[Python-Dev] Adding NewType() to PEP 484

[Steve Dower]

Did anyone suggest "distinct type alias"?

Regardless of what name, I'm fairly sure people will call it whatever the function to create it is called. So if the function is typings.distinguish_type(...), then distinguished will stick.

Top-posted from my Windows Phone

-----Original Message-----
From: "Guido van Rossum" <guido at python.org>
Sent: ‎5/‎28/‎2016 7:38
To: "Steven D'Aprano" <steve at pearwood.info>
Cc: "Python-Dev" <python-dev at python.org>
Subject: Re: [Python-Dev] Adding NewType() to PEP 484

Just to add to the list of options, Twitter also came up with

- invention

- DomainType

- TypedAlias

But seriously I think we should just decide between Derived Type and
Distinguished Type [Alias].

The latter comes from the idea that when you write e.g.

UserId = int

then UserId is a type alias (that's existing PEP 484 terminology) and
the type checker doesn't distinguish it from int -- you can use it in
places where you logically expect a UserId but to the type checker
those variables have the type int.

There is even a neat potential "origin story" that would explain why
we'd call it Distinguished Type Alias.

The story is about gradually converting a large code base to being
consistent: initially you make UserId a regular type alias and you
start putting it in your code incrementally, making sure it has no
mypy errors as you go (but this just treats it as an int). After days,
when you think you are done, you change UserId to a distinguished type
alias and then mypy will point out the places where you've missed
something or you're doing something questionable with user IDs.

And yes, in the wider context of subclassing, Derived Type is probably
confusing because a subclass is also called a derived class.


On Sat, May 28, 2016 at 5:24 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Fri, May 27, 2016 at 09:26:29PM -0700, Guido van Rossum wrote:
>
>> We discussed this over dinner at PyCon, some ideas we came up with:
>>
>> - Dependent types, harking back to a similar concept in Ada
>> (https://en.wikibooks.org/wiki/Ada_Programming/Type_System#Derived_types)
>> which in that language is also spelled with "new".
>
> I started to explain this to my non-programmer wife, I got as far as
> explaining types, and that we need a name for this thing, and she
> stopped me and said
>
> "Please don't tell me this is leading to TypyMcTypeCheck."
>
> [...]
>> - BoatyMcBoatType
>
>> The nice thing about "distinguished" is that it's a relatively rare
>> word so it is easy to remember or look up.
>
> I would have thought that being rare, it would be *harder* to remember.
>
>
>> Personally I'm still in favor of Derived type (but I'm more into
>> ancient programming languages than most folks here). I could also live
>> with Distinguished Type.
>
> I think Derived Type is the nicest of the options. It accurately
> describes what it is: a type derived from another. And its shorter and
> easy to both say and write than "Distinguished type" (which sounds like
> "distinguished gentlemen" -- is it wearing a monocle and a top hat?).
>
> "Distinguished" is too vague for my tastes, it might as well be
> "flibblegubble type". *All* types are distinguished, the type checker
> has to distinguish int from float from list from str, so to call
> NewType("userid", int) a "distinguished type" is only to call it a type.
>
>
>
> --
> Steve
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-- 
--Guido van Rossum (python.org/~guido)
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