Either instance for IsForeign

docs/Data/Foreign/Class.md

@@ -25,6 +25,7 @@ instance booleanIsForeign :: IsForeign Boolean
 instance numberIsForeign :: IsForeign Number
 instance intIsForeign :: IsForeign Int
 instance arrayIsForeign :: (IsForeign a) => IsForeign (Array a)
+instance eitherIsForeign :: (IsForeign l, IsForeign r) => IsForeign (Either l r)
 instance nullIsForeign :: (IsForeign a) => IsForeign (Null a)
 instance undefinedIsForeign :: (IsForeign a) => IsForeign (Undefined a)
 instance nullOrUndefinedIsForeign :: (IsForeign a) => IsForeign (NullOrUndefined a)

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

@@ -56,6 +56,15 @@ instance arrayIsForeign :: (IsForeign a) => IsForeign (Array a) where
     readElement :: forall a. (IsForeign a) => Int -> Foreign -> F (Array a)
     readElement i value = readWith (ErrorAtIndex i) value
 
+instance eitherIsForeign :: (IsForeign l, IsForeign r) => IsForeign (Either l r) where
+  read value =
+    case (read value) :: F r of
+         Right rval -> pure (Right rval)
+         Left e ->
+           case (read value) :: F l of
+                Right lval -> pure (Left lval)
+                Left e' -> Left e'
+
 instance nullIsForeign :: (IsForeign a) => IsForeign (Null a) where
   read = readNull read