injuly proposed a talk on defunctionalisation in compilers - this rehashes his post defunctionalisation, but in OCaml. We recommend you read his post first and switch to this article to compare the OCaml implementation.

Higher order functions​

let rec fold f acc = function
| [] -> acc
| x::xs -> f x (fold f acc xs)

let sum xs = fold (+) 0 xs
let add n xs = fold (fun x acc -> (x + n)::acc) [] xs

fold is the higher order function because it takes another function (possibly higher order), f.

The GADT representation of functions like function arguments​

type ('env, 'params, 'return) arrow =
  | FnPlus : int * int -> (unit, (int * int), int) arrow
  | FnPlusCons : int * (int * int list) -> (int, (int * int list), int list) arrow

('env, 'params, 'return) arrow encodes functions that can be passed around as arguments to other functions.

fold (+) 0 xs could be expressed as fold (FnPlus (0, xs)) fold (fun x acc -> (x + n :: acc) could be expressed as fold (FnPlusCons (n, (x, acc)) where n is the free variable.

These representations can be evaluated with apply

let apply : type env params return.  (env, params, return) arrow -> return = fun arrow ->
  match arrow with
  | FnPlus (a, b)  -> a + b
  | FnPlusCons (n, (x, xs)) -> (n + x)::xs

Example output

utop[41]> apply (FnPlus (1, 2));;
- : int = 3
utop[42]> apply (FnPlusCons (10, (1, [2;3])));;
- : int args = [11; 2; 3]
utop[43]> 

When is the next meetup?​

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We discussed what's needed by the package manager to make it's project bootstrap without itself.

Notably,

1. Trace the complete transitive closure of the dependencies - so package manager must build such dependencies in isolation to catching missing dependencies.

2. Trace the complete build environment variables

3. A parser for .install files that ocaml build systems create to install built artifacts

A PoC with esy package manager can be found here

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We discussed encoding the rules of the popular card game Monopoly Deal in OCaml's type system with @theteachr.

Highlights

• Briefly discussed the gameplay.
• Incrementally built modules around relevant data structures.
• Explored whether constructors could clash.
  • Initially, the general understanding was that it wouldn't work. Either the compilation would fail, or one of them would not be usable. But found out that it will just need some extra [type] hinting on some occasions to get the correct type.
• Focussed on the principal of making invalid states impossible to represent.
• Created an incomplete game state just to start getting some feedback.

You can find the code on Github

Once again, the discussion after lasted an hour. Srijan Paul shared how dithering in images can improve GIF color depths.

Next Meetup​

We will be continue last week's Canvas tutorial, lead by AC Sreedhar in a meetup. More details here

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Discord: https://discord.com/invite/RamP7SCKcU

We discussed Effect Handlers in OCaml 5 and how runtime switches stacks. You can find notes here.

1. Stack switching in OCaml 5
2. Deep Dive into OCaml effect handlers

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@Korven led the session explaining Tree sitter, how to write a grammar, debug tree-sitter-ocaml issies on helix. We even explored extending the Reason tree sitter grammar with a class definition. Here's the WIP snippet.

diff --git a/grammar.js b/grammar.js
index 9f587a4..7bc6748 100644
--- a/grammar.js
+++ b/grammar.js
@@ -56,6 +56,7 @@ module.exports = grammar(require("./embedded/ocaml"), {
         $.expression_statement,
         $.open_statement,
         $.module_definition,
+        $.class_definition,
         $.mutable_record_update,
       ),
 
@@ -71,6 +72,22 @@ module.exports = grammar(require("./embedded/ocaml"), {
         $._semicolon,
       ),
 
+    class_definition: ($) =>
+      seq(
+        "class",
+        // TODO optional($._attribute),
+        $.class_binding,
+        $._semicolon,
+      ),
+
+    class_binding: ($) =>
+      seq(
+        field("name", choice($._class_name, alias("_", $.class_name))),
+        // optional($._module_typed),
+        seq("=", field("body", $._module_expression)),
+        // repeat($.item_attribute),
+      ),
+
     module_binding: ($) =>
       seq(
         field("name", choice($._module_name, alias("_", $.module_name))),

Next Meetup​

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@theteachr showed us the new game loop using Seq.of_dispenser making the game loop look like this.

let () =
  let game =
    [ "ocaml"; "reason"; "melange"; "dune" ]
    |> List.map Player.make
    |> Game.start
  in
  read_command
  |> Seq.of_dispenser
  |> Seq.filter_map Command.parse
  |> Seq.scan Command.exec game
  |> Seq.take_while Game.is_not_over
  |> Seq.iter Tui.draw

Highlights

We discussed,

1. Possible use of GADTs for handling propert card transformations. You can find the git branch here
2. use of Seq.unfold. Code
3. Modular card transformations. Code

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@theteachr showed us the minimal TUI he wrote for the game and explained the game loop.

Highlights

1. We discussed how game loop had to be kept a pure function of game state and player moves
2. Possible improvements with a Seq modules

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We discussed the last pieces essential for a web framework - routers. Last session, we had written a router that worked only in the client. We ported for server side rendering.

Highlights

1. We discussed CSS rendering that occurs on the server on different routes
2. We discussed the possibility of writing static site generators with the knowledge so far

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We discussed the gameplay, possible UI for the game, and went over the stack to implement the game.

Highlights

1. I setup Melange for @theteachr's repository. You can find the pull request here
2. @theteachr explained the codebase and explained interesting abstractions used.
3. Possiblilty of use of Lenses and how it improves abstractions
4. Release of new winget package for opam - OCaml.opam

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AC Sreedhar Reddy led the session on HTML5 canvas tutorial in Reason. You can find the code (and specifically the commit) here. Follow the readme in the repo to setup the project.

Other topics that were discussed included,

1. The recent modular explicits PR upstream
2. Pure FP graphics API
3. Persistent datastructures
4. Avoiding invalid states (ghost types etc)

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