Hi Jonathon!
You only catch some specific type of exception, everything else is simply
past onwards. See end of p. 2/beginning of p. 3 here:
http://community.haskell.org/~simonmar/papers/ext-exceptions.pdf
On Thu, Aug 17, 2017 at 2:24 PM, Jonathon Delgado
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? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
-- Markus Läll