On Wed, Jun 5, 2019 at 6:08 PM Yanghao Hua <yanghao...@gmail.com> wrote:
>
> On Wed, Jun 5, 2019 at 12:52 AM Greg Ewing <greg.ew...@canterbury.ac.nz>
> wrote:
> >
> > Yanghao Hua wrote:
> > > Did Guido say "user defined syntax" or "user defined operator"?
> > > ... for me defining new operator is different
> >
> > To my mind it's not that much different. A reader encountering an
> > unfamiliar operator is pretty much faced with a new piece of
> > syntax to learn. Its spelling gives little to no clue as to its
> > meaning, it precedence in relation to other operators is not
> > obvious, etc.
> >
> > > The way scala allows user to define new operator is
> > > really elegant,
> >
> > There are some differences between Python and Scala that present
> > practical difficulties here. Some kind of declaration would be
> > needed to define its arity, precedence and corresponding dunder
> > method. This is no problem in Scala because it analyses imported
> > code at compile time. But the Python compiler only sees the module
> > being compiled, so it has no ability to import these kinds of
> > declarations from another module.
>
> My understood how scala achieves this is, whenenver it sees "taken1
> token2 token3" kind of expression, it does token1.token2(token3). So
> it was assumed token2 is a method of token1 and it always takes one
> argument.
>
> With my very limited understanding of cpython internals (at least when
> I implement <==) it seems cpython is searching for an operator and
> then translates it into a method call on the objects, so what if
> instead of telling e.g. python to search __add__ method call, it
> should search int.+ (the "+" method call) instead? Not sure if this
> helps at all I believe there are tons of other problems too. This is
> definitely NOT an easy thing to add post-fact if not thought through
> from the very beginning.
Part of the reason you can't just treat the + operator as a method
call is that there are reflected methods. Consider:
class Int(int):
def __radd__(self, other):
print("You're adding %s to me!" % other)
return 1234
x = Int(7)
print(x + 1)
print(1 + x)
If these were implemented as x.__add__(1) and (1).__add__(x), the
second one would use the default implementation of addition. The left
operand would be the only one able to decide how something should be
implemented.
ChrisA
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