In article
<2E9B33CE230409489A7ED37E5E34090F0541A61D@EUR-MSG-20.europe.corp.microso
ft.com>,
"Simon Marlow"
I don't think we need more extensions to do a reasonable job of extensible exceptions:
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