Merge pull request #17 from passy/maybe-combinator

Add a combinator for decoding maybe fields

Author: garyb

src/Data/Argonaut/Decode/Combinators.purs

@@ -5,7 +5,7 @@ import Prelude
 import Data.Argonaut.Core (JObject)
 import Data.Argonaut.Decode.Class (class DecodeJson, decodeJson)
 import Data.Either (Either(..))
-import Data.Maybe (maybe)
+import Data.Maybe (Maybe(Just, Nothing), maybe)
 import Data.StrMap as SM
 
 getField :: forall a. DecodeJson a => JObject -> String -> Either String a
@@ -16,3 +16,14 @@ getField o s =
     (SM.lookup s o)
 
 infix 7 getField as .?
+
+getFieldOptional :: forall a. DecodeJson a => JObject -> String -> Either String (Maybe a)
+getFieldOptional o s =
+  maybe
+    (pure Nothing)
+    decode
+    (SM.lookup s o)
+  where
+    decode json = Just <$> decodeJson json
+
+infix 7 getFieldOptional as .??

test/Test/Main.purs

@@ -143,6 +143,7 @@ data User
   | Guest String
   | Registered
       { name :: String
+      , bio :: Maybe String
       , age :: Int
       , balance :: Number
       , banned :: Boolean
@@ -169,6 +170,7 @@ genericsCheck = do
   logShow $ gEncodeJson $ Guest "guest's handle"
   logShow $ gEncodeJson $ Registered
     { name: "user1"
+    , bio: Just "Ordinary User"
     , age: 5
     , balance: 26.6
     , banned: false
@@ -178,6 +180,7 @@ genericsCheck = do
         , Guest "someGuest"
         , Registered
             { name: "user2"
+            , bio: Nothing
             , age: 6
             , balance: 32.1
             , banned: false