Raphael Descamps wrote: > Am Freitag, den 10.07.2009, 17:06 -0700 schrieb Jon Lang: >> How about this: in role composition, "mandate" causes methods to take >> precedence over other methods with which they would normally conflict, >> and to conflict with methods that would normally take precedence over >> them. > > I really dislike this because it is contrary to the original idea of the > "stateless traits" as defined in the original paper from Nathanael > Schärli.
Agreed. OTOH, Roles are already contrary in this respect, because they can provide attributes as well as methods. Note also that this was my first proposal; I have since abandoned it in favor of (I hope) a more intuitive approach. > The main reason why "traits" have been introduced was to solve the > problems inherent to mixins. In mixins the main problem is that the > class using the mixin is not able to control the composition (which is > simply done sequencially) and that lend to fragile hierarchies. > > The brilliant idea with "traits" is that it bring back the control to > the class consuming the "trait" and conflicts have to be solved > explicitly. The traits paper propose 3 different operators to solve such > conflicts: overriding, excluding or aliasing. > > I definitively think that perl 6 roles should also have an excluding > operator because I think that *every* composition conflicts arising > should be solvable by the class comsuming the role. > > What you propose here is a step behind: you reintroduce the problem > existing with mixins by bringing back precedence rules in the way > composition is made. Well, yes and no. The class still has the final say on how a given method is to be implemented; the only thing being debated here is whether or not the class should have to explicitly pull rank to redefine a method being provided by a role, or if it does so silently. The latter approach is how things currently stand, and is being criticized as a source of bugs as authors of classes inadvertently override method definitions that they didn't intend to override. > So far, I have only seen reference to the original paper decribing the > "stateless traits". As roles are an implementation of "stateful traits", > maybe we should start to point to the paper formalising it: > http://scg.unibe.ch/archive/papers/Berg07eStatefulTraits.pdf Thanks for the link. -- Jonathan "Dataweaver" Lang
