still known as Objective Caml, and Mirada was still something being discussed on programming language lectures on my university.

From what I see, I also kind of find the same, too many people rush out for Rust thinking that ML type systems is something new introduced by Rust, without having the background where all comes from.

Just Arc, clone and panic your way to success! :)

* OPAM is quite buggy and extremely confusing.

* Windows support is very bad. If you ever tried to use Perl on Windows back in the day... it's worse than that.

* Documentation is terse to the point of uselessness.

* The syntax style is quite hard to mentally parse and also not very recoverable. If you miss some word or character the error can be "the second half of the file has a syntax error". Not very fun. Rust's more traditional syntax is much easier to deal with.

Rust basically has none of those issues. Really the only advantage I can see with OCaml today is compile time, which is important, but it's definitely not important enough to make me want to use OCaml.

OCaml had its act together. It was significantly nicer than Python when I used it professionally in 2010. Just look at what JaneStreet achieved with it.

The main impediment to OCaml was always that it was not American nor mainly developed from the US.

People like to believe there is some technical merit to language popularity but the reality it’s all fashion based. Rust is popular because they did a ton of outreach. They used to pay someone full time to mostly toot their horn.

Granted, this may just be an argument for being more comfortable reading/writing code in a particular style, but even without the advantages of LLMs adoption of functional paradigms and tools has been a struggle.

And then I didn't even make this "experiment" with Java or another managed, more imperative language which could have shed some weight due to not caring about manual memory management.

So not sure how much truth is in there - I think it differs based on the given program: some lend itself better for an imperative style, others prefer a more functional one.

Aside from the obvious problem that there's not enough FP in the training corpus, it seems like terser languages don't work all that well with LLMs.

My guess is that verbosity actually helps the generation self-correct... if it predicts some "bad" tokens it can pivot more easily and still produce working code.

Claude Code’s Haskell style is very verbose; if-then-elsey, lots of nested case-ofs, do-blocks at multiple levels of intension, very little naming things at top-level.

Given a sample of a simple API client, and a request to do the same but for another API, it did very well.

I concluded that I just have more opinions about Haskell than Java or Rust. If it doesn’t look nice, why even bother with Haskell.

I reckon that you could seed it with style examples that take up very little context space. Also, remind it to not enable language pragmas per file when they’re already in .cabal, and similar.

My experience in the past with something like cats-effect has been that there are straightforward things that aren't obvious, and if you haven't been using it recently, and maybe even if you've been using it but haven't solved a similar problem recently, you can get stuck trawling through the docs squinting at type signatures looking for what turns out to be, in hindsight, an elegant and simple solution. LLMs have vastly reduced this kind of friction. I just ask, "In cats-effect, how do I...?" and 80% of the time the answer gets me immediately unstuck. The other 20% of the time I provide clarifying context or ask a different LLM.

I haven't done enough maintenance coding yet to know if this will radically shift my view of the cost/benefit of functional programming with effects, but I'm very excited. Writing cats-effect code has always been satisfying and frustrating in equal measure, and so far, I'm getting the confidence and correctness with a fraction of the frustration.

I haven't unleashed Claude Code on any cats-effect code yet. I'm curious to see how well it will do.

For instance, dependent types allow us to say something like "this function will return a sorted list", or even "this function will return a valid Sudoku solution", and these things will be checked at compile time--again, at compile time.

Combine this with an effect system and we can suddenly say things like "this function will return a valid Sudoku solution, and it will not access the network or filesystem", and then you let the LLM run wild. You don't even have to review the LLM output, if it produces code that compiles, you know it works, and you know it doesn't access the network or filesystem.

Of course, if LLMs get a lot better, they can probably just do all this in Python just as well, but if they only get a little better, then we might want to build better deterministic systems around the unreliable LLMs to make them reliable.

Rather, immutability/purity is a huge advantage because it plays better with the small context window of LLM's. An LLM then doesn't have to worry about side effects or mutable references to data outside the scope currently being considered.

The expressive type system catches a lot of mistakes, and the fact that they are compile errors which can be fed right into the LLM again means that incorrect code is caught early.

The second is property based testing. With it I have had the LLM generate amazingly efficient, correct code, by iteratively making it more and more efficient – running quickcheck on each pass. The LLM is not super good at writing the tests, but if you add some yourself, you quickly root out any mistakes in the generated code.

Python does it best from what I've seen so far, with its __repr__ method.

But OCaml sadly can't replace F# for all my use cases. F# does get access to many performance-oriented features that the CLR supports and OCaml simply can't, such as value-types. Maybe OxCaml can fix that long term, but I'm currently missing a performant ML-like with a simple toolchain.

