On Mon, Feb 27, 2006 at 10:57:17PM -0500, Cale Gibbard wrote:
Well, this is an issue. Perhaps a version of error which makes the line/column number available to its parameter would help... something along the lines of
type SourcePos = (Integer, Integer) -- possibly a data/newtype with a nicer Show instance errorPos :: (SourcePos -> String) -> a
Yes, this is what jhc's SRCLOC_ANNOTATE addreses, more or less.
This would give all the benefits normally acquired from the expansion of the syntax sugar while allowing you to additionally add any extra messages you'd like. Further, you'd not be required to work in, say the identity monad, in order to get line number messages for failures (though in GHC at least, irrefutable pattern match failures in lambdas and let also get line numbered).
Well, the benefit of the Identity monad is so that the user of a routine can choose to recover gracefully by using a different monad, you only use the Identity monad when you are making a choice to bottom out on errors. using 'error' directly is not an option in said cases because it would take away the ability of the user of a routine to catch errors properly. error should only be used for reporting bugs that should never happen, not user visible failure. The writer of a library shouldn't decide how (non-buggy) failure should be handled, the user of it should.
I'm actually really against the inclusion of fail in the Monad class, so finding a reasonable replacement for any constructive uses it might have had is important to me.
I know you keep saying this, We start with the exact same premises and goals, yet somehow come to the exact opposite conclusion. I have not quite figured out why. However, a quick survey shows that _every single_ monad defined in the standard and fptools libraries has an interesting non-error 'fail' method other than Identity, whose sole purpose is to turn 'fail's into errors. Separating out a MonadError with 'fail' seems rather odd as every monad will be an instance of it! (including Identity, since turning fails into errors is its main purpose) (the monads like 'Reader' and 'Writer' are actually just shorthand for ReaderT a Identity, the inner monad determines the failure mode) John -- John Meacham - ⑆repetae.net⑆john⑈