An implementation of TOML for Elixir projects, compliant with the latest specification
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TOML for Elixir

Master Version

This is a TOML library for Elixir projects. It is compliant with version 0.5.0 of the official TOML specification. You can find a brief overview of the feature set below, but you are encouraged to read the full spec at the link above (it is short and easy to read!).


  • Decode from string, file, or stream
  • Fully compliant with the latest version of the TOML spec
  • Is tested against toml-test, a test suite for spec-compliant TOML encoders/decoders, used by implementations in multiple languages. The test suite has been integrated into this project to be run under Mix so that we get better error information and so it can run as part of the test suite.
  • Decoder produces a map with values using the appropriate Elixir data types for representation
  • Supports extension via value transformers (see Toml.Transform docs for details)
  • Supports use as a configuration provider in Distillery 2.x+ (use TOML files for configuration!)
  • Decoder is written by hand to take advantage of various optimizations.
  • Library passes Dialyzer checks

Comparison To Other Libraries

I compared toml to four other libraries:

  • toml_elixir
  • tomlex
  • jerry
  • etoml

Of these four, none correctly implement the 0.5.0 specification. Either they are targeting older versions of the spec (in etoml, it is built against pre-0.1), are not fully implemented (i.e. don't support all features), or have bugs which prevent them from properly parsing a 0.5.0 example file (the test/fixtures/example.toml file in this repository).

If you are looking for a TOML library, at present toml is the only one which full implements the spec and correctly decodes example.toml.


This library is available on Hex as :toml, and can be added to your deps like so:

def deps do
    {:toml, "~> 0.1.0"}

Type Conversions

In case you are curious how TOML types are translated to Elixir types, the following table provides the conversions.

NOTE: The various possible representations of each type, such as hex/octal/binary integers, quoted/literal strings, etc., are considered to be the same base type (e.g. integer and string respectively in the examples given).

TOML Elixir
String String.t (binary)
Integer integer
inf :infinity
+inf :infinity
-inf :negative_infinity
nan :nan
+nan :nan
-nan :negative_nan
Boolean boolean
Offset Date-Time DateTime.t
Local Date-Time NaiveDateTime.t
Local Date Date.t
Local Time Time.t
Array list
Table map
Table Array list(map)

Implementation-specific Behaviors

Certain features of TOML have implementation-specific behavior:

  • -inf, inf, and +inf are all valid infinity values in TOML. In Erlang/Elixir, these don't have exact representations. Instead, by convention, :infinity is used for positive infinity, as atoms are always larger than integers when using comparison operators, so :infinity > <any integer> will always be true. However, negative infinity cannot be represented, as numbers are always considered smaller than every other type in term comparisons. Instead, we represent it with :negative_infinity, so that the type information is not lost, but you must be careful to deal with it specifically in comparisons/sorting/etc.
  • -nan, nan, and +nan are all valid NaN (not a number) values in TOML. In Erlang/Elixir, NaN is traditionally represented with :nan, but there is no representation for negative NaN, and no API actually produces :nan, instead invalid numbers typically raise errors, in the typical spirit of "let it crash" in the face of errors. For purposes of preserving type information though, we use the :nan convention, and :negative_nan for -NaN. You will need to take care to deal with these values manually if the values need to be preserved.
  • The maximum precision of times in the various time types is microseconds (i.e. precision to six decimal places), if you provide higher precision values (i.e. nanoseconds), the extra precision will be lost.
  • Hex, octal, and binary numbers are converted to integers, so serializing those values after decoding them from a TOML document will be in their decimal representation.

