From b67fcedfebc9a52293c77ddec516077a03c7f6ba Mon Sep 17 00:00:00 2001
From: Felipe Lessa <felipe.lessa@gmail.com>
Date: Wed, 5 Sep 2012 20:34:03 -0300
Subject: [PATCH] Allow queries to return ().

---
 src/Database/Esqueleto/Internal/Sql.hs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/Database/Esqueleto/Internal/Sql.hs b/src/Database/Esqueleto/Internal/Sql.hs
index eec506b..d1e5001 100644
--- a/src/Database/Esqueleto/Internal/Sql.hs
+++ b/src/Database/Esqueleto/Internal/Sql.hs
@@ -480,6 +480,10 @@ class SqlSelect a r | a -> r, r -> a where
   -- | Transform a row of the result into the data type.
   sqlSelectProcessRow :: [PersistValue] -> Either T.Text r
 
+instance SqlSelect () () where
+  sqlSelectCols _ _ = mempty
+  sqlSelectColCount _ = 0
+  sqlSelectProcessRow _ = Right ()
 
 instance PersistEntity a => SqlSelect (SqlExpr (Entity a)) (Entity a) where
   sqlSelectCols escape expr@(EEntity ident) = ret