G.hs

{-# LANGUAGE DeriveFoldable #-}
{-# LANGUAGE DeriveFunctor #-}

module Salmon.Op.G where

import Control.Comonad.Cofree (Cofree (..))
import Data.Aeson (FromJSON, ToJSON (..), (.:), (.=))
import qualified Data.Aeson as Aeson
import qualified Data.Aeson.Types as Aeson
import Data.Coerce (coerce)

import Salmon.Op.Graph (Graph)

-- | Helper to provide Aeson instances for (Cofree Graph a).
newtype G a = G {getCofreeGraph :: (Cofree Graph a)}
    deriving (Functor, Foldable)

instance (ToJSON a) => ToJSON (G a) where
    toJSON (G (x :< p)) =
        Aeson.object
            [ "node" .= toJSON x
            , "preds" .= (toJSON $ fmap G p)
            ]

instance (FromJSON a) => FromJSON (G a) where
    parseJSON = Aeson.withObject "cofree layer" $ \o -> do
        obj <- o .: "node"
        preds <- o .: "preds" :: Aeson.Parser (Graph (G a))
        pure $ G $ obj :< fmap coerce preds