Documentation

Monad for bidirectional conversion. Contains pair of functions:

1. How to read value of type a from immutable environment context r?
2. How to store value of type a in stateful context w?

In practice instead of r we will use some Reader Toml and instead of w we will use State Toml. This approach with the bunch of utility functions allows to have single description for from/to TOML conversion.

In practice this type will always be used in the following way:

type BiCodec r w a = Codec r w a a

Type parameter c if fictional. Here some trick is used. This trick is implemented in the codec package and described in more details in related blog post: https://blog.poisson.chat/posts/2016-10-12-bidirectional-serialization.html.

Specialized version of Codec data type. This type alias is used in practice.

This is an instance of Profunctor for Codec. But since there's no Profunctor type class in base or package with no dependencies (and we don't want to bring extra dependencies) this instance is implemented as a single top-level function.

Useful when you want to parse newtypes. For example, if you had data type like this:

data Example = Example
    { foo :: Bool
    , bar :: Text
    }

Bidirectional TOML converter for this type will look like this:

exampleCodec :: TomlCodec Example
exampleCodec = Example
    <$> Toml.bool "foo" .= foo
    <*> Toml.text "bar" .= bar

Now if you change your type in the following way:

newtype Email = Email { unEmail :: Text }

data Example = Example
    { foo :: Bool
    , bar :: Email
    }

you need to patch your TOML codec like this:

exampleCodec :: TomlCodec Example
exampleCodec = Example
    <$> Toml.bool "foo" .= foo
    <*> dimap unEmail Email (Toml.text "bar") .= bar

Bidirectional converter for Maybe a values. For example, given the data type:

data Example = Example
    { foo :: Bool
    , bar :: Maybe Int
    }

the TOML codec will look like

exampleCodec :: TomlCodec Example
exampleCodec = Example
    <$> Toml.bool "foo" .= foo
    <*> dioptional (Toml.int "bar") .= bar

Combinator used for newtype wrappers. For example, given the data types:

newtype N = N Int

data Example = Example
    { foo :: Bool
    , bar :: N
    }

the TOML codec can look like

exampleCodec :: TomlCodec Example
exampleCodec = Example
    <$> Toml.bool "foo" .= foo
    <*> diwrap (Toml.int "bar") .= bar

Alternative instance for function arrow but without empty.

Operator to connect two operations:

1. How to get field from object?
2. How to write this field to toml?

In code this should be used like this:

data Foo = Foo
    { fooBar :: Int
    , fooBaz :: String
    }

fooCodec :: TomlCodec Foo
fooCodec = Foo
    <$> Toml.int "bar" .= fooBar
    <*> Toml.str "baz" .= fooBaz