On Fri, Feb 24, 2006 at 12:39:27PM -0500, Cale Gibbard wrote:
I look at the above as generating a proof obligation for me as the programmer that the lookup will never fail, or at least the ability to convince myself. :) If you want to handle errors, you should actually handle them, not let your users get "Irrefutable pattern failed" messages. Also, if someone else later comes along and wants to catch that error, they have to either do it in IO, which can be fiddly if the error occurs deep in the evaluation of some structure, or they refactor your code so that it returns the error explicitly. Sure, irrefutable pattern matches are useful, but they shouldn't be used if you expect they'll ever fail.
Ah, perhaps I wasn't clear. I don't ever expect these to fail. The reason I prefer irrefutable pattern matches to handwritten 'error' messages (at first) is so many months later when I introduce a subtle heisenbug I don't get a error: This shouldn't happen or worse error: Prelude.undefined but rather a nice error pointing right to the line number. anything I ever expect to fail for any reason other than a bug I put in a failing Monad with a suitably user digestable error message. So, I was comparing them to handwritten 'error' messages for announcing programming bugs. not handwritten 'error' messages for users to see (which really should be using 'fail' in a monad anyway). John -- John Meacham - ⑆repetae.net⑆john⑈