[Python-ideas] [Python-3000] PEP 3107 Function Annotations: overloadable ->

[Josiah Carlson]

Tony Lownds <tony at pagedna.com> wrote:
> On Jan 1, 2007, at 10:34 PM, Josiah Carlson wrote:
> > -1 on the -> operator as specified in the email you link.  There is no
> > reason to add aribtrary operators for call-site annotations.
> 
> What was the arbitrary operator? -> is not arbitrary, and there was a  
> reason given to add it.

I misunderstood portions of the post.

> > PEP 3107
> > has already defined the syntax for function definition site  
> > annotations
> > as follows...
> >
> >     def name(argument=default:annotation) -> annotation:
> >         body
> 
> You mean
> 
> def name(argument:annotation=default)

Thank you for the correction.  Maybe I'm the only one, but I would
prefer the default value to come before the annotation.


> > The mail you link, talks about an arbitrary -> operator for call-site
> > annotations.  Without seeing actual use-cases where a -> operator  
> > would
> > be useful in the real-world, I can't help but be -1 on the -> operator
> > and -1 on the __returns__ method.
> 
> That email had a use case... The gain is being able to write
> 
>    def wrap(text: str, split: Function(str)-> list):
> 
> instead of
> 
>    def wrap(text: str, split: Function(str) == list):
> 
> or
> 
>    def wrap(text: str, split: Function(str, returns=list)):
> 
> or one of the other methods that Kay came up with.

Kay's mail specified:

    maptype = Function( Function(object)->object, list) -> list

That looks to me like the -> operator being used anywhere the user
desires.

With your examples above, I see the use-case, but I'm still -1 (for the
same reasons).

For other reasons to be -1 on ->, we can look at how it would be abused
to violate the one obvious way to do things...

    Stack() -> 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8

    t = Tree()
    t -> (t -> (1,2), t -> (3,4))

Or for a concrete example...

    class mylist(list):
        def __returns__(self, arg):
            self.append(arg)
            return self

    mylist() -> 1 -> 2 -> 3 -> 4 -> 5

Right now those things aren't done because the punctuation for doing it
isn't as natural as -> would be.


 - Josiah