Example Usage

The following is a brief overview of how to use this library. First, let's take a look at an example TOML file, as borrowed from the TOML homepage:

# This is a TOML document.

title = "TOML Example"

name = "Tom Preston-Werner"
dob = 1979-05-27T07:32:00-08:00 # First class dates

server = ""
ports = [ 8001, 8001, 8002 ]
connection_max = 5000
enabled = true


  # Indentation (tabs and/or spaces) is allowed but not required
  ip = ""
  dc = "eqdc10"

  ip = ""
  dc = "eqdc10"

data = [ ["gamma", "delta"], [1, 2] ]

# Line breaks are OK when inside arrays
hosts = [


iex> input = """
server = ""
...> {:ok, %{"database" => %{"server" => ""}}} = Toml.decode(input)
...> {:ok, %{database: %{server: ""}}} = Toml.decode(input, keys: :atoms)
...> stream =!("example.toml")
...> {:ok, %{"database" => %{"server" => ""}}} = Toml.decode_stream(stream)
...> {:ok, %{"database" => %{"server" => ""}}} = Toml.decode_file("example.toml")
...> invalid = """
a = 1 b = 2
...> {:error, {:invalid_toml, reason}} = Toml.decode(invalid); IO.puts(reason)
expected '\n', but got 'b' in nofile on line 2:

    a = 1 b = 2



Support for extending value conversions is provided by the Toml.Transform behavior. An example is shown below:

Given the follwing TOML document:

ip = ""
ports = [8080, 8081]

ip = ""
ports = [8082, 8083]

And the following modules:

defmodule Server do
  defstruct [:name, :ip, :ports]

defmodule IPStringToCharlist do
  use Toml.Transform
  def transform(:ip, v) when is_binary(v) do
  def transform(_k, v), do: v

defmodule CharlistToIP do
  use Toml.Transform
  def transform(:ip, v) when is_list(v) do
    case :inet.parse_ipv4_address(v) do
      {:ok, address} ->
      {:error, reason} ->
        {:error, {:invalid_ip_address, reason}}
  def transform(:ip, v), do: {:error, {:invalid_ip_address, v}}
  def transform(_k, v), do: v

defmodule ServerMapToList do
  use Toml.Transform
  def transform(:servers, v) when is_map(v) do
    for {name, server} <- v, do: struct(Server, Map.put(server, :name, name))
  def transform(_k, v), do: v

You can convert the TOML document to a more strongly-typed version using the above transforms like so:

iex> transforms = [IPStringToCharlist, CharlistToIP, ServerMapToList]
...> {:ok, result} = Toml.decode("example.toml", keys: :atoms, transforms: transforms)
%{servers: [%Server{name: :alpha, ip: {192,168,1,1}}, ports: [8080, 8081] | _]}

The transforms given here are intended to show how they can be composed: they are applied in the order provided, and the document is transformed using a depth-first, bottom-up traversal. Put another way, you transform the leaves of the tree before the branches; as shown in the example above, this means the :ip key is converted to an address tuple before the :servers key is transformed into a list of Server structs.

Using with Distillery

To use this library as a configuration provider in Distillery, add the following to your rel/config.exs:

release :myapp do
  # ...snip...
  set config_providers: [
    {Toml.Provider, [path: "${XDG_CONFIG_DIR}/myapp.toml", transforms: [...]]}

This will result in Toml.Provider being invoked during boot, at which point it will evaluate the given path and read the TOML file it finds. If one is not found, or is not accessible, the provider will raise an error, and the boot sequence will terminate unsuccessfully. If it succeeds, it persists settings in the file to the application environment (i.e. you access it via Application.get_env/2).

You can pass the same options in the arguments list for Toml.Provider as you can to Toml.decode/2, but :path is required, and :keys only supports :atoms and :atoms! values.

The config provider expects a certain format to the TOML file, namely that keys at the root of the document correspond to applications which need to be configured. If it encounters keys at the root of the document which are not tables, they are ignored.

# This is an example of something that would be ignored
title = "My config file"

# We're expecting something like this:
key = "value"

# To use a bit of Phoenix config, you translate to TOML like so:
cache_static_manifest = "priv/static/cache_manifest.json"

port = "4000"

hsts = true

# Or logger..
level = "info"

format = "[$level] $message \n"


  • Add benchmarking suite
  • Provide options for converting keys to atom, similar to Jason/Poison/etc.
  • Optimize lexer to always send offsets to decoder, rather than only in some cases
  • Try to find pathological TOML files to test


This project is licensed Apache 2.0, see the LICENSE file in this repo for details.