countDistinct, countDistinctRows.

src/Database/Esqueleto.hs

@@ -41,8 +41,9 @@ module Database.Esqueleto
     Esqueleto( where_, on, groupBy, orderBy, rand, asc, desc, limit, offset
              , distinct, distinctOn, don, distinctOnOrderBy, having, locking
              , sub_select, sub_selectDistinct, (^.), (?.)
-             , val, isNothing, just, nothing, joinV, countRows, count, not_
-             , (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)
+             , val, isNothing, just, nothing, joinV
+             , countRows, count, countDistinct
+             , not_, (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)
              , (+.), (-.), (/.), (*.)
              , random_, round_, ceiling_, floor_
              , min_, max_, sum_, avg_, castNum, castNumM

src/Database/Esqueleto/Internal/Language.hs

@@ -327,7 +327,12 @@ class (Functor query, Applicative query, Monad query) =>
   countRows :: Num a => expr (Value a)
 
   -- | @COUNT@.
-  count :: (Num a) => expr (Value typ) -> expr (Value a)
+  count :: Num a => expr (Value typ) -> expr (Value a)
+
+  -- | @COUNT(DISTINCT x)@.
+  --
+  -- /Since: 2.4.1/
+  countDistinct :: Num a => expr (Value typ) -> expr (Value a)
 
   not_ :: expr (Value Bool) -> expr (Value Bool)
 

src/Database/Esqueleto/Internal/Sql.hs

@@ -55,15 +55,15 @@ import Control.Exception (throw, throwIO)
 import Control.Monad (ap, MonadPlus(..), liftM)
 import Control.Monad.IO.Class (MonadIO(..))
 import Control.Monad.Trans.Class (lift)
-import qualified Control.Monad.Trans.Reader as R
+import Control.Monad.Trans.Resource (MonadResource)
+import Data.Acquire (with, allocateAcquire, Acquire)
 import Data.Int (Int64)
 import Data.List (intersperse)
 import Data.Monoid (Last(..), Monoid(..), (<>))
 import Data.Proxy (Proxy(..))
 import Database.Esqueleto.Internal.PersistentImport
-import Database.Persist.Sql.Util (
-    entityColumnNames, entityColumnCount, parseEntityValues, isIdField
-  , hasCompositeKey)
+import Database.Persist.Sql.Util (entityColumnNames, entityColumnCount, parseEntityValues, isIdField, hasCompositeKey)
+import qualified Control.Monad.Trans.Reader as R
 import qualified Control.Monad.Trans.State as S
 import qualified Control.Monad.Trans.Writer as W
 import qualified Data.Conduit as C
@@ -72,8 +72,6 @@ import qualified Data.HashSet as HS
 import qualified Data.Text as T
 import qualified Data.Text.Lazy as TL
 import qualified Data.Text.Lazy.Builder as TLB
-import Data.Acquire (with, allocateAcquire, Acquire)
-import Control.Monad.Trans.Resource (MonadResource)
 
 import Database.Esqueleto.Internal.Language
 
@@ -444,10 +442,9 @@ instance Esqueleto SqlQuery SqlExpr SqlBackend where
   nothing = unsafeSqlValue "NULL"
   joinV (ERaw p f)        = ERaw p f
   joinV (ECompositeKey f) = ECompositeKey f
-  countRows = unsafeSqlValue "COUNT(*)"
-  count (ERaw _ f) = ERaw Never $ \info -> let (b, vals) = f info
-                                           in ("COUNT" <> parens b, vals)
-  count (ECompositeKey _) = unsafeSqlValue "COUNT(*)" -- Assumes no NULLs on a PK
+  countRows     = unsafeSqlValue "COUNT(*)"
+  count         = countHelper ""           ""
+  countDistinct = countHelper "(DISTINCT " ")"
 
   not_ (ERaw p f) = ERaw Never $ \info -> let (b, vals) = f info
                                           in ("NOT " <> parensM p b, vals)
@@ -558,6 +555,10 @@ ifNotEmptyList :: SqlExpr (ValueList a) -> Bool -> SqlExpr (Value Bool) -> SqlEx
 ifNotEmptyList EEmptyList b _ = val b
 ifNotEmptyList (EList _)  _ x = x
 
+countHelper :: Num a => TLB.Builder -> TLB.Builder -> SqlExpr (Value typ) -> SqlExpr (Value a)
+countHelper open close (ERaw _ f) = ERaw Never $ first (\b -> "COUNT" <> open <> parens b <> close) . f
+countHelper _ _ (ECompositeKey _) = countRows -- Assumes no NULLs on a PK
+
 
 ----------------------------------------------------------------------
 

test/Test.hs

@@ -21,7 +21,7 @@ module Main (main) where
 import Control.Applicative ((<$>))
 import Control.Arrow ((&&&))
 import Control.Exception (IOException)
-import Control.Monad (replicateM, replicateM_, void)
+import Control.Monad (forM_, replicateM, replicateM_, void)
 import Control.Monad.IO.Class (MonadIO(liftIO))
 import Control.Monad.Logger (MonadLogger(..), runStderrLoggingT, runNoLoggingT)
 import Control.Monad.Trans.Control (MonadBaseControl(..))
@@ -1330,6 +1330,23 @@ main = do
       it "looks sane for ForShare"        $ sanityCheck ForShare        "FOR SHARE"
       it "looks sane for LockInShareMode" $ sanityCheck LockInShareMode "LOCK IN SHARE MODE"
 
+    describe "counting rows" $ do
+      forM_ [ ("count (test A)",    count . (^. PersonAge),         4)
+            , ("count (test B)",    count . (^. PersonWeight),      5)
+            , ("countRows",         const countRows,                5)
+            , ("countDistinct",     countDistinct . (^. PersonAge), 2) ] $
+        \(title, countKind, expected) ->
+        it (title ++ " works as expected") $
+          run $ do
+            mapM_ insert
+              [ Person "" (Just 1) (Just 1) 1
+              , Person "" (Just 2) (Just 1) 1
+              , Person "" (Just 2) (Just 1) 1
+              , Person "" (Just 2) (Just 2) 1
+              , Person "" Nothing  (Just 3) 1]
+            [Value n] <- select $ from $ return . countKind
+            liftIO $ (n :: Int) `shouldBe` expected
+
     describe "PostgreSQL module" $ do
       it "should be tested on the PostgreSQL database" $
 #if !defined(WITH_POSTGRESQL)