The List module in OCaml

map, filter, sort, fold_left: the daily toolkit.

(* The List module is the daily toolkit: map, filter, sort. *)
let survey = [("north", 12); ("ledge", 31); ("cove", 5)]

let () =
  survey
  |> List.sort (fun (_, a) (_, b) -> compare b a)
  |> List.iter (fun (pool, n) -> Printf.printf "%-6s %3d\n" pool n);

  let total = List.fold_left (fun acc (_, n) -> acc + n) 0 survey in
  Printf.printf "Total animals: %d\n" total;

  let busy = List.filter (fun (_, n) -> n > 10) survey in
  busy |> List.map fst |> String.concat ", "
       |> Printf.printf "Above ten: %s\n"

How it works

1. List.sort takes a comparator; flipping it sorts descending.
2. fold_left sums a field across tuples.
3. filter then map fst extracts the busy pool names.

Keywords and builtins used here

• fun
• in
• let

The run, in numbers

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Tokens

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Three-star pace

60 tpm

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Step 2 of 4 in Lists, step 12 of 29 in Language basics.

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The List module in other languages

• F#