Monad.hs

-- | Contains general underlying monad for bidirectional conversion.

module Toml.Bi.Monad
       ( Codec (..)
       , BiCodec
       , dimap
       , dioptional
       , diwrap
       , (<!>)
       , (.=)
       ) where

import Control.Applicative (Alternative (..), optional)
import Control.Monad (MonadPlus (..))
import Data.Coerce (Coercible, coerce)


{- | 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](http://hackage.haskell.org/package/codec) package and
described in more details in related blog post:
<https://blog.poisson.chat/posts/2016-10-12-bidirectional-serialization.html>.
-}
data Codec r w c a = Codec
    { -- | Extract value of type @a@ from monadic context @r@.
      codecRead  :: r a

      -- | Store value of type @c@ inside monadic context @w@ and returning
      -- value of type @a@. Type of this function actually should be @a -> w ()@ but with
      -- such type it's impossible to have 'Monad' and other instances.
    , codecWrite :: c -> w a
    }

-- | Specialized version of 'Codec' data type. This type alias is used in practice.
type BiCodec r w a = Codec r w a a

instance (Functor r, Functor w) => Functor (Codec r w c) where
    fmap :: (a -> b) -> Codec r w c a -> Codec r w c b
    fmap f codec = Codec
        { codecRead  = f <$> codecRead codec
        , codecWrite = fmap f . codecWrite codec
        }

instance (Applicative r, Applicative w) => Applicative (Codec r w c) where
    pure :: a -> Codec r w c a
    pure a = Codec
        { codecRead  = pure a
        , codecWrite = \_ -> pure a
        }

    (<*>) :: Codec r w c (a -> b) -> Codec r w c a -> Codec r w c b
    codecf <*> codeca = Codec
        { codecRead  = codecRead codecf <*> codecRead codeca
        , codecWrite = \c -> codecWrite codecf c <*> codecWrite codeca c
        }

instance (Monad r, Monad w) => Monad (Codec r w c) where
    (>>=) :: Codec r w c a -> (a -> Codec r w c b) -> Codec r w c b
    codec >>= f = Codec
        { codecRead  = codecRead codec >>= \a -> codecRead (f a)
        , codecWrite = \c -> codecWrite codec c >>= \a -> codecWrite (f a) c
        }

instance (Alternative r, Alternative w) => Alternative (Codec r w c) where
    empty :: Codec r w c a
    empty = Codec
        { codecRead  = empty
        , codecWrite = \_ -> empty
        }

    (<|>) :: Codec r w c a -> Codec r w c a -> Codec r w c a
    codec1 <|> codec2 = Codec
        { codecRead  = codecRead codec1 <|> codecRead codec2
        , codecWrite = \c -> codecWrite codec1 c <|> codecWrite codec2 c
        }

instance (MonadPlus r, MonadPlus w) => MonadPlus (Codec r w c) where
    mzero = empty
    mplus = (<|>)

-- | Alternative instance for function arrow but without 'empty'.
infixl 3 <!>
(<!>) :: Alternative f => (a -> f x) -> (a -> f x) -> (a -> f x)
f <!> g = \a -> f a <|> g a

{- | 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 @newtype@s. 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
    \<$\> Toml.bool "foo" '.=' foo
    \<*\> 'dimap' unEmail Email (Toml.text "bar") '.=' bar
@
-}
dimap
    :: (Functor r, Functor w)
    => (c -> d)       -- ^ Mapper for consumer
    -> (a -> b)       -- ^ Mapper for producer
    -> Codec r w d a  -- ^ Source 'Codec' object
    -> Codec r w c b  -- ^ Target 'Codec' object
dimap f g codec = Codec
    { codecRead  = g <$> codecRead codec
    , codecWrite = fmap g . codecWrite codec . f
    }

{- | 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
@
-}
dioptional
    :: (Alternative r, Applicative w)
    => Codec r w c a
    -> Codec r w (Maybe c) (Maybe a)
dioptional Codec{..} = Codec
    { codecRead  = optional codecRead
    , codecWrite = traverse codecWrite
    }

{- | 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
@
-}
diwrap
    :: forall b a r w .
       (Coercible a b, Functor r, Functor w)
    => BiCodec r w a
    -> BiCodec r w b
diwrap = dimap coerce coerce

{- | 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
@
-}
infixl 5 .=
(.=) :: Codec r w field a -> (object -> field) -> Codec r w object a
codec .= getter = codec { codecWrite = codecWrite codec . getter }