On Mon, Jun 17, 2019 at 7:23 AM Rhodri James <rho...@kynesim.co.uk> wrote:
> On 16/06/2019 03:34, Guido van Rossum wrote: > > I don't actually know how viable this proposal is, but given that it's > > being debated at some length, I'd like to put in my opinion that *if* > > we're going to define an operator that's (roughly) synonymous with > > issubclass(), it should be '<:', which is used in other languages (e.g. > > Scala) and notational systems (https://en.wikipedia.org/wiki/Subtyping). > > Overloading '<=' would be easier to implement, but would also cause > enough > > confusion that I think we should avoid it at all cost. > > I rather strongly disagree. We are describing a fairly standard > incomplete ordering, which makes the comparison operators completely > appropriate. Adding new syntax for something that doesn't need it is > the thing likely to cause confusion. > Indeed it would work the same way as set comparison does, if you interpret a type as the set of its instances. But I imagine that many Python users are not so familiar with this abstraction level (even if they use isinstance() and issubclass() all day long). Sometimes using a *different* operator is helpful to send a signal to the reader that a *different* kind of thing is being manipulated -- sort of like how you can tell that a collection is a list or a set by noticing whether values are added using append() or add(). Type theorists apparently have chosen to use the <: notation, and presumably for the same reason. -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him/his **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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