Bools and explicitness [was Re: PyWart: The problem with "print"]

[rusi]

On Jun 7, 8:24 am, rusi <rustompm... at gmail.com> wrote:
> On Jun 7, 8:14 am, Mark Janssen <dreamingforw... at gmail.com> wrote:
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> > >> I am aware of what it means, but Python doesn't really have it (although
> > >> it may evolve to it with annotations).
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> > > No polymorphism huh?
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> > > py> len([1, 2, 3])  # len works on lists
> > > 3
> > > py> len((1, 2))  # and on tuples
> > > 2
> > > py> len({})  # and on dicts
> > > 0
> > > py> len('I pity the fool')  # and on strings
> > > 15
> > > py> len(b'\x23')  # and on bytes
> > > 1
> > > py> len(set(range(2)))  # and on sets
> > > 2
> > > py> len(frozenset(range(4)))  # and on frozensets
> > > 4
> > > py> len(range(1000))  # and on range objects
> > > 1000
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> > Okay, wow, it looks like we need to define some new computer science
> > terms here.
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> Fairly definitive terms have existed since 1985:http://lucacardelli.name/Papers/OnUnderstanding.A4.pdf
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> > You are making an "outside view of a function" (until a better term is
> > found).  So that give you one possible view of polymorphism.  However,
> > *within* a class that I would write, you would not see polymorphism
> > like you have in C++,  where it is within the *function closure*
> > itself.   Instead you would see many if/then combinations to define
> > the behavior given several input types.  I would call this simulated
> > polymorphism.
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> Cardelli and Wegner cited above call this ad-hoc polymorphism.
> What you are calling polymorphism, they call universal polymorphism.
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> See sect 1.3 for a summary diagram

I should have added that python has the universal polymorphism that
you want:

$ python
Python 2.7.5 (default, May 20 2013, 13:49:25)
[GCC 4.7.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> len([1,2])
2
>>> len([[1,2]])
1
>>> len([[[1,2]],[[3]],[[4,5]]])
3
>>>

The main thing to note about universal -> parametric polymorphism is
that one definition works for an infinite set of types, without any
extra code(ing)