Collect all errors in envy

app/Main.hs

@@ -79,8 +79,8 @@ main :: IO ()
 main = do
   cfg <-
     Envy.load @ConfigT >>= \case
-      Left err -> do
-        Log.error $ printf "failed to read config: %s" err
+      Left errs -> do
+        forM_ errs $ Log.error . printf "failed to read config: %s"
         exitFailure
       Right c ->
         pure c

src/Envy.hs

@@ -56,13 +56,13 @@ type t ? d = Optional t d
 load
   :: forall (m :: ConfigVariant -> Type)
    . (Generic (m Value), GFromEnv (Rep (m Spec)) (Rep (m Value)))
-  => IO (Either String (m Value))
+  => IO (Either [String] (m Value))
 load = do
   env <- getEnvironment
   pure $ to <$> gFromEnv @(Rep (m Spec)) @(Rep (m Value)) Proxy env
 
 class GFromEnv spec value where
-  gFromEnv :: Proxy (spec c) -> [(String, String)] -> Either String (value c)
+  gFromEnv :: Proxy (spec c) -> [(String, String)] -> Either [String] (value c)
 
 instance
   (GFromEnv i o)
@@ -77,7 +77,15 @@ instance
   gFromEnv Proxy env = M1 <$> gFromEnv @i @o Proxy env
 
 instance (GFromEnv i1 o1, GFromEnv i2 o2) => GFromEnv (i1 :*: i2) (o1 :*: o2) where
-  gFromEnv Proxy env = (:*:) <$> gFromEnv @i1 @o1 Proxy env <*> gFromEnv @i2 @o2 Proxy env
+  gFromEnv Proxy env =
+    case (eLhs, eRhs) of
+      (Left errs1, Right _) -> Left errs1
+      (Right _, Left errs2) -> Left errs2
+      (Left errs1, Left errs2) -> Left $ errs1 ++ errs2
+      (Right lhs, Right rhs) -> Right $ lhs :*: rhs
+    where
+      eLhs = gFromEnv @i1 @o1 Proxy env
+      eRhs = gFromEnv @i2 @o2 Proxy env
 
 instance
   (KnownSymbol sym, EnvVarSpec s t)
@@ -86,7 +94,7 @@ instance
       (M1 S meta2 (Rec0 t))
   where
   gFromEnv Proxy env =
-    M1 . K1 <$> decodeEnvVar @s @t Proxy varName (lookup varName env)
+    M1 . K1 <$> first (: []) (decodeEnvVar @s @t Proxy varName (lookup varName env))
     where
       varName = selectorNameToEnvVarName $ symbolVal $ Proxy @sym