better error message propagation when parsing List

src/Hpack/Util.hs

@@ -42,7 +42,9 @@ newtype List a = List {fromList :: [a]}
   deriving (Eq, Show, Data, Typeable)
 
 instance FromJSON a => FromJSON (List a) where
-  parseJSON v = List <$> (parseJSON v <|> (return <$> parseJSON v))
+  parseJSON v = List <$> case v of
+    Array _ -> parseJSON v
+    _ -> return <$> parseJSON v
 
 type GhcOption = String
 

test/Hpack/UtilSpec.hs

@@ -1,10 +1,14 @@
 {-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE QuasiQuotes #-}
 module Hpack.UtilSpec (main, spec) where
 
-import           Helper
 import           Data.Aeson
+import           Data.Aeson.QQ
+import           Data.Aeson.Types
+import           Helper
 import           System.Directory
 
+import           Hpack.Config
 import           Hpack.Util
 
 main :: IO ()
@@ -53,11 +57,26 @@ spec = do
           ]
 
   describe "List" $ do
-    it "can be a single value" $ do
-      fromJSON (toJSON $ Number 23) `shouldBe` Success (List [23 :: Int])
+    let invalid = [aesonQQ|{
+          name: "hpack",
+          gi: "sol/hpack",
+          ref: "master"
+        }|]
+        parseError :: Either String (List Dependency)
+        parseError = Left "neither key \"git\" nor key \"github\" present"
+    context "when parsing single values" $ do
+      it "returns the value in a singleton list" $ do
+        fromJSON (toJSON $ Number 23) `shouldBe` Success (List [23 :: Int])
 
-    it "can be a list of values" $ do
-      fromJSON (toJSON [Number 23, Number 42]) `shouldBe` Success (List [23, 42 :: Int])
+      it "returns error messages from element parsing" $ do
+        parseEither parseJSON invalid `shouldBe` parseError
+
+    context "when parsing a list of values" $ do
+      it "returns the list" $ do
+        fromJSON (toJSON [Number 23, Number 42]) `shouldBe` Success (List [23, 42 :: Int])
+
+      it "propagates parse error messages of invalid elements" $ do
+        parseEither parseJSON (toJSON [String "foo", invalid]) `shouldBe` parseError
 
   describe "tryReadFile" $ do
     it "reads file" $ do