[Python-3000] Use cases for type annotations? (WAS: Type parameterization)

Phillip J. Eby pje at telecommunity.com
Sun May 21 18:41:59 CEST 2006

At 08:51 PM 5/20/2006 -0600, Steven Bethard wrote:
>I'd particularly be interested in an example where overloading
>implies() to do something other than issubclass comparisons solves a
>real problem.

The prototype overloads implies() to allow a generic function to imply any 
type that can be passed to the generic function.

The flatten() example then uses this by defining a dummy 'iter()' generic 

RuleDispatch is of course another example; it has its own 'implies()' 
protocol that is used in basically the same way.  Of course, it has lots of 
types that implement implies(), since it supports isinstance(), 
issubclass(), comparison operators, object identity checks, etc.  So 
anything that people use RuleDispatch for is essentially a "real problem" 
being solved by overloading of implies().

The Overload3K implementation is a restricted subset of what RuleDispatch 
does, in that Overload3K never performs tests directly on arguments, only 
on the arguments' types.  However, a RuleDispatch implementation based on 
Overload3K could still use the same implies() function that Overload3K 
does, albeit only for inter-signature priority comparisons.

Anyway, the main thrust of the prototype was to answer the then-current 
discussion about interfaces and typeclasses and the like.  I wanted to show 
that an open-ended system (based on the 4 concepts in my post immediately 
previous to this one) would allow interfaces, typeclasses, or whatever the 
heck else people want, to be implemented through a common base system that 
would also be extensible to full predicate dispatching for people who want 

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