Hashtbl and assoc lists in OCaml

The mutable map, and the immutable habit for small data.

(* Hashtbl is the mutable map; assoc lists are the immutable habit. *)
let tally = Hashtbl.create 8

let () =
  Hashtbl.replace tally "limpet" 40;
  Hashtbl.replace tally "anemone" 12;
  Hashtbl.replace tally "limpet" 43;  (* replace overwrites *)
  Printf.printf "Limpets: %d\n" (Hashtbl.find tally "limpet");
  Printf.printf "Species tracked: %d\n" (Hashtbl.length tally);

  (* An assoc list does the same job immutably for small data. *)
  let notes = [("north", "clear"); ("ledge", "surge")] in
  match List.assoc_opt "ledge" notes with
  | Some note -> print_endline ("Ledge: " ^ note)
  | None -> print_endline "Ledge unsampled"

How it works

1. Hashtbl.replace writes; a second replace overwrites.
2. Hashtbl.find throws; find_opt returns an option.
3. List.assoc_opt reads an association list safely.

Keywords and builtins used here

• None
• Some
• in
• let
• match
• with

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Step 1 of 4 in Collections, step 18 of 29 in Language basics.

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