From c601613162c77b905508d996ec72595ae451683d Mon Sep 17 00:00:00 2001
From: Felipe Lessa <felipe.lessa@gmail.com>
Date: Tue, 4 Sep 2012 18:12:25 -0300
Subject: [PATCH] isNothing, just, nothing.

---
 src/Database/Esqueleto.hs                   |  3 ++-
 src/Database/Esqueleto/Internal/Language.hs | 11 +++++++++++
 src/Database/Esqueleto/Internal/Sql.hs      |  6 ++++++
 test/Test.hs                                | 22 ++++++++++++++++++---
 4 files changed, 38 insertions(+), 4 deletions(-)

diff --git a/src/Database/Esqueleto.hs b/src/Database/Esqueleto.hs
index 76da173..e16bdd9 100644
--- a/src/Database/Esqueleto.hs
+++ b/src/Database/Esqueleto.hs
@@ -16,7 +16,8 @@
 -- @
 module Database.Esqueleto
   ( -- * Esqueleto's Language
-    Esqueleto( where_, sub, (^.), val, not_, (==.), (>=.)
+    Esqueleto( where_, sub, (^.), val, isNothing, just, nothing
+             , not_, (==.), (>=.)
              , (>.), (<=.), (<.), (!=.), (&&.), (||.)
              , (+.), (-.), (/.), (*.) )
   , from
diff --git a/src/Database/Esqueleto/Internal/Language.hs b/src/Database/Esqueleto/Internal/Language.hs
index 9fa9688..2911036 100644
--- a/src/Database/Esqueleto/Internal/Language.hs
+++ b/src/Database/Esqueleto/Internal/Language.hs
@@ -30,6 +30,17 @@ class (Functor query, Applicative query, Monad query) =>
   -- | Lift a constant value from Haskell-land to the query.
   val  :: PersistField typ => typ -> expr (Single typ)
 
+  -- | @IS NULL@ comparison.
+  isNothing :: PersistField typ => expr (Single (Maybe typ)) -> expr (Single Bool)
+
+  -- | Analog to 'Just', promotes a value of type @typ@ into one
+  -- of type @Maybe typ@.  It should hold that @val . Just ===
+  -- just . val@.
+  just :: expr (Single typ) -> expr (Single (Maybe typ))
+
+  -- | @NULL@ value.
+  nothing :: expr (Single (Maybe typ))
+
   not_ :: expr (Single Bool) -> expr (Single Bool)
 
   (==.) :: PersistField typ => expr (Single typ) -> expr (Single typ) -> expr (Single Bool)
diff --git a/src/Database/Esqueleto/Internal/Sql.hs b/src/Database/Esqueleto/Internal/Sql.hs
index 8448a41..8b19b25 100644
--- a/src/Database/Esqueleto/Internal/Sql.hs
+++ b/src/Database/Esqueleto/Internal/Sql.hs
@@ -117,6 +117,12 @@ instance Esqueleto SqlQuery SqlExpr SqlPersist where
 
   val = ERaw . const . (,) "?" . return . toPersistValue
 
+  isNothing (ERaw f) = ERaw $ first ((<> " IS NULL") . parens) . f
+  isNothing _        = error "Esqueleto/Sql/isNothing: never here (see GHC #6124)"
+  just (ERaw f) = ERaw f
+  just _        = error "Esqueleto/Sql/just: never here (see GHC #6124)"
+  nothing = ERaw $ \_ -> ("NULL", mempty)
+
   not_ (ERaw f) = ERaw $ \esc -> let (b, vals) = f esc
                                  in ("NOT " <> parens b, vals)
   not_ _ = error "Esqueleto/Sql/not_: never here (see GHC #6124)"
diff --git a/test/Test.hs b/test/Test.hs
index 83aa668..1dd8351 100644
--- a/test/Test.hs
+++ b/test/Test.hs
@@ -50,6 +50,11 @@ main = do
           ret <- select $ return $ val (3 :: Int)
           liftIO $ ret `shouldBe` [ Single 3 ]
 
+      it "works for a single NULL value" $
+        run $ do
+          ret <- select $ return $ nothing
+          liftIO $ ret `shouldBe` [ Single (Nothing :: Maybe Int) ]
+
     describe "select/from" $ do
       it "works for a simple example" $
         run $ do
@@ -124,7 +129,7 @@ main = do
                  return p
           liftIO $ ret `shouldBe` [ Entity p1k p1, Entity p2k p2 ]
 
-      it "works for a simple example with (>.)" $
+      it "works for a simple example with (>.) [uses val . Just]" $
         run $ do
           p1k <- insert p1
           _   <- insert p2
@@ -135,17 +140,28 @@ main = do
                  return p
           liftIO $ ret `shouldBe` [ Entity p1k p1 ]
 
-      it "works for a simple example with (>.) and not_" $
+      it "works for a simple example with (>.) and not_ [uses just . val]" $
         run $ do
           _   <- insert p1
           _   <- insert p2
           p3k <- insert p3
           ret <- select $
                  from $ \p -> do
-                 where_ (not_ $ p ^. PersonAge >. val (Just 17))
+                 where_ (not_ $ p ^. PersonAge >. just (val 17))
                  return p
           liftIO $ ret `shouldBe` [ Entity p3k p3 ]
 
+      it "works with isNothing" $
+        run $ do
+          _   <- insert p1
+          p2k <- insert p2
+          _   <- insert p3
+          ret <- select $
+                 from $ \p -> do
+                 where_ $ isNothing (p ^. PersonAge)
+                 return p
+          liftIO $ ret `shouldBe` [ Entity p2k p2 ]
+
 
 ----------------------------------------------------------------------