From 5018a34abd68f90bf7dc33c1a8d264c051f40270 Mon Sep 17 00:00:00 2001
From: Denis Redozubov <denis.redozubov@gmail.com>
Date: Tue, 22 Sep 2015 14:02:52 +0300
Subject: [PATCH] fix Elem type family to enable sane error reporting

---
 servant-foreign/src/Servant/Foreign.hs | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/servant-foreign/src/Servant/Foreign.hs b/servant-foreign/src/Servant/Foreign.hs
index cdfebc31..c42ccdc3 100644
--- a/servant-foreign/src/Servant/Foreign.hs
+++ b/servant-foreign/src/Servant/Foreign.hs
@@ -1,13 +1,14 @@
-{-# LANGUAGE CPP                  #-}
-{-# LANGUAGE ConstraintKinds      #-}
-{-# LANGUAGE DataKinds            #-}
-{-# LANGUAGE FlexibleContexts     #-}
-{-# LANGUAGE FlexibleInstances    #-}
-{-# LANGUAGE ScopedTypeVariables  #-}
-{-# LANGUAGE TemplateHaskell      #-}
-{-# LANGUAGE TypeFamilies         #-}
-{-# LANGUAGE TypeOperators        #-}
-{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE CPP                   #-}
+{-# LANGUAGE ConstraintKinds       #-}
+{-# LANGUAGE DataKinds             #-}
+{-# LANGUAGE FlexibleContexts      #-}
+{-# LANGUAGE FlexibleInstances     #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE ScopedTypeVariables   #-}
+{-# LANGUAGE TemplateHaskell       #-}
+{-# LANGUAGE TypeFamilies          #-}
+{-# LANGUAGE TypeOperators         #-}
+{-# LANGUAGE UndecidableInstances  #-}
 
 -- | Generalizes all the data needed to make code generation work with
 -- arbitrary programming languages.
@@ -140,7 +141,12 @@ captureArg                  _  = error "captureArg called on non capture"
 defReq :: Req
 defReq = Req defUrl "GET" [] False []
 
+-- | To be used exclusively as a "negative" return type/constraint
+-- by @'Elem`@ type family.
+class NotFound
+
 type family Elem (a :: *) (ls::[*]) :: Constraint where
+  Elem a '[]         = NotFound
   Elem a (a ': list) = ()
   Elem a (b ': list) = Elem a list