PureScript by Example: Chapter 10 (The Foreign Function Interface) exercise 10.19.4

Tagged.purs

module Tagged where

import Prelude

import Data.Either
import Data.Foreign
import Data.Foreign.Class

newtype Tagged a b = Tagged (Either a b)

tagKey :: String
tagKey = "tag"

valueKey :: String
valueKey = "value"

instance taggedIsForeign :: (IsForeign a, IsForeign b) => IsForeign (Tagged a b) where
    read value = do
        tag <- readProp tagKey value
        case tag of
             "Left"  -> (Tagged <<< Left)  <$> readProp valueKey value
             "Right" -> (Tagged <<< Right) <$> readProp valueKey value
             _       -> Left $ JSONError $ "Unrecoginzed tag " ++ tag

instance showTagged :: (Show a, Show b) => Show (Tagged a b) where
    show (Tagged e) = "Tagged " ++ show e