Either instance for IsForeign

examples/Either.purs

@@ -0,0 +1,27 @@
+module Examples.Either where
+
+import Prelude
+import Data.Either
+
+import Data.Foreign
+import Data.Foreign.Class
+
+import Control.Monad.Eff.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 = do
+  print $
+    readJSON """{ "x":1, "y": 2, "z": 3}""" :: F Response
+  print $ 
+    readJSON """["Invalid parse", "Not a valid y point"]""" :: F Response

src/Data/Foreign/Class.purs

@@ -10,6 +10,7 @@ module Data.Foreign.Class
 
 import Prelude
 
+import Control.Alt ((<|>))
 import Data.Array (range, zipWith, length)
 import Data.Either (Either(..), either)
 import Data.Foreign (F, Foreign, ForeignError(..), parseJSON, readArray, readInt, readNumber, readBoolean, readChar, readString)
@@ -75,3 +76,13 @@ readWith f value = either (Left <<< f) Right (read value)
 -- | Attempt to read a property of a foreign value at the specified index
 readProp :: forall a i. (IsForeign a, Index i) => i -> Foreign -> F a
 readProp prop value = value ! prop >>= readWith (errorAt prop)
+
+-- | 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