On 20/02/06, John Meacham
There is a more straightforward way to get localized error messages rather than using 'maybe' and hand-writing an appropriate error, and that is to rely on irrefutable bindings.
f x = ... y ... where Just y = Map.lookup x theMap
now if the lookup fails you automatically get an error message pointing to the exact line number of the failure. or if the failure message of the routine is more important than the source location you can do
f x = ... y ... where Identity y = Map.lookup x theMap
it is anoying you have to make a choice between these two possibilities, but this can be mitigated with CPP magic or the SRCLOC_ANNOTATE pragma.
John
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. - Cale