And the best way I can describe why is that my code generally ends up with a few heavy functions that do too much; I can fix it once I notice it, but that's the direction my code tends to go in.

In my OCaml code, I would look for the big function and... just not find it. No single workhorse that does a lot - for some reason it was just easier for me to write good code.

Now I do Rust for side projects because I like the type system - but I would prefer OCaml.

I keep meaning to checkout F# though for all of these reasons.

Do you have a ballpark value of how much faster Rust is? Also I wonder if OxCaml will be roughly as fast with less effort.

OCaml does have an okay LSP implementation though, and it's getting better; certainly more stable than F#'s in my experience, since that comparison is coming up a lot in this comment section.

A sum type has as many possible values as the sum of its cases. E.g. `A of bool | B of bool` has 2+2=4 values. Similarly for product types and exponential types. E.g. the type bool -> bool has 2^2=4 values (id, not, const true, const false) if you don't think about side effects.

Here is my uneducated guess:

In math, after sum and product, comes exponent :)

So they may have used that third term in an analogous manner in the example.

The type A → (B → C) is isomorphic to (A × B) → C (via currying). This is analogous to the rule (cᵇ)ᵃ = cᵇ˙ᵃ.

The type (A + B) → C is isomorphic to (A → C) × (B → C) (a function with a case expression can be replaced with a pair of functions). This is analogous to the rule cᵃ⁺ᵇ = cᵃ·cᵇ.

Correct. This is not the case when you talk about Java/Kotlin. Just ugliness and typical boilerplate heavy approach of JVM languages.

A case of a sum-type is an expression (of the variety so-called a type constructor), of course it has a type.

  datatype shape =
      Circle of real
    | Rectangle of real * real
    | Point

   Circle : real -> shape
   Rectangle : real * real -> shape
   Point : () -> shape

The moment you start ripping cases as distinct types out of the sum-type, you create the ability to side-step exhaustiveness and sum-types become useless in making invalid program states unrepresentable. They're also no longer sum-types. If you have a sum-type of nominally distinct types, the sum-type is contingent on the existence of those types. In a class hierarchy, this relationship is bizarrely reversed and there are knock-on effects to that.

> You declare the sum type once, and use it many times.

And you typically write many sum-types. They're disposable. And more to the point, you also have to read the code you write. The cost of verbosity here is underestimated.

> Slightly more verbose sum type declaration is worth it when it makes using the cases cleaner.

C#/Java don't actually have sum-types. It's an incompatible formalism with their type systems.

Anyways, let's look at these examples:

C#:

  public abstract record Shape;
  public sealed record Circle(double Radius) : Shape;
  public sealed record Rectangle(double Width, double Height) : Shape;
  public sealed record Point() : Shape;
  
  double Area(Shape shape) => shape switch
  {
      Circle c => Math.PI * c.Radius * c.Radius,
      Rectangle r => r.Width * r.Height,
      Point => 0.0,
      _ => throw new ArgumentException("Unknown shape", nameof(shape))
  };

  datatype shape =
      Circle of real
    | Rectangle of real * real
    | Point
  
  val result =
    case shape of
        Circle r => Math.pi * r * r
      | Rectangle (w, h) => w * h
      | Point => 0.0

Operationally these systems and philosophies are quite different, but mathematically we are all working in more work less an equivalent category and all the type system shenanigans you have in FP are possible in OOP modulo explicit limits placed on the language and vice versa.

ReasonML has custom operators that allows for manipulating monads somewhat sanely (>>= operators and whatnot). rescript (reasonml’s “fork”) did not last time I checked. But it does have an async/await syntax which helps a lot with async code. reasonml did not last time I checked, so you had to use raw promises.

I believe Melange (which the article briefly talks about) supports let bindings with the reason syntax.

And this kinda changes everything if you React. Because you can now have sane JSX with let bindings. Which you could not until melange. Indeed, you can PPX your way out of it in ocaml syntax, but I’m not sure the syntax highlight works well in code editors. It did not on mine anyway last time I checked.

So for frontend coding, Melange’s reason ml is great as you have both, and let bindings can approximate quite well async syntax on top of writing readable monadic code.

For backend code, as a pythonista, I hate curlies. and I do like parenthesis-less function calls and definitions a lot. But I still have a lot of trouble, as a beginner ocamler, with non-variable function argument as I need to do “weird” parenthesis stuff.

Hope this “helps”!

