On Thu, Jul 9, 2020 at 1:42 PM Eric Nieuwland <[email protected]>
wrote:
> Much of the discussion seems to focus on how to distinguish between a
> variable as a provider of a value and a variable as receiver of a matched
> value.
>
> In normal Python syntax a variable in an expression provides a value,
> please let’s keep that unchanged.
>
For patterns, these are no different than parameters for a function (either
a lambda expression or with `def`); or target assignments in unpacking
assignments. So just like I wouldn't wonder where `a` and `b` materialized
in the parameters for the function definition below
def sum2(a, b):
return a + b
I think it will be straightforward to understand this in the context of a
`case` using a capture pattern:
match x:
case (a, b):
return a + b
...
(This commonality between cases and function definitions is further used in
Scala for example, but I don't see that approach for defining an idea of
partial functions -- not like functools.partial functions! -- as being that
useful in Python.)
> So it seems to me we should explicitly mark a variable to receive a
> matched value.
> I have seen ‘?’ suggested as a prefix to do this, ‘\’ would also do fine.
>
> This would solve the single variable issue, too:
> case foo:
> matches the value of ‘foo’, while
> case \foo:
> matches anything and stores it in ‘foo’.
>
>
Explicit namespacing (if a constant) or using a guard (if a variable) seems
to be the right solution, as Ethan demonstrated earlier. No need for . or ^
or \ or ... to disambiguate. Also it seems to me that structural pattern
matching will build on two common usages of namespaces for constants:
1. Constants used from other modules are almost always used in the module
namespace. Eg, socket.AF_UNIX or signal.SIGTERM.
2. New code often tends to use constants defined within an Enum namespace.
Hopefully we will see more of this convention in usage.
(Very much an aside: Interestingly with the socket module we see both used
- it defines its constants with IntEnum and exports them traditionally. The
namespace specifics it uses with IntEnum._convert_ to make this happen --
strictly speaking EnumMeta._convert, not documented, and a bit hard to
follow -- might be possibly debatable, but it works out quite well in
practice in providing backwards compatibility while continuing to work with
a C source of these constants.)
> This would also mean
> case Point(x=\x, y=\y):
> should be used to obtain x and y from the Point instance.
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