WIP matchMaybe

Author: Tom

src/Opaleye/Internal/MaybeFields.hs

@@ -35,7 +35,7 @@ import           Opaleye.Internal.Operators ((.&&), (.||), (.==), restrict, not,
                                              ifExplict, IfPP, EqPP(EqPP))
 import qualified Opaleye.Internal.Lateral
 import qualified Opaleye.SqlTypes
-import           Opaleye.SqlTypes (SqlBool, IsSqlType)
+import           Opaleye.SqlTypes (SqlBool, IsSqlType, SqlInt4)
 
 import           Control.Monad (replicateM_)
 
@@ -92,6 +92,15 @@ justFields = pure
 maybeFields :: PP.Default IfPP b b => b -> (a -> b) -> MaybeFields a -> b
 maybeFields = maybeFieldsExplicit PP.def
 
+matchMaybe :: PP.Default IfPP b b => MaybeFields a -> (Maybe a -> b) -> b
+matchMaybe mf f = maybeFields (f Nothing) (f . Just) mf
+
+example :: MaybeFields (Field SqlInt4)
+        -> Field SqlInt4
+example mf = matchMaybe mf $ \case
+  Nothing -> 0
+  Just x  -> x * 100
+
 -- | The Opaleye analogue of 'Data.Maybe.fromMaybe'
 fromMaybeFields :: PP.Default IfPP b b => b -> MaybeFields b -> b
 fromMaybeFields = fromMaybeFieldsExplicit PP.def