toml-parser: TOML 1.0.0 parser

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Dependencies array (>=0.5 && <0.6), base (>=4.14 && <4.21), containers (>=0.5 && <0.8), prettyprinter (>=1.7 && <1.8), text (>=0.2 && <3), time (>=1.9 && <1.13 || >=1.14 && <1.15), toml-parser, transformers (>=0.5 && <0.7) [details]
License ISC
Copyright 2023 Eric Mertens
Author Eric Mertens
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Source repo head: git clone main)
Uploaded by EricMertens at 2024-05-31T22:57:05Z
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Reverse Dependencies 6 direct, 167 indirect [details]
Executables toml-benchmarker
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Readme for toml-parser-

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TOML Parser

This package implements a validating parser for TOML 1.0.0.

This package uses an alex-generated lexer and happy-generated parser.

It also provides a pair of classes for serializing into and out of TOML.

Package Structure

title: Package Structure
    classDef important font-weight:bold;

    TOML:::important --> ApplicationTypes:::important : decode
    ApplicationTypes --> TOML : encode
    TOML --> [Token]: Lexer
    [Token] --> [Expr]: Parser
    [Expr] --> Table : Semantics
    Table --> ApplicationTypes : FromValue
    ApplicationTypes --> Table : ToValue
    Table --> TOML : Pretty

Most users will only need to import Toml or Toml.Schema. Other top-level modules are for low-level hacking on the TOML format itself. All modules below these top-level modules are exposed to provide direct access to library implementation details.

  • Toml - Basic encoding and decoding TOML
  • Toml.Schema - TOML schemas for application types
  • Toml.Semantics - Low-level semantic operations on TOML syntax
  • Toml.Syntax - Low-level parsing of text into TOML raw syntax


This file uses markdown-unlit to ensure that its code typechecks and stays in sync with the rest of the package.

{-# Language OverloadedStrings #-}
import Data.Text (Text)
import GHC.Generics (Generic)
import QuoteStr (quoteStr)
import Test.Hspec (Spec, hspec, it, shouldBe)
import Toml
import Toml.Schema

main :: IO ()
main = hspec (parses >> decodes >> encodes >> warns >> errors)

Using the raw parser

Consider this sample TOML text from the TOML specification.

fruitStr :: Text
fruitStr = [quoteStr|
name = "apple"

[fruits.physical]  # subtable
color = "red"
shape = "round"

[[fruits.varieties]]  # nested array of tables
name = "red delicious"

name = "granny smith"

name = "banana"

name = "plantain"

Parsing using this package generates the following unstructured value

parses :: Spec
parses = it "parses" $
    forgetTableAnns <$> parse fruitStr
    Right (table [
        ("fruits", List [
            Table (table [
                ("name", Text "apple"),
                ("physical", Table (table [
                    ("color", Text "red"),
                    ("shape", Text "round")])),
                ("varieties", List [
                    Table (table [("name", Text "red delicious")]),
                    Table (table [("name", Text "granny smith")])])]),
            Table (table [
                ("name", Text "banana"),
                ("varieties", List [
                    Table (table [("name", Text "plantain")])])])])])

Defining a schema

We can define a schema for our TOML format in the form of instances of FromValue, ToValue, and ToTable in order to read TOML directly into structured data form. This example manually derives some of the instances as a demonstration.

newtype Fruits = Fruits { fruits :: [Fruit] }
    deriving (Eq, Show, Generic)
    deriving (ToTable, ToValue, FromValue) via GenericTomlTable Fruits

data Fruit = Fruit { name :: String, physical :: Maybe Physical, varieties :: [Variety] }
    deriving (Eq, Show, Generic)
    deriving (ToTable, ToValue, FromValue) via GenericTomlTable Fruit

data Physical = Physical { color :: String, shape :: String }
    deriving (Eq, Show, Generic)
    deriving (ToTable, ToValue, FromValue) via GenericTomlTable Physical

newtype Variety = Variety String
    deriving (Eq, Show)

instance FromValue Variety where
    fromValue = parseTableFromValue (Variety <$> reqKey "name")
instance ToValue Variety where
    toValue = defaultTableToValue
instance ToTable Variety where
    toTable (Variety x) = table ["name" .= x]

We can run this example on the original value to deserialize it into domain-specific datatypes.

decodes :: Spec
decodes = it "decodes" $
    decode fruitStr
    Success [] (Fruits [
            (Just (Physical "red" "round"))
            [Variety "red delicious", Variety "granny smith"],
        Fruit "banana" Nothing [Variety "plantain"]])

encodes :: Spec
encodes = it "encodes" $
    show (encode (Fruits [Fruit
            (Just (Physical "red" "round"))
            [Variety "red delicious", Variety "granny smith"]]))
    `shouldBe` [quoteStr|
        name = "apple"

        color = "red"
        shape = "round"

        name = "red delicious"

        name = "granny smith"|]

Useful errors and warnings

This package takes care to preserve source information as much as possible in order to provide useful feedback to users. These examples show a couple of the message that can be generated when things don't go perfectly.

warns :: Spec
warns = it "warns" $
    decode [quoteStr|
        name = "simulated"
        typo = 10|]
        ["2:1: unexpected key: typo in <top-level>"] -- warnings
        (Variety "simulated")

errors :: Spec
errors = it "errors" $
    decode [quoteStr|
        # Physical characteristics table
        color = "blue"
        shape = []|]
        ["3:9: expected string but got array in shape"]
        :: Result String Physical)

More Examples

A demonstration of using this package at a more realistic scale can be found in HieDemoSpec. The various unit test files demonstrate what you can do with this library and what outputs you can expect.

See the low-level operations used to build a TOML syntax highlighter in TomlHighlighter.