[Haskell] Re: [Haskell-cafe] ANNOUNCE: enumerator, an alternative iteratee package

[John Millikin]

After fielding some more questions regarding error handling, it turns
out that my earlier mail was in error (hah) -- error handling is much
more complicated than I thought.

When I gave each iteratee its own error type, I was expecting that
each pipeline would have only one or two sources of errors -- for
example. a parser, or a file reader. However, in reality, it's likely
that every single element in a pipeline can produce an error. For
example, in a JSON/XML/etc reformatter (enumFile, parseEvents,
formatEvents, iterFile), errors could be SomeException, ParseError, or
FormatError.

Futhermore, while it's easy to change an iteratee's error type with
just (e1 -> e2), changing an enumerator or enumeratee *also* requires
(e2 -> e1). In other words, to avoid loss of error information, the
two types have to be basically the same thing anyway.

I would like to avoid hard-coding the error type to SomeException,
because it forces libraries to use unsafe/unportable language features
(dynamic typing and casting). However, given the apparent practical
requirement that all iteratees have the same error type, it seems like
there's no other choice.

So, my questions:

1. Has anybody here successfully created / used / heard of an iteratee
implementation with independent error types?
2. Do alternative Haskell implementations (JHC, UHC, Hugs, etc)
support DeriveDataTypeable? If not, is there any more portable way to
define exceptions?
3. Has anybody actually written any libraries which use the existing
"enumerator" error handling API? I don't mind rewriting my own
uploads, since this whole mess is my own damn fault, but I don't want
to inconvenience anybody else.