YAMLx

A pure-OCaml YAML 1.2 and 1.1 library with a lossless, comment-preserving AST.

This is AI-assisted software, fully owned and maintained by Martin Jambon.

👉 API Documentation

Features

• Full YAML 1.2 compliance — passes all 371 tests from the yaml-test-suite.
• Pure OCaml — no C bindings, no external runtime dependencies.
• Lossless AST — the parsed node tree preserves scalar styles (plain, 'single-quoted', "double-quoted", | literal, > folded), flow vs. block collection style, tags, anchors, and source positions.
• Best-effort comment preservation — standalone (head) comments before a node, inline (line) comments after a value, and trailing (foot) comments after the last item of a block collection are attached to the nearest node and re-emitted by the printer.
• Pretty-printer — Nodes.to_yaml serializes a node list back to a YAML string, preserving all of the above.
• Plain-YAML printer — Nodes.to_plain_yaml_exn produces a restricted subset with no anchors, no aliases (expanded inline), no tags, no flow collections, and no complex mapping keys — the fragment of YAML that most people recognize on sight.
• Typed-value resolver — Values.of_yaml applies the YAML 1.2 JSON schema and returns value list with Null | Bool | Int | Float | String | Seq | Map constructors.
• Programmatic value construction — Value.Build provides convenience constructors (null, bool, int, float, string, seq, map) that fill in zero_loc automatically, for building value trees without a source file.
• Duplicate-key detection on export — Values.to_yaml and friends raise Duplicate_key_error by default when a map contains repeated keys, catching bugs in programmatically-constructed values before they reach the output. Pass ~strict_keys:false to allow duplicates through.
• Multi-document streams — both the node and value APIs handle streams containing more than one ----separated document.
• Correct anchor scoping — anchors are document-local; an alias in document N cannot refer to an anchor defined in document N−1.
• Structured errors — Scan_error and Parse_error carry a pos record with line, column, and byte offset. catch_errors wraps any of these into a (_, string) result with a human-readable message, optionally prefixed with a file name.
• Command-line tool — the yamlx binary reads YAML from a file or stdin and prints it in one of several formats (see below).

Quick start

(* Single-document config file — most common pattern *)
match YAMLx.Value.of_yaml_file "config.yaml" with
| Ok value  -> ...
| Error msg -> prerr_endline msg  (* "file config.yaml, line 3, col 5: ..." *)

(* Parse a YAML string into a single typed value *)
match YAMLx.Value.of_yaml "answer: 42\nflag: true" with
| Ok (Map (_, [(_, String (_, "answer"), Int (_, 42L));
               (_, String (_, "flag"),   Bool (_, true))])) -> ...
| _ -> ...

(* Multi-document stream *)
match YAMLx.Values.of_yaml input with
| Ok values -> ...
| Error msg -> ...

(* Build a value tree programmatically and serialize it *)
let v =
  YAMLx.Value.Build.(
    map [ "name", string "Alice"; "scores", seq [ int 95; int 87 ] ])
in
print_string (YAMLx.Value.to_yaml v)

(* Round-trip through the lossless AST *)
match YAMLx.Nodes.of_yaml input with
| Ok nodes -> print_string (YAMLx.Nodes.to_yaml nodes)
| Error msg -> ...

(* Strip YAML-specific features *)
match YAMLx.Nodes.of_yaml input with
| Ok nodes ->
    (match YAMLx.catch_errors (fun () ->
         YAMLx.Nodes.to_plain_yaml_exn nodes) with
    | Ok plain -> print_string plain
    | Error msg -> ...)
| Error msg -> ...

Command-line tool

yamlx [-f FORMAT] [--schema VERSION] [FILE]

Output formats (-f FORMAT):
  yaml         Pretty-printed YAML — scalar styles and block/flow mode
               preserved (default)
  plain        Simplified YAML — aliases expanded, tags stripped, flow
               collections converted to block; merge keys expanded in
               YAML 1.1 mode
  reformat     Normalized YAML — reads input as typed values, then
               re-serializes. Drops comments, anchors, and tags. Converts
               flow collections to block. Long strings use literal (|) or
               folded (>) block style as appropriate.
  value        Typed-value tree: Null / Bool / Int / Float / String / Seq /
               Map. Useful for checking how scalars are resolved.
  value-loc    Same as value but with source locations
  node         Full AST without source locations or heights
  node-loc     Same as node but with source locations and heights
  events       yaml-test-suite event-tree notation (mainly for parser testing)

YAML schema (--schema VERSION):
  1.2  YAML 1.2 JSON schema — default. Booleans: true/false only.
  1.1  YAML 1.1 schema — extended booleans (yes/no/on/off), 0755-style
       octal, sexagesimal, merge keys (<<).

Options:
  --strict          With -f plain: error on tags instead of stripping them
  --strict-schema   Error if the document's %YAML directive disagrees with
                    --schema
  --reject-ambiguous
                    With --schema 1.2: error on plain scalars that would
                    resolve differently under YAML 1.1 (e.g. yes, 0755, <<)
  --plain           With -f value or value-loc: error on anchors, aliases,
                    explicit tags, or (with --schema 1.1) merge keys
  --strict-keys     With -f value or value-loc: error on duplicate mapping
                    keys instead of silently keeping the last occurrence

Comment preservation

Comments are captured on a best-effort basis during scanning and attached to the AST after parsing:

# head comment (attached to the node that follows)
key: value  # line comment (attached to the scalar)
list:
  - a
  - b
  # foot comment (attached to the sequence, after the last item)

Comments inside flow collections ([...], {...}) and on block scalar header lines (key: | # this) are not captured.

Pipeline

Reader → Scanner → Parser → Composer → Resolver

• Reader — UTF-8 / UTF-16 / UTF-32 input normalization.
• Scanner — tokenization, indentation, flow-level tracking, comment capture.
• Parser — token stream → event stream (YAML grammar).
• Composer — events → node graph (anchor/alias resolution).
• Resolver — nodes → typed value tree (YAML 1.2 JSON schema).

License

YAMLx is currently released under the AGPL. There is an ongoing fundraiser: once a funding goal is reached, the license will switch to the permissive ISC license for everyone. Donors above a certain threshold receive an immediate commercial license. See FUNDING.md for details.

References

• YAML 1.2 specification (1.2.2)
• YAML 1.1 specification
• Anil Madhavapeddy's AoAH Day 6: Getting a YAML 1.2 implementation in pure OCaml