module Example.Union where

import Prelude

import Control.Monad.Except (Except, runExcept)
import Data.List.NonEmpty (NonEmptyList)
import Effect (Effect)
import Effect.Console (logShow)
import Example.Util.Value (foreignValue)
import Foreign (Foreign, ForeignError, readBoolean, readString)
import Foreign.Index ((!))

data StringList = Nil | Cons String StringList

instance showStringList :: Show StringList where
  show Nil = "Nil"
  show (Cons s l) = "(Cons " <> show s <> " " <> show l <> ")"

readStringList :: Foreign -> Except (NonEmptyList ForeignError) StringList
readStringList value =
  value ! "nil" >>=
    readBoolean >>=
      if _
      then pure Nil
      else
        Cons
          <$> (value ! "head" >>= readString)
          <*> (value ! "tail" >>= readStringList)

main :: Effect Unit
main = do

  logShow $ runExcept $
    readStringList =<< foreignValue
      """
      { "nil": false
      , "head": "Hello"
      , "tail":
        { "nil": false
        , "head": "World"
        , "tail":
          { "nil": true }
        }
      }
      """

  logShow $ runExcept $
    readStringList =<< foreignValue
      """
      { "nil": false
      , "head": "Hello"
      , "tail":
        { "nil": false
        , "head": 0
        , "tail":
          { "nil": true }
        }
      }
      """