From 78d317c7bd0bcd599fe21e2cc0fea83564e68d9f Mon Sep 17 00:00:00 2001
From: Paul Brinkmeier <brinkmeier@meqo.de>
Date: Wed, 17 Sep 2025 19:57:25 +0200
Subject: [PATCH] Make gFromEnv return Either String (...)

---
 app/Main.hs | 12 +++++++++---
 src/Envy.hs | 19 ++++++++++---------
 2 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/app/Main.hs b/app/Main.hs
index d373052..e01f3eb 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -37,6 +37,7 @@ import Servant
   , (:>)
   , pattern MkHandler
   )
+import System.Exit (exitFailure)
 import System.FilePath ((</>))
 import Text.Blaze.Html.Renderer.Utf8 (renderHtml)
 import Text.Blaze.Html5 ((!))
@@ -80,8 +81,13 @@ deriving instance Show Config
 
 main :: IO ()
 main = do
-  cfg <- Envy.load @ConfigT
-  -- _ <- exitSuccess
+  cfg <-
+    Envy.load @ConfigT >>= \case
+      Left err -> do
+        Log.error $ printf "failed to read config: %s" err
+        exitFailure
+      Right c ->
+        pure c
 
   let
     settings =
@@ -95,7 +101,7 @@ main = do
         ]
 
   db <- DB.initDB cfg.yoreDb
-  Warp.runSettings settings $ serve (Proxy :: Proxy API) $ hoistServer (Proxy :: Proxy API) nt $ server cfg db
+  Warp.runSettings settings $ serve (Proxy @API) $ hoistServer (Proxy @API) nt $ server cfg db
   where
     logger req status _ = do
       Log.info $
diff --git a/src/Envy.hs b/src/Envy.hs
index f732210..6e23301 100644
--- a/src/Envy.hs
+++ b/src/Envy.hs
@@ -54,28 +54,30 @@ type v =@? t = v =@@ OptionalMaybe t
 type t ? d = Optional t d
 
 load
-  :: forall (m :: ConfigVariant -> Type). (Generic (m Value), GFromEnv (Rep (m Spec)) (Rep (m Value))) => IO (m Value)
+  :: forall (m :: ConfigVariant -> Type)
+   . (Generic (m Value), GFromEnv (Rep (m Spec)) (Rep (m Value)))
+  => IO (Either String (m Value))
 load = do
   env <- getEnvironment
-  pure $ to $ gFromEnv @(Rep (m Spec)) @(Rep (m Value)) Proxy env
+  pure $ to <$> gFromEnv @(Rep (m Spec)) @(Rep (m Value)) Proxy env
 
 class GFromEnv spec value where
-  gFromEnv :: Proxy (spec c) -> [(String, String)] -> value c
+  gFromEnv :: Proxy (spec c) -> [(String, String)] -> Either String (value c)
 
 instance
   (GFromEnv i o)
   => GFromEnv (M1 D b i) (M1 D d o)
   where
-  gFromEnv Proxy env = M1 $ gFromEnv @i @o Proxy env
+  gFromEnv Proxy env = M1 <$> gFromEnv @i @o Proxy env
 
 instance
   (GFromEnv i o)
   => GFromEnv (M1 C b i) (M1 C d o)
   where
-  gFromEnv Proxy env = M1 $ gFromEnv @i @o Proxy env
+  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 = (:*:) <$> gFromEnv @i1 @o1 Proxy env <*> gFromEnv @i2 @o2 Proxy env
 
 instance
   (KnownSymbol sym, EnvVarSpec s t)
@@ -83,9 +85,8 @@ instance
       (M1 S (MetaSel (Just sym) su ss lz) (Rec0 s))
       (M1 S meta2 (Rec0 t))
   where
-  gFromEnv Proxy env = case decodeEnvVar @s @t Proxy varName $ lookup varName env of
-    Left err -> error err
-    Right value -> M1 $ K1 value
+  gFromEnv Proxy env =
+    M1 . K1 <$> decodeEnvVar @s @t Proxy varName (lookup varName env)
     where
       varName = selectorNameToEnvVarName $ symbolVal $ Proxy @sym