include key in error message from getField functions

Author: Chris Cornwell

src/Data/Argonaut/Decode/Combinators.purs

@@ -4,6 +4,7 @@ import Prelude
 
 import Data.Argonaut.Core (Json)
 import Data.Argonaut.Decode.Class (class DecodeJson, decodeJson)
+import Data.Bifunctor (lmap)
 import Data.Either (Either(..))
 import Data.Maybe (Maybe(..), fromMaybe, maybe)
 import Foreign.Object as FO
@@ -12,7 +13,7 @@ getField :: forall a. DecodeJson a => FO.Object Json -> String -> Either String
 getField o s =
   maybe
     (Left $ "Expected field " <> show s)
-    decodeJson
+    (elaborateFailure s <<< decodeJson)
     (FO.lookup s o)
 
 infix 7 getField as .?
@@ -24,11 +25,17 @@ getFieldOptional o s =
     decode
     (FO.lookup s o)
   where
-    decode json = Just <$> decodeJson json
+    decode json = Just <$> (elaborateFailure s <<< decodeJson) json
 
 infix 7 getFieldOptional as .??
 
 defaultField :: forall a. Either String (Maybe a) -> a -> Either String a
 defaultField parser default = fromMaybe default <$> parser
 
 infix 6 defaultField as .?=
+
+elaborateFailure :: ∀ a. String -> Either String a -> Either String a
+elaborateFailure s e =
+  lmap msg e
+  where
+    msg m = "Failed to decode key '" <> s <> "': " <> m