[Python-3000] Type Coersion Syntax

[Calvin Spealman]

Well one thing this fixes is removal of unneeded overloading, which is
an admitted problem with builtin types doubling as factories. The
special method doesn't have to be __coerce__, and as a matter of fact
something better named would probably be more appropriate.

On 7/6/06, Brett Cannon <brett at python.org> wrote:
>
>
>
> On 7/6/06, Calvin Spealman <ironfroggy at gmail.com> wrote:
> > I would like to propose a syntax that would explicitly be read as
> > "Create an object of Foo converted from object Bar". Being explicit,
> > as such, could bring about some interesting optimizations in the
> > future or immediately, and reduce redundant signatures (such as the
> > type(foo) syntax). I understand this is likely to be given some
> > negative comments, but I am interested in everyone's thoughts on the
> > idea.
> >
> >     # Coerce an object into its type
> >     type from foo
> >
> >     # Create a list from some iterable
> >     list from bar
> >
> > Obviously you can see I propose the use of the 'from' keyword to mean
> > "create this from that". This would not translate directly to
> > type(foo) and list(bar), however, but would look for a new special
> > method, named '__coerce__', and would pass the original object to this
> > method to request a coerced instance of the desired type (or
> > interface). This special method, which is explicitly for coercing from
> > other types, can do more explicit things when we know exactly what we
> > are coercing, rather than just creating a new instance.
> >
> >     # Gives a mutable string interface to a file-like object
> >     string from some_file
> >     # Would create a new string, iterating over the file-like object
> >     string(some_file)
> >     # Where 'string' is some generic mutable string type.
> >
> > The same syntax could be used to implement other things, such as
> > key-from-value operations:
> >
> >     # Get the first index of a value in a list
> >     idx = 'b' from ['a', 'b', 'c']
> >     assert idx == 'b'
> >
> > In this example, 'b' would not have a __coerce__ method of its own as
> > an instance, and the list instance's __rcoerce__ would be called to
> > acquire the index.
>
>
> We just removed __coerce__ from Py3K.  =)
>
> Anyway, I don't see how this will allow for any special optimization since
> it will still be calling a method on the object (constructor or otherwise)
> and that cannot be optimized away easily.  Plus, what is wrong with passing
> on object to a type/class' constructor: ``str(42)``?  Not redundant and it's
> still clean, obvious, and does not require new syntax.
>
>
> -Brett