package miniyaml

MiniYaml — A simple YAML parser in pure OCaml

MiniYaml reads and writes the subset of YAML that people actually hand-write in configuration files: block mappings and sequences, inline flow collections, quoted scalars and comments.

It is written in pure OCaml and has no dependencies — not even a lexer or parser generator. The whole library is a single module of about 650 lines.

Installation

opam install miniyaml

The opam package and the dune library are named miniyaml, the module is Yaml:

(executable
 (name my_program)
 (libraries miniyaml))

Usage

let () =
  match
    Yaml.of_string
      {|
name: miniyaml
tags: [yaml, parser]
server:
  host: localhost
  port: 8080
|}
  with
  | Ok v -> print_string (Yaml.to_string v)
  | Error e -> prerr_endline e

which prints

name: miniyaml
tags:
  - yaml
  - parser
server:
  host: localhost
  port: 8080

Values are built and inspected directly:

Yaml.to_string (Assoc [ "ports", List [ Float 80.; Float 443. ] ])
(* "ports:\n  - 80\n  - 443\n" *)

API

type t =
  | Null
  | Bool of bool
  | Float of float          (** any number, integers included *)
  | String of string
  | List of t list
  | Assoc of (string * t) list

val of_string : string -> (t, string) result
val to_string : t -> string

Assoc entries keep the order in which they occur, and duplicate keys are preserved. Errors are reported with the line at which they were detected, for instance line 3: unexpected content.

Printing and parsing are inverse to each other: of_string (to_string v) = Ok v holds for every value v. Strings that would otherwise be read back as something else are quoted on output, so String "true", String "1.5" and String "" survive the trip.

Supported subset

• block mappings (key: value) and sequences (- item), nested by indentation, including sequences written at the indentation of their key and entries written compactly after a dash (- name: alice with further entries below);
• flow collections, [1, 2] and {a: b}, nested and mixed with block style;
• plain, single-quoted ('' escapes a quote) and double-quoted scalars, the latter with the usual escapes (\n, \t, \uXXXX, ...);
• comments, and the --- / ... document markers;
• the plain scalars ~, null, true, false, .inf, .nan and decimal numbers.

The following are not supported, and are rejected with an explicit error rather than misparsed: block scalars (| and >), anchors and aliases (&, *), tags (!), directives (%), complex mapping keys (?), multi-document streams, and tabulations used for indentation. Flow collections have to fit on a single line.

Building

dune build
dune runtest

Tool disclosure

Most of the project was generated with Claude code (Opus 5).

License

MiniYaml is distributed under the GNU General Public License version 3, see LICENSE.