Not a complicated or advanced language feature but one that I don’t use a lot
and so keep forgetting about: as-patterns.
When pattern matching you would typically do something like this (an admittedly contrived example)
data Person = Person String Int
getName (Person s _) = sOnce you’ve matched against the constructor you no longer have a reference to
the Person as a whole. as-patterns are simply a way to keep a reference to
the thing you’re pattern matching against. It makes the following possible
displayPerson person@(Person s i) = getName person ++ "is " show i