tomland
Bidirectional TOML serialization
https://github.com/kowainik/tomland
| LTS Haskell 24.16: | 1.3.3.3@rev:3 |
| Stackage Nightly 2025-10-24: | 1.3.3.3@rev:3 |
| Latest on Hackage: | 1.3.3.3@rev:3 |
tomland-1.3.3.3@sha256:5a1883fbe1a59bc3e70b3058ca2e1d0fa4add0727f1f3154fbb2f69339fd9429,9404Module documentation for 1.3.3.3
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
tomland is a Haskell library for Bidirectional TOML
Serialization. It provides the composable interface for implementing
TOML codecs. If you want to use
TOML as a configuration for your tool or application, you can use
tomland to easily convert in both ways between textual TOML
representation and Haskell types.
✍️ tomland supports TOML spec version 0.5.0.
The following blog post has more details about the library design and internal implementation details:
This README contains a basic usage example of the tomland library. All code
below can be compiled and run with the following command:
cabal 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 Data.Text (Text)
import Data.Time (Day)
import Toml (TomlCodec, (.=))
import qualified Data.Text.IO as TIO
import qualified Toml
tomland is 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. The configuration
contains the following description of our data:
server.port = 8080
server.codes = [ 5, 10, 42 ]
server.description = """
This is production server.
Don't touch it!
"""
[mail]
host = "smtp.gmail.com"
send-if-inactive = false
[[user]]
guestId = 42
[[user]]
guestId = 114
[[user]]
login = "Foo Bar"
createdAt = 2020-05-19
The above static configuration describes Settings for some
server. It has several top-level fields, a table with the name mail
and an array of tables with the name user that stores list of
different types of users.
We can model such TOML using the following Haskell data types:
data Settings = Settings
{ settingsPort :: !Port
, settingsDescription :: !Text
, settingsCodes :: [Int]
, settingsMail :: !Mail
, settingsUsers :: ![User]
}
data Mail = Mail
{ mailHost :: !Host
, mailSendIfInactive :: !Bool
}
data User
= Guest !Integer -- id of guest
| Registered !RegisteredUser -- login and createdAt of registered user
data RegisteredUser = RegisteredUser
{ registeredUserLogin :: !Text
, registeredUserCreatedAt :: !Day
}
newtype Port = Port Int
newtype Host = Host Text
Using the tomland library, you can write bidirectional converters for these types
with the following guidelines and helper functions:
- If your fields are some simple primitive 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 it requires to specify this data type as TOML table in the.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 the 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
matchGuest :: User -> Maybe Integer
matchGuest = \case
Guest i -> Just i
_ -> Nothing
matchRegistered :: User -> Maybe RegisteredUser
matchRegistered = \case
Registered u -> Just u
_ -> Nothing
userCodec :: TomlCodec User
userCodec =
Toml.dimatch matchGuest Guest (Toml.integer "guestId")
<|> Toml.dimatch matchRegistered Registered registeredUserCodec
registeredUserCodec :: TomlCodec RegisteredUser
registeredUserCodec = RegisteredUser
<$> Toml.text "login" .= registeredUserLogin
<*> Toml.day "createdAt" .= registeredUserCreatedAt
And now we are ready to parse our TOML and print the result back to see whether everything is okay.
main :: IO ()
main = do
tomlRes <- Toml.decodeFileEither settingsCodec "examples/readme.toml"
case tomlRes of
Left errs -> TIO.putStrLn $ Toml.prettyTomlDecodeErrors errs
Right settings -> TIO.putStrLn $ Toml.encode settingsCodec settings
Benchmarks and comparison with other libraries
You can find benchmarks of the tomland library in the following repository:
Since tomland 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 |
In addition to the above numbers, tomland has several features that
make it unique:
tomlandis the only Haskell library that has pretty-printing.tomlandis compatible with the latest TOML spec while other libraries are not.tomlandis bidirectional, which means that your encoding and decoding are consistent with each other by construction.tomlandprovides abilities forGenericandDerivingViaderiving out-of-the-box.- Despite being the fastest,
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. In addition,toml-parserdoesn’t have ways to convert TOML AST to custom Haskell types andhtoml*libraries use typeclasses-based approach viaaesonlibrary.
Acknowledgement
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