Recursion in OCaml

rec allows self-reference; accumulators keep calls in tail position.

(* rec allows self-reference; an accumulator keeps the call a tail. *)
let rec count_animals acc = function
  | [] -> acc
  | n :: rest -> count_animals (acc + n) rest

(* Mutual recursion joins definitions with and. *)
let rec ebbs n = if n = 0 then "slack" else flows (n - 1)
and flows n = if n = 0 then "running" else ebbs (n - 1)

let () =
  Printf.printf "Survey total: %d\n" (count_animals 0 [12; 5; 31]);
  Printf.printf "Tide after 6 turns: %s\n" (ebbs 6);
  Printf.printf "Tide after 7 turns: %s\n" (ebbs 7)

How it works

1. let rec count_animals acc folds by hand, stack-safe.
2. function matches the list argument directly.
3. and joins mutually recursive definitions.

Keywords and builtins used here

• and
• else
• function
• if
• let
• rec
• then

The run, in numbers

Lines

13

Characters to type

506

Tokens

112

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 103 seconds.

Type this snippet

Step 3 of 3 in Idioms, step 26 of 29 in Language basics.

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Recursion in other languages

• C
• Erlang
• F#
• Python
• WebAssembly