[Python-3000] multi-dispatch again

[tomer filiba]

i just thought of a so-to-speak counter-example for ABCs... it's not really
a counter-example, but i believe it shows a deficiency in the concept.

theoretically speaking, objects are a made of type and state. ABCs,
isinstance()
and interfaces at general only check the type part. for example:

@dispatch
def log_to_file(text: str, device: file):
    file.write(text)

this will constrain the *type* of the device, but not its *state*.
practically speaking, i can pass a closed file, or a file open for
reading-only,
and it would pass silently.

basing multi-dispatch on types is of course a leap forward, but if we
already plan to take this leap, why not make it general enough to support
more complex use-cases?

this way we could rewrite the snippet above as

@dispatch
def log_to_file(text: str, device: open_file):
    file.write(text)

where open_file isn't a type, but rather a "checker" that may also examine
the
state. by default, type objects would check for inheritance (via a special
method),
but checkers could extend this behavior.

for efficiency purposes, we can have two decorators:
@type_dispatch - dispatches based on type only
@full_dispatch - dispatches based on type and state

bottom line -- we can't just look at the type of the object for dispatching,

overlooking its state. the state is meaningful, and we'd want the function
not to be called at all if the state of the object is wrong.


-tomer
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