tomland: Bidirectional TOML serialization

[ configuration, library, mpl, text, toml ] [ Propose Tags ]

Implementation of bidirectional TOML serialization. Simple codecs look like this:

data User = User
    { userName :: Text
    , userAge  :: Int

userCodec :: TomlCodec User
userCodec = User
    <$> Toml.text "name" .= userName
    <*>  "age"  .= userAge

The following blog post has more details about library design:

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Versions [RSS] [faq] 0.0.0, 0.1.0, 0.2.0, 0.2.1, 0.3, 0.3.1, 0.4.0, 0.5.0, 1.0.0,,,,,,,,,,
Change log
Dependencies base (>=4.10 && <4.13), bytestring (==0.10.*), containers (>=0.5.7 && <0.7), deepseq (==1.4.*), hashable (==1.2.*), megaparsec (>=7.0.5 && <7.1), mtl (==2.2.*), parser-combinators (>=1.1.0 && <1.2), text (==1.2.*), time (>=1.8 && <1.10), tomland, transformers (==0.5.*), unordered-containers (>=0.2.7 && <0.3) [details]
License MPL-2.0
Copyright 2018-2019 Kowainik
Author Dmitrii Kovanikov, Veronika Romashkina
Maintainer Kowainik <>
Category TOML, Text, Configuration
Home page
Bug tracker
Source repo head: git clone
Uploaded by shersh at 2019-07-08T14:07:51Z
Distributions Arch:, LTSHaskell:, NixOS:
Executables play-tomland, readme
Downloads 10328 total (125 in the last 30 days)
Rating 2.5 (votes: 4) [estimated by Bayesian average]
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Status Hackage Matrix CI
Docs uploaded by user [build log]
All reported builds failed as of 2019-07-08 [all 1 reports]


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Readme for tomland-

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palm Build status Hackage Stackage LTS Stackage Nightly MPL-2.0 license

“A library is like an island in the middle of a vast sea of ignorance, particularly if the library is very tall and the surrounding area has been flooded.”

― Lemony Snicket, Horseradish

Bidirectional TOML serialization. The following blog post has more details about library design:

This README contains a basic usage example of the tomland library. All code below can be compiled and run with the following command:

cabal new-run readme

Preamble: imports and language extensions

Since this is a literate haskell file, we need to specify all our language extensions and imports up front.

{-# OPTIONS -Wno-unused-top-binds #-}

{-# LANGUAGE LambdaCase        #-}
{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative ((<|>))
import Control.Category ((>>>))
import Data.Text (Text)
import Toml (TomlBiMap, TomlCodec, (.=))

import qualified Data.Text.IO as TIO
import qualified Toml

tomland is mostly designed for qualified imports and intended to be imported as follows:

import Toml (TomlCodec, (.=))  -- add 'TomlBiMap' and 'Key' here optionally
import qualified Toml

Data type: parsing and printing

We're going to parse TOML configuration from examples/readme.toml file.

This static configuration is captured by the following Haskell data type:

data Settings = Settings
    { settingsPort        :: !Port
    , settingsDescription :: !Text
    , settingsCodes       :: [Int]
    , settingsMail        :: !Mail
    , settingsUsers       :: ![User]

data Mail = Mail
    { mailHost           :: !Host
    , mailSendIfInactive :: !Bool

data User
    = Admin  Integer  -- id of admin
    | Client Text     -- name of the client
    deriving (Show)

newtype Port = Port Int
newtype Host = Host Text

Using tomland library, you can write bidirectional converters for these types using the following guidelines and helper functions:

  1. If your fields are some simple basic types like Int or Text you can just use standard codecs like and Toml.text.
  2. If you want to parse newtypes, use Toml.diwrap to wrap parsers for underlying newtype representation.
  3. For parsing nested data types, use Toml.table. But this requires to specify this data type as TOML table in .toml file.
  4. If you have lists of custom data types, use Toml.list. Such lists are represented as array of tables in TOML. If you have lists of primitive types like Int, Bool, Double, Text or time types, that you can use Toml.arrayOf and parse arrays of values.
  5. tomland separates conversion between Haskell types and TOML values from matching values by keys. Converters between types and values have type TomlBiMap and are named with capital letter started with underscore. Main type for TOML codecs is called TomlCodec. To lift TomlBiMap to TomlCodec you need to use Toml.match function.
settingsCodec :: TomlCodec Settings
settingsCodec = Settings
    <$> Toml.diwrap (  "server.port")       .= settingsPort
    <*> Toml.text              "server.description" .= settingsDescription
    <*> Toml.arrayOf Toml._Int ""       .= settingsCodes
    <*> Toml.table mailCodec   "mail"               .= settingsMail
    <*> Toml.list  userCodec   "user"               .= settingsUsers

mailCodec :: TomlCodec Mail
mailCodec = Mail
    <$> Toml.diwrap (Toml.text "host") .= mailHost
    <*> Toml.bool "send-if-inactive"   .= mailSendIfInactive

_Admin :: TomlBiMap User Integer
_Admin = Toml.prism Admin $ \case
    Admin i -> Right i
    other   -> Toml.wrongConstructor "Admin" other

_Client :: TomlBiMap User Text
_Client = Toml.prism Client $ \case
    Client n -> Right n
    other    -> Toml.wrongConstructor "Client" other

userCodec :: TomlCodec User
userCodec =
        Toml.match (_Admin >>> Toml._Integer) "id"
    <|> Toml.match (_Client >>> Toml._Text) "name"

And now we're ready to parse our TOML and print the result back to see whether everything is okay.

main :: IO ()
main = do
    tomlExample <- TIO.readFile "examples/readme.toml"
    let res = Toml.decode settingsCodec tomlExample
    case res of
        Left err -> print err
        Right settings -> TIO.putStrLn $ Toml.encode settingsCodec settings

Benchmarks and comparison with other libraries

tomland is compared with other libraries. Since it uses 2-step approach with converting text to intermediate AST and only then decoding Haskell type from this AST, benchmarks are also implemented in a way to reflect this difference.

Library parse :: Text -> AST transform :: AST -> Haskell
tomland 305.5 μs 1.280 μs
htoml 852.8 μs 33.37 μs
htoml-megaparsec 295.0 μs 33.62 μs
toml-parser 164.6 μs 1.101 μs

You may see that tomland is not the fastest one (though still very fast). But performance hasn’t been optimized so far and:

  1. toml-parser doesn’t support the array of tables and because of that it’s hardly possible to specify the list of custom data types in TOML with this library.
  2. tomland supports latest TOML spec while htoml and htoml-megaparsec don’t have support for all types, values and formats.
  3. tomland is the only library that has pretty-printing.
  4. toml-parser doesn’t have ways to convert TOML AST to custom Haskell types and htoml* libraries use typeclasses-based approach via aeson library.
  5. tomland is bidirectional 🙂


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