At 2002-07-22 04:58, Simon Peyton-Jones wrote:
>This business of having 'foralls' in a context (whether for a class or
>a function) is not hard to implement in principle, but in practice it'd
>take few days of hacking I guess. And, more to the point, thinking.
>What are the interactions with functional dependencies or implicit
>parameters?
>(Maybe none, but I've been caught before.)
>
>So it's really waiting for someone to say "having foralls in contexts
>would change my life and I would love you for ever", rather than "it'd
>be nice".
>
>Simon "lazy evaluation" PJ
OK, I can't say it would change my life but I did come up with a rather
more motivating example. I guess the best I can offer is "I'll buy you a
drink next time I'm back in Cambridge" (I have family there, so it does
happen).
I want to implement typed exceptions somewhat similar to Java. Code would
declare type using an "ExceptionMonad" of kind (* -> * -> *):
class (forall ex. Monad (mm ex)) =>
ExceptionMonad mm where
{
throwX :: x -> mm x a
catchX :: mm xa a -> (xa -> mm xb a) -> mm xb a;
}
Notice how the types of throwX and catchX precisely mirror those of
return and (>>=).
Now I could write code that throws different kinds of exceptions like
this:
data FooException = MkFooException String;
foo :: (ExceptionMonad mm) => Char -> mm FooException Integer;
foo c = ...
data BarException = MkBarException | FooBarException FooException;
bar :: (ExceptionMonad mm) => mm BarException Integer;
bar = do
{
a <- catchX (foo 'q') (\fx -> throwX (FooBarException fx));
return (a + 1);
}
This allows you to have typed structured exceptions more easily, which I
think is preferable in many cases to relying on "fail" with its String
argument...
--
Ashley Yakeley, Seattle WA
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