BSD-3-Clause licensed by Cies Breijs
Maintained by Cies Breijs <cies % kde ! nl>
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Module documentation for


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htoml on Stackage LTS 5 htoml on Stackage LTS 6 htoml on Stackage Nightly

A TOML parser library in Haskell.

TOML is the obvious, minimal configuration language by Tom Preston-Werner. It is an alternative to the XML, YAML and INI formats mainly for the purpose of configuration files. Many will find that XML and YAML are too heavy for the purpose of configuration files prupose while INI is underspecified. TOML is to configuration files, like what Markdown is for rich-text.

This library aims to be compatible with the latest version of the TOML spec. Compatibility between htoml version and TOML (as proven by BurntSushi’s language agnostic TOML test suite) is as follows:

  • TOML v0.4.0 is implemented by htoml >=
  • (currently only one item in this mapping, more will follow)


Apart from this README, documentation for this package may (or may not) be found on Hackage.

Quick start

Installing htoml is easy. Either by using Stack (recommended):

stack install htoml

Or by using Cabal:

cabal install htoml

In order to make your project depend on it you can add it as a dependency in your project’s .cabal file, and since it is not yet on Stackage you will also have to add it to the extra-deps section of your stack.yaml file when using Stack.

To quickly show some features of htoml we use Stack to start a GHCi-based REPL. It picks up configuration from the .ghci file in the root of the repository.

git clone
cd htoml
stack init
stack --install-ghc ghci

Add a --resolver flag to the stack init command to specify a specific package snapshot, e.g.: --resolver lts-4.1.

In case you have missing dependencies (possibly file-embed), they can be added to the extra-deps in stack.yaml automatically with:

stack solver --update-config

We can now start exploring htoml from a GHCi REPL. From the root of this repository run:

stack ghci

Now read a .toml file from the benchmark suite, with:

txt <- readFile "benchmarks/example.toml"
let r = parseTomlDoc "" txt

…which prints:

Right (fromList [("database",VTable (fromList [("enabled",VBoolean True),("po [...]

Then convert it to Aeson (JSON), with:

let Right toml = r
toJSON toml

…which prints:

Object (fromList [("database",Object (fromList [("enabled",Bool True),("po [...]

Finally trigger a parse error, with:

let Left err = parseTomlDoc "" "== invalid toml =="

…it errors out (as it should), showing:

(line 1, column 1):
unexpected '='
expecting "#", "\n", "\r\n", letter or digit, "_", "-", "\"", "'", "[" or end of input

Note: Some of the above outputs are truncated, indicated by [...].

How to pull data from a TOML file after parsing it

Once you have sucessfully parsed a TOML file you most likely want to pull some piecces of data out of the resulting data structure.

To do so you have two main options. The first is to use pattern matching. For example let’s consider the following parseResult:

Right (fromList [("server",VTable (fromList [("enabled",VBoolean True)] ) )] )

Which could be pattern matched with:

case parseResult of
  Left  _ -> "Could not parse file"
  Right m -> case m ! "server" of
    VTable mm -> case mm ! "enabled" of
      VBoolean b -> "Server is " ++ (if b then "enabled" else "disabled")
      _ -> "Could not parse server status (Boolean)"
    _ -> "TOML file does not contain the 'server' key"

The second main option is to use the toJSON function to transform the data to an Aeson data structure, after which you can use your Aeson toolbelt to tackle the problem. Since TOML is intended to be a close cousin of JSON this is a very practical approach.

Other ways to pull data from a parsed TOML document will most likely exist; possible using the lens library as documented here.


Currently we are testing against several versions of GHC with Travis CI as defined in the env section of our .travis.yml. lts-2 implies GHC 7.8.4, lts-3 implies GHC 7.10.2, lts-4/lts-5 imply GHC 7.10.3, and nightly is build with a regularly updated version of GHC.

Version contraints of htoml’s dependencies

If you encounter any problems because htoml’s dependecies are constrained either too much or too little, please file a issue for that. Or off even better submit a PR.

Tests and benchmarks

Tests are build and run with:

stack test

BurntSushi’s language agnostic test suite is embedded in the test suite executable. Using a shell script (that lives in test/BurntSushi) the latest tests can be fetched from its Github repository.

The benchmarks, that use the amazing criterion library, are build and run with:

stack build :benchmarks


Most welcome! Please raise issues, start discussions, give comments or submit pull-requests. This is one of the first Haskell libraries I wrote, feedback is much appreciated.


  • Compatibility to the TOML spec is proven by an extensive test suite
  • Incorporates BurntSushi’s language agnostic test suite
  • Has an internal representation that easily maps to JSON
  • Provides an Aeson-style JSON interface (suggested by Greg Weber)
  • Useful error messages (thanks to using Parsec over Attoparsec)
  • Understands arrays as described in this issue
  • Fails on mix-type arrays (as per spec)
  • Comes with a benchmark suite to make performance gains/regressions measurable
  • Tries to be well documented (please raise an issue if you find documentation lacking)
  • Available on Stackage (see top of this README for badges indicating TOMLs inclusion in Stackage status)


  • More documentation and start to use the proper Haddock idioms
  • Add property tests with QuickCheck (the internet says it’s possible for parsers)
  • Extensively test error cases (probably improving error reporting along the way)
  • See how lenses may (or may not) fit into this package, or an additional package
  • Consider moving to one of the more modern parser combinators in Haskell (megaparsec maybe?) – possibly wait until a clear winner shows

Do you see todo that looks like fun thing to implement and you can spare the time? Please knoe that PRs are welcome :)


Originally this project started off by improving the toml package by Spiros Eliopoulos.

HuwCampbell helped a lot by making tests pass and implementing “explicitness tracking” in Parsec’s parser state.

Copyright and licensing

This package includes BurntSushi’s language agnostic TOML tests, which are WTFPL licensed.

The TOML examples that are used as part of the benchmarks are copied from Tom Preston-Werner’s TOML spec which is MIT licensed.

For all other files in this project the copyrights are specified in the htoml.cabal file, they are distributed under the BSD3 license as found in the LICENSE file.


Change log


  • Improve docs

  • Use Vector over List internally, as per discussion in issue 13

  • Expose ToJSON implementation
  • Remove unused .cabal dependency (thanks @tmcgilchrist)

  • Compatible with TOML 0.4.0
  • Improve test suite (all test now pass – thanks @HuwCampbell)
  • Slight API breakage (therefore major version bump)
  • Use Parsec’s parser state to track explicitness of table definitions (thanks @HuwCampbell)
  • Clean up docs and code

  • GHC 7.10 compatibility fix (thanks @erebe)
  • Allow time >= 1.5.0, by using some CPP trickery
  • Improve README based on feedback on Reddit

  • Update the REAMDE
  • Add/relax dependency version contraints where applicable
  • Fix all warnings
  • Add

  • Fix cabal configure error in cabal file

  • Initial upload to Hackage