Formata

Composable data codecs, schemas, and validation for Elixir.

Formata decodes and encodes JSON, TOML, and YAML, validates data against native schemas (Formata.Backplane.Schema) or JSON Schema documents (Draft 7 and Draft 2020-12, via Formata.Backplane.JsonSchema), and keeps every lossy step explicit.

It is a plain functional library: no processes, no global state, no caches.

Installation

def deps do
  [
    {:formata, "~> 0.1.0"}
  ]
end

Requires Elixir 1.18 or later (the forked schema backends use Elixir 1.18's built-in JSON module) and Erlang/OTP 25 or later (the minimum of Elixir 1.18). CI covers Elixir 1.18 and 1.20 on OTP 27/28.

Formata has no runtime dependencies. Its backends are maintained forks, renamed into the Formata namespace so they never clash with the original packages in your project: Formata.Jason (from jason), Formata.Toml (gsmlg_toml), Formata.YamlElixir and formata_yamerl (yaml_elixir, yamerl), Formata.Ymlr (ymlr), Formata.Backplane.Schema (backplane_schema), Formata.Backplane.JsonSchema (backplane_json_schema), and Formata.ExJSONPointer (ex_json_pointer). Only Formata.Backplane.Schema is public API (the native schema vocabulary); the other forks are internal. Versions, licenses, and modifications are listed in NOTICE.md.

The workflow

Create a schema, decode a document, validate it, inspect errors, and encode or convert the data:

iex> {:ok, schema} =
...>   Formata.Schema.native(%{
...>     name: {:required, {:string, min: 1}},
...>     port: {:required, {:integer, range: {1, 65_535}}},
...>     tags: {:list, :string}
...>   })
iex> {:ok, config} = Formata.decode(~s({"name": "api", "port": 8080, "tags": ["web"]}), :json)
iex> Formata.validate(config, schema)
:ok
iex> Formata.encode(config, :toml)
{:ok, "name = \"api\"\nport = 8080\ntags = [\"web\"]\n"}
iex> {:error, [error]} = Formata.validate(%{config | "port" => 0}, schema)
iex> {error.kind, error.code, error.path}
{:validation, :range, ["port"]}

The same with a JSON Schema document:

iex> {:ok, schema} =
...>   Formata.Schema.json_schema(%{
...>     "type" => "object",
...>     "required" => ["port"],
...>     "properties" => %{"port" => %{"type" => "integer", "minimum" => 1}},
...>     "additionalProperties" => false
...>   })
iex> schema.dialect
:draft2020_12
iex> {:ok, config} = Formata.decode("port: 8080\n", :yaml)
iex> Formata.validate(config, schema)
:ok
iex> {:error, [%Formata.Error{kind: :validation, path: ["extra"]}]} =
...>   Formata.validate(Map.put(config, "extra", true), schema)

Converting between formats, with an explicit transformation for a value the target cannot represent:

iex> Formata.convert(~s({"name": "api", "debug": null}), :json, :yaml)
{:ok, "---\ndebug:\nname: api\n"}
iex> {:error, [%Formata.Error{kind: :encode, code: :unrepresentable, path: ["debug"]}]} =
...>   Formata.convert(~s({"name": "api", "debug": null}), :json, :toml)
iex> Formata.convert(~s({"name": "api", "debug": null}), :json, :toml, transforms: [:drop_nils])
{:ok, "name = \"api\"\n"}

Public API

format is :json, :toml, :yaml, or a module implementing Formata.Codec. kind is :native or :json_schema (Formata.Schema.native/2 and Formata.Schema.json_schema/2 are shortcuts).

Every expected failure is {:error, errors} with a non-empty list of %Formata.Error{}.

Schemas

Formata keeps the two schema systems separate. A JSON Schema document is always validated by Formata.Backplane.JsonSchema itself, never converted into a native schema; native schemas are never exported to JSON Schema in order to validate data. Formata does not offer schema export or general schema conversion.

A compiled %Formata.Schema{} records its kind, dialect (JSON Schema only), backend, and the backend's compiled representation. Compile once and reuse it.

Native schemas

Native definitions are Formata.Backplane.Schema data: maps of field names to types such as :string, {:integer, gte: 0}, {:required, t}, {:list, t}, {:enum, values}, {:either, {a, b}}, {:custom, fun}, and nested maps. Atom schema keys match string data keys. Reusable and recursive definitions use Formata.Backplane.Schema.defschema/3 with {:ref, {Module, name}}; definitions are data, so they can also be composed with Formata.Schema.Native.merge/2, select/2, and except/2.

iex> defmodule MyApp.Schemas do
...>   import Formata.Backplane.Schema
...>   defschema :node, %{name: {:required, :string}, children: {:list, {:ref, :node}}}
...> end
iex> {:ok, tree} = Formata.Schema.native({:ref, {MyApp.Schemas, :node}})
iex> Formata.validate(%{"name" => "root", "children" => [%{"name" => "leaf"}]}, tree)
:ok
iex> {:error, [%Formata.Error{code: :required, path: ["children", 0, "name"]}]} =
...>   Formata.validate(%{"name" => "root", "children" => [%{}]}, tree)

JSON Schema

Documents are JSON-compatible maps or true/false. The dialect comes from a recognized $schema, from dialect: :draft7 | :draft2020_12, or defaults to Draft 2020-12; conflicts are errors. format is an annotation unless format_assertion: true. Local references resolve without I/O; remote references require an explicit :loader function. Formata never accesses the network. See Formata.Schema.JSONSchema.

