Add placeholder instance to the otherwise empty type family (fix GHC …

…7 builds)
fizruk · Aug 31, 2016 · 570353a · 570353a
1 parent 4bc8d3e
commit 570353a
Showing 1 changed file with 2 additions and 0 deletions.
diff --git a/src/Web/Internal/FormUrlEncoded.hs b/src/Web/Internal/FormUrlEncoded.hs
@@ -255,6 +255,8 @@ data Proxy3 a b c = Proxy3
 
 type family NotSupported (cls :: k1) (a :: k2) (reason :: Symbol) :: Constraint where
 #if __GLASGOW_HASKELL__ < 800
+  -- this is just a placeholder case for older GHCs to not freak out on an empty closed type family
+  NotSupported cls a "this type family is actually empty" = ()
 #else
   NotSupported cls a reason = TypeError
     ( 'Text "Cannot derive a Generic-based " ':<>: 'ShowType cls ':<>: 'Text " instance for " ':<>: 'ShowType a ':<>: 'Text "." ':$$: