tomland
Bidirectional TOML serialization
https://github.com/kowainik/tomland
| Version on this page: | 1.2.1.0@rev:1 |
| LTS Haskell 24.16: | 1.3.3.3@rev:3 |
| Stackage Nightly 2025-10-25: | 1.3.3.3@rev:3 |
| Latest on Hackage: | 1.3.3.3@rev:3 |
tomland-1.2.1.0@sha256:7912ba07d22b24c17ede72c29e5e8d0409a5e41e5196ef2cf645669c77248b02,6717Module documentation for 1.2.1.0
tomland
“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 stock (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:
- If your fields are some simple basic types like
IntorTextyou can just use standard codecs likeToml.intandToml.text. - If you want to parse
newtypes, useToml.diwrapto wrap parsers for underlyingnewtyperepresentation. - For parsing nested data types, use
Toml.table. But this requires to specify this data type as TOML table in.tomlfile. - 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 likeInt,Bool,Double,Textor time types, that you can useToml.arrayOfand parse arrays of values. - If you have sets of custom data types, use
Toml.setorToml.HashSet. Such sets are represented as array of tables in TOML. - For parsing sum types, use
Toml.dimatch. This requires writing matching functions for the constructors of the sum type. tomlandseparates conversion between Haskell types and TOML values from matching values by keys. Converters between types and values have typeTomlBiMapand are named with capital letter started with underscore. Main type for TOML codecs is calledTomlCodec. To liftTomlBiMaptoTomlCodecyou need to useToml.matchfunction.
settingsCodec :: TomlCodec Settings
settingsCodec = Settings
<$> Toml.diwrap (Toml.int "server.port") .= settingsPort
<*> Toml.text "server.description" .= settingsDescription
<*> Toml.arrayOf Toml._Int "server.codes" .= settingsCodes
<*> Toml.table mailCodec "mail" .= settingsMail
<*> Toml.list userCodec "user" .= settingsUsers
mailCodec :: TomlCodec Mail
mailCodec = Mail
<$> 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:
toml-parserdoesn’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.tomlandsupports latest TOML spec whilehtomlandhtoml-megaparsecdon’t have support for all types, values and formats.tomlandis the only library that has pretty-printing.toml-parserdoesn’t have ways to convert TOML AST to custom Haskell types andhtoml*libraries use typeclasses-based approach viaaesonlibrary.tomlandis bidirectional :slightly_smiling_face:
Acknowledgement
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