[Haskell-beginners] Ambiguous type variable

[Michael Snoyman]

On Thu, Aug 17, 2017 at 4:29 PM, Francesco Ariis <fa-ml at ariis.it> wrote:

> 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`.
>
>
This is dangerous: `catch` with `e :: SomeException` will catch all
asynchronous exceptions, breaking things like timeout, race, and the async
library in general. That's why my message about mentioned the
safe-exceptions package.

Michael


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