Re: [Python-ideas] Generics Syntax

[אלעזר]

Yes the "class A[T]:" syntax requires on the ability to express variance as
an operator, but not the other way around.

It might be an argument in favor of switching to the + syntax: to make
possible future syntax change in class definition somewhat easier to
swallow.

~Elazar

On Thu, Sep 15, 2016 at 2:03 PM Nick Coghlan  wrote:

> On 15 September 2016 at 19:53, Ivan Levkivskyi 
> wrote:
> >
> >
> > On 15 September 2016 at 11:46, אלעזר  wrote:
> >>
> >> And that thread is only about variance. What about the generic syntax?
> >
> >
> >  If you mean code like this:
> >
> > class Container[+T]:
> > @abstractmethod
> > def __contains__(self, x: T) -> bool: ...
> >
> > then there is little chance that this will be accepted because it
> requires
> > changes to Python syntax.
>
> If the proposed spelling is tweaked to be "class
> Container(Generic[+T]):", then it doesn't require a syntax change, as
> that's merely a matter of implementing unary plus on type vars:
>
> >>> +object()
> Traceback (most recent call last):
>  File "", line 1, in 
> TypeError: bad operand type for unary +: 'object'
> >>> class UnaryPlus:
> ... def __pos__(self):
> ... return self
> ...
> >>> +UnaryPlus()
> <__main__.UnaryPlus object at 0x7f5e0fe91c50>
>
> (I have no opinion on the value of providing a simpler spelling for
> covariance, I'm just noting that if you keep the "Generic[T]" aspect
> of the current spelling it wouldn't require any changes to Python's
> syntax and will work as far back as you care to support it)
>
> Cheers,
> Nick.
>
> --
> Nick Coghlan   |   ncogh...@gmail.com   |   Brisbane, Australia
>
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Re: [Python-ideas] Generics Syntax

[Nick Coghlan]

On 15 September 2016 at 19:53, Ivan Levkivskyi  wrote:
>
>
> On 15 September 2016 at 11:46, אלעזר  wrote:
>>
>> And that thread is only about variance. What about the generic syntax?
>
>
>  If you mean code like this:
>
> class Container[+T]:
> @abstractmethod
> def __contains__(self, x: T) -> bool: ...
>
> then there is little chance that this will be accepted because it requires
> changes to Python syntax.

If the proposed spelling is tweaked to be "class
Container(Generic[+T]):", then it doesn't require a syntax change, as
that's merely a matter of implementing unary plus on type vars:

>>> +object()
Traceback (most recent call last):
 File "", line 1, in 
TypeError: bad operand type for unary +: 'object'
>>> class UnaryPlus:
... def __pos__(self):
... return self
...
>>> +UnaryPlus()
<__main__.UnaryPlus object at 0x7f5e0fe91c50>

(I have no opinion on the value of providing a simpler spelling for
covariance, I'm just noting that if you keep the "Generic[T]" aspect
of the current spelling it wouldn't require any changes to Python's
syntax and will work as far back as you care to support it)

Cheers,
Nick.

-- 
Nick Coghlan   |   ncogh...@gmail.com   |   Brisbane, Australia
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Re: [Python-ideas] Generics Syntax

[אלעזר]

And that thread is only about variance. What about the generic syntax?

‪On Thu, Sep 15, 2016 at 12:44 PM ‫אלעזר‬‎  wrote:‬

> Does that mean that if I did, it would be reconsidered?
>
> On Thu, Sep 15, 2016 at 12:43 PM Ivan Levkivskyi 
> wrote:
>
>> On 15 September 2016 at 11:21, אלעזר  wrote:
>>
>>> This suggestion is so obvious that it's likely has been discussed, but I
>>> can't find any reference (It's not what PEP-3124 talks about).
>>>
>>>
>> You might want to read this tread
>> https://github.com/python/typing/issues/211 and another tread mentioned
>> there at the start.
>> In short, this idea has been discussed, but nobody had seen it as good
>> enough to sacrifice his time for implementing the changes.
>>
>> --
>> Ivan
>>
>>
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Re: [Python-ideas] Generics Syntax

[אלעזר]

Does that mean that if I did, it would be reconsidered?

On Thu, Sep 15, 2016 at 12:43 PM Ivan Levkivskyi 
wrote:

> On 15 September 2016 at 11:21, אלעזר  wrote:
>
>> This suggestion is so obvious that it's likely has been discussed, but I
>> can't find any reference (It's not what PEP-3124 talks about).
>>
>>
> You might want to read this tread
> https://github.com/python/typing/issues/211 and another tread mentioned
> there at the start.
> In short, this idea has been discussed, but nobody had seen it as good
> enough to sacrifice his time for implementing the changes.
>
> --
> Ivan
>
>
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Re: [Python-ideas] Generics Syntax

[Ivan Levkivskyi]

On 15 September 2016 at 11:21, אלעזר  wrote:

> This suggestion is so obvious that it's likely has been discussed, but I
> can't find any reference (It's not what PEP-3124 talks about).
>
>
You might want to read this tread
https://github.com/python/typing/issues/211 and another tread mentioned
there at the start.
In short, this idea has been discussed, but nobody had seen it as good
enough to sacrifice his time for implementing the changes.

--
Ivan
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[Python-ideas] Generics Syntax

[אלעזר]

This suggestion is so obvious that it's likely has been discussed, but I
can't find any reference (It's not what PEP-3124 talks about).

Generic class syntax, now:

_T_co = TypeVar('_T', covariant=True)

class Container(Generic[_T_co]):
@abstractmethod
def __contains__(self, x: _T_co) -> bool: ...

(yes it's object in reality)
Generic class syntax, suggested:

class Container[+T]:
@abstractmethod
def __contains__(self, x: T) -> bool: ...

The + signifies covariant type, as in Scala. The need for underscore prefix
and explicit name for covariant types is gone, since it's class-scoped.
The + is a bit cryptic, but so are term "covariant" and Generic[_T_co]. The
syntax by large is exactly what users coming from statically-typed
languages will expect.

Giving a bound can be done using "T <: AnyStr", "T: AnyStr" or any other
syntax.

Again, I assume it was discussed before, but my Google skills are failing
me; a pointer will be helpful.

~Elazar
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