[Haskell-beginners] Mixing pattern matching and guards

[Francesco Ariis]

On Tue, Dec 22, 2015 at 06:29:32AM +0100, martin wrote:
> 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

The pattern does match (and any variable bound by it is made available
to the corresponding guards).
Then guards are tried in order: if no guard succeed, the next pattern
match is found (in your case purge x = x, irrefutable).

If you want you can add a catch all guard as last guard:

          | ps' == S.empty = Pempty
          | otherwise      = undefined -- handle ps' /= S.empty here