[Haskell-beginners] Mixing pattern matching and guards

[martin]

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?