On 2012-01-19 23:52, Michael Snoyman wrote:
maybe I should file a feature request: provide an extra warning flag (turned on by -Wall) that will warn when you match on a failable pattern.
I fully agree if it's IO, so that a failed pattern match leads to an exception. The "nice" implementations of fail in the List and Maybe monads are a different story. Ideally one would want to be able to turn on a warning whenever IO is used in a way which could generate a pattern match exception. This would call for a type distinction, as you said, "reinstate the MonadZero constraint". Here's an idea that might address SPJ's "killer". b) if you add an extra constructor to a single-constructor type then pattern matches on the original constructor suddenly become failable Another binding operator might be introduced so that the code would show the intention either to have a failable or non-failable pattern match. do (x,y) <- pair failable, requires MonadZero do (x,y) <=- pair requires non-failable pattern supports Monads that should not fail