Make error message more useful on failure to parse ApiAsArray.

In response to review feedback:

https://github.com/input-output-hk/cardano-wallet/pull/3251/files#r859371410

lib/core/src/Cardano/Wallet/Api/Types.hs

@@ -452,7 +452,7 @@ import Fmt
 import GHC.Generics
     ( Generic, Rep )
 import GHC.TypeLits
-    ( Nat, Symbol )
+    ( KnownSymbol, Nat, Symbol, symbolVal )
 import Numeric.Natural
     ( Natural )
 import Quiet
@@ -1146,8 +1146,9 @@ data ApiTransaction (n :: NetworkDiscriminant) = ApiTransaction
     , inputs :: ![ApiTxInput n]
     , outputs :: ![AddressAmount (ApiT Address, Proxy n)]
     , collateral :: ![ApiTxCollateral n]
-    , collateralOutputs
-        :: !(ApiAsArray (Maybe (AddressAmount (ApiT Address, Proxy n))))
+    , collateralOutputs ::
+        !(ApiAsArray "collateral_outputs"
+            (Maybe (AddressAmount (ApiT Address, Proxy n))))
     , withdrawals :: ![ApiWithdrawal n]
     , mint :: !(ApiT W.TokenMap)
     , status :: !(ApiT TxStatus)
@@ -1275,7 +1276,8 @@ data ApiDecodedTransaction (n :: NetworkDiscriminant) = ApiDecodedTransaction
     , inputs :: ![ApiTxInputGeneral n]
     , outputs :: ![ApiTxOutputGeneral n]
     , collateral :: ![ApiTxInputGeneral n]
-    , collateralOutputs :: !(ApiAsArray (Maybe (ApiTxOutputGeneral n)))
+    , collateralOutputs ::
+        !(ApiAsArray "collateral_outputs" (Maybe (ApiTxOutputGeneral n)))
     , withdrawals :: ![ApiWithdrawalGeneral n]
     , mint :: !ApiAssetMintBurn
     , burn :: !ApiAssetMintBurn
@@ -4144,19 +4146,23 @@ instance ToJSON (ApiT TxScriptValidity) where
 --
 -- The number of items permitted in the array is dependent on the wrapped type.
 --
-newtype ApiAsArray a = ApiAsArray a
+newtype ApiAsArray (s :: Symbol) a = ApiAsArray a
     deriving (Eq, Generic, Show, Typeable)
     deriving newtype (Monoid, Semigroup)
     deriving anyclass NFData
 
-instance FromJSON a => FromJSON (ApiAsArray (Maybe a)) where
+instance (KnownSymbol s, FromJSON a) => FromJSON (ApiAsArray s (Maybe a)) where
     parseJSON json = parseJSON @[a] json >>= \case
         [a] ->
             pure $ ApiAsArray $ Just a
         [] ->
             pure $ ApiAsArray Nothing
         _  ->
-            fail "Expected at most one item."
+            fail $ mconcat
+                [ "Expected at most one item for "
+                , show $ symbolVal $ Proxy @s
+                , "."
+                ]
 
-instance ToJSON a => ToJSON (ApiAsArray (Maybe a)) where
+instance ToJSON a => ToJSON (ApiAsArray s (Maybe a)) where
     toJSON (ApiAsArray m) = toJSON (maybeToList m)

lib/core/test/unit/Cardano/Wallet/Api/TypesSpec.hs

@@ -486,7 +486,7 @@ spec = parallel $ do
     describe "JSON golden roundtrip" $ do
         jsonTest @(AddressAmount (ApiT Address, Proxy ('Testnet 0)))
         jsonTest @(AddressAmountNoAssets (ApiT Address, Proxy ('Testnet 0)))
-        jsonTest @(ApiAsArray (Maybe Word64))
+        jsonTest @(ApiAsArray "Test" (Maybe Word64))
         jsonTest @(ApiBalanceTransactionPostData ('Testnet 0))
         jsonTest @(ApiCoinSelection ('Testnet 0))
         jsonTest @(ApiCoinSelectionChange ('Testnet 0))
@@ -662,7 +662,9 @@ spec = parallel $ do
 
         describe "ApiAsArray (Maybe t)" $ do
             let parseApiAsArray = Aeson.parseEither $
-                    parseJSON @(ApiAsArray (Maybe Word64))
+                    parseJSON @(ApiAsArray "test_field" (Maybe Word64))
+            let expectedFailureMessage =
+                    "Error in $: Expected at most one item for \"test_field\"."
             it "element count = 0 (should succeed)" $
                 parseApiAsArray [aesonQQ|[]|]
                     `shouldBe` Right (ApiAsArray Nothing)
@@ -671,10 +673,10 @@ spec = parallel $ do
                     `shouldBe` Right (ApiAsArray (Just 0))
             it "element count = 2 (should fail)" $
                 parseApiAsArray [aesonQQ|[0, 1]|]
-                    `shouldBe` Left "Error in $: Expected at most one item."
+                    `shouldBe` Left expectedFailureMessage
             it "element count = 3 (should fail)" $
                 parseApiAsArray [aesonQQ|[0, 1, 2]|]
-                    `shouldBe` Left "Error in $: Expected at most one item."
+                    `shouldBe` Left expectedFailureMessage
 
         it "ApiT (Passphrase \"user\") (too short)" $ do
             let minLength = passphraseMinLength (Proxy :: Proxy "user")
@@ -2470,7 +2472,7 @@ selectFromPreparedBinaries = elements $ toByteString <$>
 genWits :: Gen ByteString
 genWits = BS.pack <$> Test.QuickCheck.scale (min 32) (listOf arbitrary)
 
-deriving instance Arbitrary a => Arbitrary (ApiAsArray a)
+deriving instance Arbitrary a => Arbitrary (ApiAsArray s a)
 
 instance Arbitrary (ApiBytesT base ByteString) where
     arbitrary = ApiBytesT <$> selectFromPreparedBinaries