[Haskell-cafe] throwDyn typing fun

2005-11-11 Thread Einar Karttunen 
Hello

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 ().

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

- Einar Karttunen
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Re: [Haskell-cafe] throwDyn typing fun

2005-11-11 Thread Tomasz Zielonka 
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
Tomasz
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