use a compatibility class

src/Database/Esqueleto/Internal/Language.hs

@@ -53,6 +53,11 @@ import qualified Data.ByteString as B
 import qualified Data.Text as T
 import qualified Data.Text.Lazy as TL
 
+class BackendCompatible sup sub
+
+instance BackendCompatible SqlBackend SqlBackend
+instance BackendCompatible SqlBackend SqlReadBackend
+instance BackendCompatible SqlBackend SqlWriteBackend
 
 -- | Finally tagless representation of @esqueleto@'s EDSL.
 class (Functor query, Applicative query, Monad query) =>
@@ -72,12 +77,12 @@ class (Functor query, Applicative query, Monad query) =>
   --   @JOIN@.
   fromStart
     :: ( PersistEntity a
-       , PersistEntityBackend a ~ backend )
+       , BackendCompatible backend (PersistEntityBackend a) )
     => query (expr (PreprocessedFrom (expr (Entity a))))
   -- | (Internal) Same as 'fromStart', but entity may be missing.
   fromStartMaybe
     :: ( PersistEntity a
-       , PersistEntityBackend a ~ backend )
+       , BackendCompatible backend (PersistEntityBackend a) )
     => query (expr (PreprocessedFrom (expr (Maybe (Entity a)))))
   -- | (Internal) Do a @JOIN@.
   fromJoin
@@ -1041,13 +1046,13 @@ class Esqueleto query expr backend => FromPreprocess query expr backend a where
 
 instance ( Esqueleto query expr backend
          , PersistEntity val
-         , PersistEntityBackend val ~ backend
+         , BackendCompatible backend (PersistEntityBackend val)
          ) => FromPreprocess query expr backend (expr (Entity val)) where
   fromPreprocess = fromStart
 
 instance ( Esqueleto query expr backend
          , PersistEntity val
-         , PersistEntityBackend val ~ backend
+         , BackendCompatible backend (PersistEntityBackend val)
          ) => FromPreprocess query expr backend (expr (Maybe (Entity val))) where
   fromPreprocess = fromStartMaybe