Pure validation versus casting

Formata.validate/3 checks data exactly as given and returns :ok or errors. It never coerces, inserts defaults, or removes fields, and it never validates a modified copy of the data.

iex> {:ok, strict} = Formata.Schema.native(%{"n" => :integer})
iex> {:error, [%Formata.Error{code: :type}]} = Formata.validate(%{"n" => "42"}, strict)
iex> {:ok, coercing} = Formata.Schema.native(%{"n" => {:coerce, :integer}})
iex> Formata.cast(%{"n" => "42"}, coercing)
{:ok, %{"n" => 42}}
iex> {:error, [%Formata.Error{kind: :unsupported, code: :pure_validation_unsupported}]} =
...>   Formata.validate(%{"n" => "42"}, coercing)

For native schemas, pure validation supports a documented non-transforming subset of Formata.Backplane.Schema. Defaults, coercions, transforms, encoders, and run-time-computed schemas are rejected before anything runs, with the schema path of the construct. Custom validators run in the native engine and must return :ok or an error; one that returns a new value is reported as :non_pure_custom.

Formata.cast/3 runs the full native engine: defaults, coercion, constraint checks, then transforms (see Formata.Schema.Native for the exact order). JSON Schema annotations such as default are never applied; cast/3 with a JSON Schema returns :unsupported.

Unknown fields (native schemas)

JSON Schema documents control their own additional properties; there is no facade option to override additionalProperties or unevaluatedProperties.

Codecs and value domains

Decoded data uses ordinary Elixir terms. Document keys stay strings and Formata never creates atoms from document content. An absent field is absent from the map; an explicit null is nil.

Policies:

• Duplicate keys are rejected by all three decoders (:duplicate_key, or the TOML decoder's syntax error).
• YAML is single-document: multiple documents are rejected (:multiple_documents); an empty stream decodes to nil. Merge keys (<<) are rejected. Anchors/aliases are expanded. Comments, formatting, anchors, and key order are not preserved by any codec.
• Encoding rejects every value outside the target's domain with an :encode error and its path (:unrepresentable, :invalid_key, :invalid_root, :out_of_range). Nothing is dropped, stringified, replaced, or rounded implicitly. YAML output is decoded again and compared; a mismatch is :roundtrip_mismatch.
• Explicit transformations in Formata.Transform: drop_nils/1, replace_nils/2, temporal_to_iso8601/1, special_floats_to_strings/1, stringify_keys/1, and apply_all/2 (used by convert/4's :transforms).
• Conversion goes straight from decoded Elixir values to the target encoder; JSON is not used as an intermediate format. To validate native date/time values with JSON Schema, convert them explicitly first (temporal_to_iso8601/1); otherwise validation reports :non_json_value.

TOML is decoded by Formata.Toml and encoded by Formata.Codec.TOML.Encoder (TOML 1.0.0). Both are checked against the TOML 1.0.0 cases of the official toml-test suite: the decoder accepts all 208 valid documents with the expected values and rejects all 509 invalid ones, and every value from the valid cases is encoded and parsed again by Formata.Toml and by Python's tomllib as an independent parser.

Errors

%Formata.Error{} fields: kind (:parse, :encode, :schema, :validation, :transform, :unsupported), a stable code, message, backend, format, path (data path), schema_path, location (only when the backend reports line/column/offset), details, and the original cause. Exception.message/1 prefixes the path:

iex> {:error, [error]} = Formata.decode(~s({"a": 1, "a": 2}), :json)
iex> Exception.message(error)
"duplicate object key \"a\""
iex> {:error, [error]} = Formata.encode(%{"a" => %{"b" => nil}}, :toml)
iex> Exception.message(error)
"at /a/b: null values cannot be represented"

Known limitations

• TOML offset date-times are normalized to UTC DateTime values; the original offset is not kept.
• Native schemas treat an explicit nil like an absent field.
• Native {:oneof, ...} is "first matching branch"; JSON Schema oneOf is exclusive.
• Unknown-field rejection is not available for native keyword-list schemas.
• YAML timestamps decode as strings; YAML encoding rejects date/time values.
• With format_assertion: true, hostname, email, idn-hostname, and idn-email values that contain an A-label (xn--...) or a non-ASCII label cannot be checked without the IDNA2008 tables. Validation returns a single :unsupported / :unsupported_format error for them (never a pass or a :format failure, and not invertible by not/anyOf/oneOf/if). Plain ASCII names are fully checked.
• Format assertion has known gaps outside the required JSON Schema test suite: some edge cases of email (quoted local parts, ipv6: literals) and uri-template (malformed % escapes, spaces, DEL) are judged incorrectly.

Documentation

• Design notes: contracts, forked backends, and decisions.
• Release procedure.
• Changelog, third-party notices.

License

Formata's own code is MIT (see LICENSE). The forked backends keep their original licenses (MIT, Apache-2.0, BSD-2-Clause); see NOTICE.md and LICENSES/.