> The idea that the example program could use pattern matching to bind to either test values or production ones is interesting, but I can’t conceptualize what that would look like with the verbal description alone.

The article appears to have described the free monad + interpreter pattern, that is, each business-logic statement doesn't execute the action (as a verb), but instead constructs it as a noun and slots it into some kind of AST. Once you have an AST you can execute it with either a ProdAstVisitor or a TestAstVisitor which will carry out the commands for real.

More specific to your question, it sounds like the pattern matching you mentioned is choosing between Test.ReadFile and Test.WriteFile at each node of the AST (not between Test.ReadFile and Prod.ReadFile.)

I think the Haskell community turned away a little from free monad + interpreter when it was pointed out that the 'tagless final' approach does the same thing with less ceremory, by just using typeclasses.

> I’d have liked to see the use of dependency injection via the effects system expanded upon.

I'm currently doing DI via effects, and I found a technique I'm super happy with:

At the lowest level, I have a bunch of classes & functions which I call capabilities, e.g

  FileOps (readTextFile, writeTextFile, ...)
  Logger (info, warn, err, ...)
  Restful (postJsonBody, ...)

At the next level up I have classes & functions which can know about the business (and the lower level capabilities)

  StoredCommands (fetchStoredCommands) - this uses the 'Restful' capability above to construct and send a payload to our business servers.

I associate CliApp to all its actual implementations (low-level and mid-level) using type classes:

  instance FileOps CliApp where
    readTextFile  = readTextFileImpl
    writeTextFile = writeTextFileImpl
    ...

  instance Logger CliApp where
    info = infoImpl
    warn = warnImpl
    err  = errImpl
    ...

  instance StoredCommands CliApp where
    fetchStoredCommands = fetchStoredCommandsImpl
    ...

I can create a CliTestApp which has a different set of bindings, e.g.

  instance Logger CliTestApp where
    info msg = -- maybe store message using in-memory list so I can assert on it?

The low-level prod code (capabilites) are allowed to do IO, as signaled by the MonadIO in the type sig:

  readTextFileImpl :: MonadIO m => FilePath -> m (Either String Text)

  readTextFileTest :: Monad m => FilePath -> m (Either String Text)

Another way of looking at it is, if I invent a new capability (e.g. Caching) and start calling it from my business logic, the CliTestApp pointing at that same business logic will compile-time error that it doesn't have its own Caching implementation. If I try to 'cheat' by wiring the CliTestApp to the prod Caching (which would make my test cases non-deterministic) I'll get another compile-time error.

Would it work in OCaml? Not sure, the article says:

> Currently, it should be noted that effect propagation is not tracked by the type system

Use opam: https://opam.ocaml.org or https://opam.ocaml.org/doc/Install.html.

Additionally, see: https://ocaml.org/install#linux_mac_bsd and https://ocaml.org/docs/set-up-editor.

It is easy to set up with Emacs, for example. VSCodium has OCaml extension as well.

All you need for the OCaml compiler is opam, it handles all the packages and the compiler.

For your project, use dune: https://dune.readthedocs.io/en/stable/quick-start.html.

Even the object system which most OCaml developers avoid, is actually very useful for some specific modelling scenarios (usually hierarchies in GUIs or IaC) that comes with similar type system guarantees and minimal type annotations.

I actually really like the syntax of OCaml, its very easy to write and when you're used to it, easy to read (easier than reasonml IMO).

Double semicolons are afaik only used in the repl.

The syntax is `let <constantname> <parameters> = <value> in <expression>;;` where `expression` can also have `let` bindings.

So you can have

  let one = 1 in
  let two = 2 in
  let three = one + two in
  print_endline (string_of_int three);;

YMMV but let expressions are one of the nice things about OCaml - the syntax is very clean in a way other languages aren't. Yes, the OCaml syntax has some warts, but let bindings aren't one of them.

It's also quite elegant if you consider how multi-argument let can be decomposed into repeated function application, and how that naturally leads to features like currying.

> Also, you need to end the declaration with `in`?

Not if it's a top level declaration.

It might make more sense if you think of the `in` as a scope operator, eg `let x = v in expr` makes `x` available in `expr`.

> Then, semicolons...

