[BangPypers] Clojure style multimethod functions in python

[Dhananjay Nene]

On Mon, Aug 23, 2010 at 8:36 PM, Baishampayan Ghose <b.ghose at gmail.com>wrote:

> Dhananjay,
>
> > Could you help explain in light of the following (the argument lists are
> in
> > bold). The switcher function is provided exactly the same arguments as
> the
> > various multimethods. Probably something about clojure I am not aware of
> ?
> >
> > Dhananjay
> >
> > def multi(switcher_func):
> >    """ Declares a multi map based method which will switch to the
> >    appropriate function based on the results of the switcher func"""
> >
> >    def dispatcher(*args, **kwargs):
> >        key = *switcher_func(*args, **kwargs)*
> >        func = dispatcher.dispatch_map[key]
> >        if func :
> >            return *func(*args,**kwargs)*
> >        else :
> >            raise Exception("No function defined for dispatch key: %s" %
> key
> > )
> >    dispatcher.dispatch_map = {}
> >    return dispatcher
>
> Multimethods are supposed to dispatch on the ``value'' returned by the
> dispatch function and not just when a specific condition is satisfied.
> What will you do when there are multiple possibilities? Ideally, I
> should be able to prefer a specific method to another one in case
> there is some ambiguity. Yours will always pick the first one that
> satisfies the condition, which is again dependent on the order in
> which the code is evaluated.
>
> For example, try implementing the following using your code -
> http://clojure.org/runtime_polymorphism
>
> Without making any change whatsoever to multi() and multi_method(),
the result of the following code :

# Declare the existence of a multi method switcher
encounter = multi(lambda x,y : (x["Species"], y["Species"]))

@multi_method(encounter, ("Bunny","Lion"))
def encounter(a1, a2):
    return "run-away"

@multi_method(encounter, ("Lion","Bunny"))
def encounter(a1, a2):
    return "eat"

@multi_method(encounter, ("Bunny","Bunny"))
def encounter(a1, a2):
    return "mate"

@multi_method(encounter, ("Lion","Lion"))
def encounter(a1, a2):
    return "fight"

b1 = {"Species" : "Bunny", "Other" : "Stuff"}
b2 = {"Species" : "Bunny", "Other" : "Stuff"}
l1 = {"Species" : "Lion", "Other" : "Stuff"}
l2 = {"Species" : "Lion", "Other" : "Stuff"}

print encounter(b1, b2)
print encounter(b1, l1)
print encounter(l1, b1)
print encounter(l1, l2)

*is
*
mate
run-away
eat
fight

Is that consistent with the expectations ?
Dhananjay


> Regards,
> BG
>
> --
> Baishampayan Ghose
> b.ghose at gmail.com
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