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? Francesco Ariis wrote:
I'm trying to use catch (...) (\e -> putStrLn $ show e) However, I get an error Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved. This goes away if I change the code to catch (...) (\e -> putStrLn $ show (e::IOException))
A couple of things I don't understand here: - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available? - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?
IOException is a concrete type while Exception is a typeclass. In the end, the compiler needs the former, the latter not being enough. The code works as any other class-based function would someFunction :: Monoid a -> [a] -> a -- ^-- in the end `Monoid a` will become something concrete, like -- a String, a Sum, etc. Does that make sense?