On 2015-06-09 at 23:56:59 +0200, Henning Thielemann wrote: [...]
You think about making MonadFail a subclass of Applicative where the name MonadFail becomes misleading - how about naming it Fail instead?
'Fail' would be a good name, if it became an independent class (w/o a superclass to either 'Applicative' or 'Monad'). But the cost for that is having to use `(Fail m, Monad m) =>` Or rely on e.g. constraint synonyms, via e.g. {-# LANGUAGE ConstraintKinds #-} type MonadFail m = (Fail m, Monad m) type ApplicativeFail m = (Fail m, Applicative m) for convenience. I've also heard the suggestion 'FailDo', which could make sense if it became a superclass of Applicative (but then we'd still have to use '(FailDo m, Monad m)' or a constraint-synonym for the majority of cases, where 'fail' is used in a 'Monad' context) Otoh, the reason I suggested to just go w/ 'MonadFail' to keep it simple was that I doubted that that 'fail' in a '-XApplicativeDo' made much sense. But maybe it does make sense? I.e. while 'fail' has laws w/ 'Monad', what would be the respective laws for 'fail' in an 'Applicative' context? Cheers, hvr