[Haskell-beginners] Mixing pattern matching and guards

Hello all, I recently wrote this piece of code import qualified Data.Set as S [...] purge :: (Eq cp, Eq ip) => Product cp ip -> Product cp ip purge (Punion ps) | ps' == S.empty = Pempty where ps' = S.filter (/= Pempty) ps purge x = x and I thought I had missed the "otherwise" case in the guard and I was prepared to see a "non exhaustive ..." error, but to my amazement it works *Main> purge $ Punion $ S.fromList [Packed 1 Pempty] Punion (fromList [Packed 1 Pempty]) As this is a Punion, it should match the first pattern, but not the guard. It seems to fall right through to the second pattern. Is this the way it works?