[Haskell-cafe] Error handling package

Michael Snoyman michael at snoyman.com
Sun Oct 18 15:45:57 EDT 2009

While working on the next release of data-object, I wanted to represent some
operations that might fail. The typical candidates were:

1) Maybe
2) Either
3) Monad

Monad is always iffy because of the often times poorly defined fail. Maybe
doesn't provide any means of reporting what the problem was. Either is not
defined as a Monad in the base library. Also, the usual candidate of Either
String only provides for an error message. It would be nice to have more
in-depth information available.

So I've put together a package I call "attempt". It defines a data type
called (surprise) Attempt with a Success and Failure constructor. The trick
here is that it using the new extensible exceptions under the surface, so
you can report any kind of exception you want. It also provides a
"FromAttempt" type class for possible conversion targets from an Attempt,
provides an attempt function with some helpers for performing the equivalent
of Control.Exception.catches, and provides two samples of functions I would
want to implement with this library: attemptJoin and attemptLookup.

My questions for the list are:

1) Is the overall approach sound, or do people have better ideas?
2) Are there any other FromAttempt instances I should provide out of the
3) I was considering adding specialized versions of the fromAttempt
function, ie ioFromAttempt, maybeFromAttempt. Thoughts?
4) Should I follow the naming scheme attemptJoin, attemptLookup, etc, or
just call the functions join, lookup and force people to import the module
5) Any other suggestions for attempt functions? I've considered
6) Include ToAttempt?

The code is available on github at
http://github.com/snoyberg/attempt/blob/master/Data/Attempt.hs . I
appreciate the review. Also, I have not yet documented the code, but I will
do so before uploading to Hackage; I just didn't want to document a changing

Thank you,
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