+ unless (givenWD ≡ correctWD)
+ $ fail
+ $ concat [ "Gregorian day "
+ , show year
+ , "-"
+ , show month
+ , "-"
+ , show day
+ , " is "
+ , longWeekDayName correctWD
+ , ", not "
+ , longWeekDayName givenWD
+ ]
+
+assertGregorianDateIsGood ∷ Monad m ⇒ Integer → Int → Int → m Day
+{-# INLINEABLE assertGregorianDateIsGood #-}
+assertGregorianDateIsGood year month day
+ = case fromGregorianValid year month day of
+ Nothing
+ → fail $ concat [ "Invalid gregorian day: "