[Haskell-cafe] throwDyn typing "fun"

Tomasz Zielonka tomasz.zielonka at gmail.com
Fri Nov 11 07:35:41 EST 2005

On Fri, Nov 11, 2005 at 01:20:05PM +0200, Einar Karttunen wrote:
> It seems that the type of throwDyn and throwDynTo are dangerously close.
> ThrowDyn works in with any of the arguments of throwDynTo, which can
> cause evil situations.
> throwDyn :: Typeable exception => exception -> b
> Which means e.g. "throwDyn someThreadId SomeException" will work 
> when you wanted to say "throwDynTo someThreadId SomeException"
> and they both have types which unify with IO ().

How evil! ;-)

> I think using a
> class Typeable => DynamicException a where ...
> and throwDyn :: DynamicException a => a -> b
> could make more sense.

You could also do something like:

    newtype Exn a = Exn a -- not Typeable

    throwDyn' :: Typeable exception => Exn exception -> b
    throwDyn' (Exn e) = throwDyn e

used as

    throwDyn' (Exn (some-typeably-thingy))

then neither (throwDyn' someThreadId SomeException) nor
(throwDyn' someThreadId (Exn SomeException)) will compile.

And you won't have to create instances of DynamicException, but it is
probably more ugly.

Best regards

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