Le 13/11/2012 19:28, Allen Wirfs-Brock a écrit :
On Nov 13, 2012, at 1:44 AM, David Bruant wrote:
I'm fine if we're discussing removing some other inconsistencies, but I'd be
more interested in seeing a principled way to decide which inconsistency we're
getting rid of and which we still allow. In other word, what makes an given
inconsistency a bad thing?
The previous answer we had was in essence "if it prevents defensive
programming" (Tom or Mark will correct me if I'm misinterpreting or miswording it)
and with the current proxy design, it's possible to offer freedom to proxies as long as
it's within the boundaries of respecting non-configurable properties and non-extensible
invariants.
What would be the new answer to my question?
footgun-ness
I totally agree with you. The possibility of inconsistency is inherent in this
style of metaprogramming. There is no way we can practically prevent somebody
from coding a proxy handler that intentionally produces inconsistent results.
The lowest layers of the specification/implementations must robust in the
presence of such inconsistencies.
But we should also try to avoid making it too easy for developers to
unintentionally create such inconsistencies. Proxies have a API model that
(intentionally) permits handlers that only implement of a subset of the traps.
This is good, up to a point but if the traps don't have orthogonal
functionality it becomes easy to unintentionally introduce inconsistencies
simply by leaving out a handler method. We can engineer the API to reduce the
occurrence of unintended inconsistency. One thing we can do is minimize
redundant traps that are expected to produce consistent results. We may also
fine it use for to merge traps in ways that centralize such semantics in one
place. We can eliminate traps (eg [[Iterate]] by promoting the functionality
out of the meta layer. Overall, the surface area of the MOP should probably be
as small as possible.
We've explored the security issues or proxies in depth. Now is the time to
explore the usability issues.
Very good point.
Proxies are not intended for novices but there still will be thousands of
developers trying to use proxies who are neither metaprogramming experts or
have a deep understand of the ES MOP. We should try to minimize the number of
footguns and other hazards that they have to face.
Ok, this rationale sounds good to me. My original impression was that
this thread was taking one inconsistency out of the blue, but I'm
completely fine with what you just stated.
David
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