[Python-Dev] PEP 246: lossless and stateless

[Clark C. Evans]

Ok.  I think we have identified two sorts of restrictions on the
sorts of adaptations one may want to have:

  `stateless'  the adaptation may only provide a result which
               does not maintain its own state
               
  `lossless'   the adaptation preserves all information available
               in the original object, it may not discard state

If we determined that these were the 'big-ones', we could possibly
allow for the signature of the adapt request to be parameterized with 
these two designations, with the default to accept any sort of adapter:

   adapt(object, protocol, alternative = None, 
         stateless = False, lossless = False)

   __conform__(self, protocol, stateless, lossless)

   __adapt__(self, object, stateless, lossless)

Then, Guido's 'Optional Static Typing',

     def f(X: Y):
         pass

   would be equivalent to

      def f(X):
          X = adapt(Y, True, True)

In other words, while calling adapt directly would allow for any adapter; 
using the 'Static Typing' short-cut one would be asking for adapters
which are both stateless and lossless.  Since __conform__ and __adapt__
would sprout two new arguments, it would make those writing adapters 
think a bit more about the kind of adapter that they are providing.

Furthermore, perhaps composite adapters can be automatically generated
from 'transitive' adapters (that is, those which are both stateless
and lossless).  But adaptations which were not stateless and lossless
would not be used (by default) in an automatic adapter construction.

Your thoughts?

Clark

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Clark C. Evans                      Prometheus Research, LLC.
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