On 2015-06-10 at 00:42:12 +0200, David Luposchainsky wrote: [...]
I think there are two important consequences of MonadFail. First of all, we can all safely write failable patterns if we so desire. Second, the compiler can ensure other people's codebases do not lie to us (knowingly or unknowingly).
...as a data-point, when turning on MonadFail during testing, I've seen at least one place in GHC's own code-base that directly calls 'fail' for a Monad instance which did *not* have its 'fail' method overridden. Moreover, I've seen at least one (other) case, where failable pattern matches occurred (intentionally or not[1]), but the respective Monad instance didn't have the 'fail' method overridden either. [1]: Failable patterns can in theory snuck in non-intentionally, e.g. they can be introduced during refactoring if e.g. a single-constructor type gets added new constructors. If the context was previously constraint to a 'Monad', after the refactoring the typechecker would point out that now there's a failable pattern not accounted for.