On Thu, Aug 17, 2017 at 12:24:07PM +0000, Jonathon Delgado wrote:
I'm sure it makes sense! I'm not really following though.
I understood typeclasses to be analogous to OO interfaces. So if a variable implements the Exception interface, and Exception implements the Show interface, then it should automatically support show.
I take it this was wrong? How does the compiler use typeclasses if they're not interfaces?
That's correct! Indeed ghc is not complaining about a lack of instances, as it would with, say λ> putStrLn 5 -- • No instance for (Num String) arising from etc etc. but about the *ambiguity* of type variable `e`. How does `catch` know _which_ exception to deal with if you don't specify the concrete type? Consider: prova = catch (print $ div (error "hey bby") 0) (\e -> print "ouch" >> print (e :: ErrorCall)) -- I want to deal with arithmetic errors here and not in `prova` palla = catch prova (\e -> print "baaah" >> print (e :: ArithException)) If I switch ArithException and ErrorCall the behaviour of invoking `palla` changes. Having a catch-all `catch` is possible by using (e :: SomeException); if you don't care about `e` and just want to do an action regardless, you are probably better off with `onException`. Ask more if needed!