Add support for date_trunc

src/Opaleye/Operators.hs

@@ -104,6 +104,8 @@ module Opaleye.Operators
  , IntervalNum
  , addInterval
  , minusInterval
+ , TimestampPrecision(..)
+ , date_trunc
  -- * Deprecated
  )
 
@@ -137,6 +139,9 @@ import qualified Opaleye.Internal.HaskellDB.PrimQuery as HPQ
 
 import qualified Data.Profunctor.Product.Default as D
 
+import qualified Data.Text as Text
+import qualified Data.Text.Encoding as TE
+
 {-| Keep only the rows of a query satisfying a given condition, using an
 SQL @WHERE@ clause. It is equivalent to the Haskell function
 
@@ -492,3 +497,25 @@ minusInterval = C.binOp (HPQ.:-)
 -- | Current date and time (start of current transaction)
 now :: F.Field T.SqlTimestamptz
 now = Column $ HPQ.FunExpr "now" []
+
+data TimestampPrecision =
+ MicrosecondsPrecision
+ | MillisecondsPrecision
+ | SecondPrecision
+ | MinutePrecision
+ | HourPrecision
+ | DayPrecision
+ | WeekPrecision
+ | MonthPrecision
+ | QuarterPrecision
+ | YearPrecision
+ | DecadePrecision
+ | CenturyPrecision
+ | MillenniumPrecision
+ deriving Show
+
+precisionToExpr :: TimestampPrecision -> HPQ.PrimExpr
+precisionToExpr p = HPQ.ConstExpr . HPQ.ByteStringLit . TE.encodeUtf8 . Text.dropEnd 9 . Text.pack $ show p
+
+date_trunc :: TimestampPrecision -> F.Field T.SqlTimestamptz -> F.Field T.SqlTimestamptz
+date_trunc p (Column e) = Column $ HPQ.FunExpr "date_trunc" [(precisionToExpr p), e]