Either decoding for IsForeign

Author: parsonsmatt

examples/Either.purs

@@ -0,0 +1,32 @@
+module Examples.Either where
+
+import Prelude
+import Control.Monad.Eff (Eff)
+import Data.Either (Either)
+
+import Data.Foreign (F, parseJSON)
+import Data.Foreign.Class (class IsForeign, readEitherR, readProp)
+
+import Control.Monad.Eff.Console (logShow, CONSOLE)
+
+data Point = Point Number Number Number
+
+instance showPoint :: Show Point where
+  show (Point x y z) = "Point " <> show [x, y, z]
+
+instance pointIsForeign :: IsForeign Point where
+  read value = Point <$> readProp "x" value
+                     <*> readProp "y" value
+                     <*> readProp "z" value
+
+type Response = Either (Array String) Point
+
+main :: forall eff. Eff (console :: CONSOLE | eff) Unit
+main = do
+  logShow do
+    json <- parseJSON """{ "x":1, "y": 2, "z": 3}"""
+    readEitherR json :: F Response
+
+  logShow do
+    json <- parseJSON """["Invalid parse", "Not a valid y point"]"""
+    readEitherR json :: F Response

src/Data/Foreign/Class.purs

@@ -9,10 +9,13 @@ module Data.Foreign.Class
   , class AsForeign
   , write
   , writeProp, (.=)
+  , readEitherR
+  , readEitherL
   ) where
 
 import Prelude
 
+import Control.Alt ((<|>))
 import Data.Array (range, zipWith, length)
 import Data.Either (Either(..), either)
 import Data.Foreign (F, Foreign, ForeignError(..), Prop(..), parseJSON, readArray, readInt, readNumber, readBoolean, readChar, readString, toForeign)
@@ -120,3 +123,13 @@ infixl 8 writeProp as .=
 
 writeProp :: forall a. AsForeign a => String -> a -> Prop
 writeProp k v = Prop { key: k, value: write v }
+
+-- | Attempt to read a value that can be either one thing or another. This
+-- | implementation is right biased.
+readEitherR :: forall l r. (IsForeign l, IsForeign r) => Foreign -> F (Either l r)
+readEitherR value = Right <$> read value <|> Left <$> read value
+
+-- | Attempt to read a value that can be either one thing or another. This
+-- | implementation is left biased.
+readEitherL :: forall l r. (IsForeign l, IsForeign r) => Foreign -> F (Either l r)
+readEitherL value = Left <$> read value <|> Right <$> read value