Add instance of UnsafeSqlFunctionArgument ()

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

@@ -1053,12 +1053,12 @@ instance FinalResult (Unique val) where
 instance (FinalResult b) => FinalResult (a -> b) where
   finalR f = finalR (f undefined)
 
--- | Convert a constructor for a 'Unique' key on a record to the 'UniqueDef' that defines it. You 
+-- | Convert a constructor for a 'Unique' key on a record to the 'UniqueDef' that defines it. You
--- can supply just the constructor itself, or a value of the type - the library is capable of figuring 
+-- can supply just the constructor itself, or a value of the type - the library is capable of figuring
 -- it out from there.
 --
 -- @since 3.1.3
-toUniqueDef :: forall a val. (KnowResult a ~ (Unique val), PersistEntity val,FinalResult a) => 
+toUniqueDef :: forall a val. (KnowResult a ~ (Unique val), PersistEntity val,FinalResult a) =>
   a -> UniqueDef
 toUniqueDef uniqueConstructor = uniqueDef
   where
@@ -1071,9 +1071,9 @@ toUniqueDef uniqueConstructor = uniqueDef
     uniqueDef = head . filter filterF . entityUniques . entityDef $ proxy
 
 -- | Render updates to be use in a SET clause for a given sql backend.
--- 
+--
 -- @since 3.1.3
-renderUpdates :: (BackendCompatible SqlBackend backend) => 
+renderUpdates :: (BackendCompatible SqlBackend backend) =>
     backend
     -> [SqlExpr (Update val)]
     -> (TLB.Builder, [PersistValue])
@@ -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 =>

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_