Single semicolons are syntactic sugar for unit return values. Eg,

  print_string "abc";

  let () = print_string "abc" in

This doesn't look or feel all that different to me:

  enum Expr {
      Const(f64),
      Var(String),
      Sum(Box<Expr>, Box<Expr>),
      Diff(Box<Expr>, Box<Expr>),
      Prod(Box<Expr>, Box<Expr>),
      Quot(Box<Expr>, Box<Expr>),
  }

  fn print_expr(exp: &Expr) {
      fn open_paren(prec: i32, op_prec: i32) {
          if prec > op_prec {
              print!("(");
          }
      }

      fn close_paren(prec: i32, op_prec: i32) {
          if prec > op_prec {
              print!(")");
          }
      }

      fn print_rec(prec: i32, exp: &Expr) {
          match exp {
              Expr::Const(c) => print!("{}", c),
              Expr::Var(v) => print!("{}", v),
              Expr::Sum(f, g) => {
                  open_paren(prec, 0);
                  print_rec(0, f);
                  print!(" + ");
                  print_rec(0, g);
                  close_paren(prec, 0);
              }
              Expr::Diff(f, g) => {
                  open_paren(prec, 0);
                  print_rec(0, f);
                  print!(" - ");
                  print_rec(1, g);
                  close_paren(prec, 0);
              }
              Expr::Prod(f, g) => {
                  open_paren(prec, 2);
                  print_rec(2, f);
                  print!(" * ");
                  print_rec(2, g);
                  close_paren(prec, 2);
              }
              Expr::Quot(f, g) => {
                  open_paren(prec, 2);
                  print_rec(2, f);
                  print!(" / ");
                  print_rec(3, g);
                  close_paren(prec, 2);
              }
          }
      }

      print_rec(0, exp);
  }

Syntax is one of those funny things. The more you look at it, the more sense it makes. The more you learn about it and the reasons behind why it's designed that way–the more sense it makes.

> Also, you need to end the declaration with `in`?

Not all of them, just the ones that need to refer to previously defined bindings. So eg you could do:

    let print_expr exp =
      (* Local function definitions *)
      let open_paren prec op_prec = if prec > op_prec then print_string "("
      and close_paren prec op_prec = if prec > op_prec then print_string ")"
      in
      let rec print prec = function (* prec is the current precedence *)
        | Const c -> print_float c
        | Var v -> print_string v
        | Sum (f, g) ->
          open_paren prec 0;
          print 0 f; print_string " + "; print 0 g;
          close_paren prec 0
        | Diff (f, g) ->
          open_paren prec 0;
          print 0 f; print_string " - "; print 1 g;
          close_paren prec 0
        | Prod (f, g) ->
          open_paren prec 2;
          print 2 f; print_string " * "; print 2 g;
          close_paren prec 2
        | Quot (f, g) ->
          open_paren prec 2;
          print 2 f; print_string " / "; print 3 g;
          close_paren prec 2
      in
      print 0 exp

Modules are like structurally-typed records that can contain both abstract types and values/functions dependent on those types; every implementation file is itself a module. When passed to functors (module-level functions), they allow you to parameterize large pieces of code, depending on multiple types and functions, all at once quite cleanly. And simply including them or narrowing their signatures is how one exports library APIs.

(The closest equivalent I can imagine to module signatures is Scala traits with abstract type members, but structurally-typed and every package is an instance.)

However, they are a bit too verbose for finer-grained generics. For example, a map with string keys needs `module String_map = Map.Make(String)`. There is limited support for passing modules as first-class values with less ceremony, hopefully with more on the way.

No. Unless you also want to be the maintainer of a UI bindings library.

Could you elaborate on the difference? I was under the impression that "latent typing" just means "values, not variables, have types", which would make Python (without type annotations) latently typed as well.

(Sort of like how OOP was introduced to solve the problem of structuring code, but then we needed a profusion of ever-more-complex design patterns to fix the issues only ever introduced by OOP in the first place.)

I'm coming to be a big fan of functional dynamic languages. You can just let go of entire categories of CS complexity. No need for generics without types, no need for locks without mutability, etc. Actual fearless concurrency.

This in turn has freed me to write actual business logic, without the constant need to stop and make sacrifices to the CS complexity god.

F# has FuncUI - based on Avalonia. All just possible because of the ecosystem.

https://github.com/fsprojects/Avalonia.FuncUI

- No HKTs "in your sense" but: ```ocaml module type S = sig type 'a t end `` `type 'a t` is an Higher Kinded type (but in the module Level). - No typeclasses, yes, for the moment but the first step of https://arxiv.org/pdf/1512.01895 is under review: https://github.com/ocaml/ocaml/pull/13275 - no call-site expansion ? https://ocaml.org/manual/5.0/attributes.html look at the attribute `inline`.

Yes, F# is a very nice language, however, it seems to me that I am making a somewhat forced comparison between OCaml and F# in the following section: https://xvw.lol/en/articles/why-ocaml.html#ocaml-and-f