Exceptions

[Ashley Yakeley]

In article 
<2E9B33CE230409489A7ED37E5E34090F0541A61D at EUR-MSG-20.europe.corp.microso
ft.com>,
 "Simon Marlow" <simonmar at microsoft.com> 
 wrote:

> I don't think we need more extensions to do a reasonable job of
> extensible exceptions:
> 
> http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf

You write:

> Compared to our approach, theirs requires new extensions to the language 
> (although not deep),

"Typeable" is an extension to Haskell, and a rather ugly one at that. 
The open datatypes extension is both cleaner and more general.

> and has difficulties with separate compilation.

They claim to solve this I think, though I haven't examined it really 
carefully. You may know better, of course.

> Arguably the open data types approach is more direct and more accessible,

Yes,

> as is often the case with extensions designed to solve a particular problem.

That's not fair. Open datatypes have other applications. A general "file 
interpreter" for instance, that given a MIME type string and a list of 
bytes yields an object. Or a collection of variable "resources" of 
various types that could be passed to a program. Or a hierarchy of UI 
widgets. Or anything that Typeable and Dynamic are currently used for, 
but more cleanly. Hs-plugins, for instance.

It's the missing piece.

> Still, the argument for adding open data types to the language is weakened by 
> the fact that they are subsumed by type classes: in fact the authors give an 
> encoding of open data types into type classes,

Well not really. The "encoding" involves lifting everything from values 
to types, which means a function still can't return a value of an open 
type determined at run-time.

-- 
Ashley Yakeley
Seattle WA