Meetup 23: Introduction to using Buck2 with OCaml

Shubham Kumar lead the discussion on using OCaml with Buck2.

Additionally, Patate started a discussion about how monads unify exceptions with regular values. They also unify IO with regular values. My take, adding to his point, was how monads let us parameterise the execution model - they help us detach the meaning of computations from their representation and simply tell us what computation depends on what. The execution model, which could be swapped, then decides if it's an IO or error handling.

Here's a snippet that Patate graciously contributed

(* The program we are interested in is below.
 * here are some preliminary definitions *)

(* IO monad *)
module IO: sig
  type 'a t
  val return: 'a -> 'a t
  val bind: 'a t -> ('a -> 'b t) -> 'b t
  val both: 'a t -> 'b t -> ('a * 'b) t
  val wrap: (unit -> 'a) -> 'a t
  val run: 'a t -> 'a
end = struct
  type 'a t = unit -> 'a
  let return x () = x
  let bind io f () = f (io ()) ()
  let both a b () = let a_result = a () in a_result, b ()
  let wrap = Fun.id
  let run io = io ()
end

(* Identity monad *)
module Id = struct
  let return = Fun.id
  let bind = ( |> )
  let both a b = a, b
  let run = Fun.id
end

(* ignores are for avoiding "unused" warnings *)
let () = ignore IO.(return, bind, both, wrap, run)
let () = ignore Id.(return, bind, both, run)

module Color = struct
  type t = Red | Green | Blue

  let to_string = function
    | Red -> "red"
    | Green -> "green"
    | Blue -> "blue"

  let of_string = function
    | "red" -> Red
    | "green" -> Green
    | "blue" -> Blue
    | s -> raise @@ Invalid_argument ("string_of_color \"" ^ s ^ "\"")
end

module IO_behaviour = struct
  include IO
  let get_name = IO.wrap (fun () -> input_line stdin)
  let get_age = IO.wrap (fun () -> input_line stdin |> int_of_string)
  let get_favorite_color = IO.wrap (fun () -> input_line stdin |> String.trim |> Color.of_string)
end
module Normal_behaviour = struct
  include Id
  let get_name = "patate"
  let get_age = 42
  let get_favorite_color = Color.Blue
end
let () = ignore (IO_behaviour.(get_name, get_age, get_favorite_color), Normal_behaviour.(get_name, get_age, get_favorite_color))
open Normal_behaviour
let () = ignore (get_name, get_age, get_favorite_color)

(* Program starts here *)

module Behaviour = Normal_behaviour
(* module Behaviour = Normal_behaviour *)

let ( let* ) = Behaviour.bind
let ( and* ) = Behaviour.both
let () = ignore (( let* ), ( and* ))

let result =
  (*let open Behaviour in*)
  let name = get_name
  and age = get_age
  and favorite_color_string = get_favorite_color in
  (*Behaviour.return
  @@*) Printf.sprintf
    "%s is %dyo and likes %s"
    name
    age
    (Color.to_string favorite_color_string)

let () = print_endline ((*Behaviour.run*) result)

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