Add instance of UnsafeSqlFunctionArgument () (#159)

* Add instance of UnsafeSqlFunctionArgument ()

* Use now, clean a warn

changelog.md

@@ -1,3 +1,10 @@
+3.2.1
+========
+
+- @parsonsmatt
+  = [#159](https://github.com/bitemyapp/esqueleto/pull/159): Add an instance of `UnsafeSqlFunction ()` for 0-argument SQL
+  functions.
+
 3.2.0
 ========
 

esqueleto.cabal

@@ -1,7 +1,7 @@
 cabal-version: 1.12
 
 name:           esqueleto
-version:        3.2.0
+version:        3.2.1
 synopsis:       Type-safe EDSL for SQL queries on persistent backends.
 description:    @esqueleto@ is a bare bones, type-safe EDSL for SQL queries that works with unmodified @persistent@ SQL backends.  Its language closely resembles SQL, so you don't have to learn new concepts, just new syntax, and it's fairly easy to predict the generated SQL and optimize it for your backend. Most kinds of errors committed when writing SQL are caught as compile-time errors---although it is possible to write type-checked @esqueleto@ queries that fail at runtime.
                 .

src/Database/Esqueleto/Internal/Internal.hs

@@ -2025,6 +2025,13 @@ unsafeSqlCastAs _ (ECompositeKey _) = throw (CompositeKeyErr SqlCastAsError)
 
 class UnsafeSqlFunctionArgument a where
   toArgList :: a -> [SqlExpr (Value ())]
+
+-- | Useful for 0-argument functions, like @now@ in Postgresql.
+--
+-- @since 3.2.1
+instance UnsafeSqlFunctionArgument () where
+  toArgList _ = []
+
 instance (a ~ Value b) => UnsafeSqlFunctionArgument (SqlExpr a) where
   toArgList = (:[]) . veryUnsafeCoerceSqlExprValue
 instance UnsafeSqlFunctionArgument a =>

src/Database/Esqueleto/PostgreSQL.hs

@@ -44,7 +44,7 @@ import           Data.List.NonEmpty                           ( NonEmpty( (:|) )
 import           Data.Int                                     (Int64)
 import           Data.Proxy                                   (Proxy(..))
 import           Control.Arrow                                ((***), first)
-import           Control.Exception                            (Exception, throw, throwIO)
+import           Control.Exception                            (throw)
 import           Control.Monad                                (void)
 import           Control.Monad.IO.Class                       (MonadIO (..))
 import qualified Control.Monad.Trans.Reader                   as R
@@ -169,7 +169,7 @@ chr :: SqlString s => SqlExpr (Value Int) -> SqlExpr (Value s)
 chr = unsafeSqlFunction "chr"
 
 now_ :: SqlExpr (Value UTCTime)
-now_ = unsafeSqlValue "NOW()"
+now_ = unsafeSqlFunction "NOW" ()
 
 upsert :: (MonadIO m,
       PersistEntity record,

test/PostgreSQL/Test.hs

@@ -25,7 +25,7 @@ import qualified Data.List as L
 import Data.Ord (comparing)
 import qualified Data.Text as T
 import qualified Data.Text.Encoding as TE
-import Data.Time.Clock (getCurrentTime, diffUTCTime)
+import Data.Time.Clock (getCurrentTime, diffUTCTime, UTCTime)
 import Database.Esqueleto hiding (random_)
 import qualified Database.Esqueleto.Internal.Sql as ES
 import Database.Esqueleto.PostgreSQL (random_)
@@ -493,6 +493,14 @@ testPostgresModule = do
         [Value (ret :: String)] <- select $ return (EP.chr (val 65))
         liftIO $ ret `shouldBe` "A"
 
+    it "allows unit for functions" $ do
+      vals <- run $ do
+        let
+          fn :: SqlExpr (Value UTCTime)
+          fn = ES.unsafeSqlFunction "now" ()
+        select $ pure fn
+      vals `shouldSatisfy` ((1 ==) . length)
+
     it "works with now" $
       run $ do
         nowDb <- select $ return EP.now_