MonadFail proposal (MFP): Moving fail out of Monad

Erik Hesselink hesselink at
Wed Jun 10 13:40:23 UTC 2015

That would make it impossible to have a failed pattern match become an
automatic 'fail' with a MonadFail constraint, right? I think that's a
pretty nice feature, actually. But perhaps I misunderstand your idea.


On Wed, Jun 10, 2015 at 3:00 PM, Johan Tibell <johan.tibell at> wrote:
> An idea. Could the new desugaring be the same as for normal functions with a
> missing case? Code that uses fail would have to add the constraint still,
> but code that really didn't want to use fail would 1) get a warning about an
> incomplete pattern match and 2) get a runtime error for the failed pattern
> match produced by GHC. Is there anything I'm not thinking of here why this
> wouldn't work?
> _______________________________________________
> Libraries mailing list
> Libraries at

More information about the Libraries mailing list