On 2020-06-23 18:20, Antoine Pitrou wrote:
Some comments:
* What you call "Constant Value Patterns" can really refer to any local
or non-local name, regardless of how complex the referred object is,
right? Also, __eq__ is called in that case, not __match__?
* If I understand correctly, these:
case b"":
print("it's an empty bytes object")
and
case bytes():
print("it's a bytes object")
have entirely different meanings. Am I right? This sounds like I have
to switch contexts when reading code, based on whether I am reading
regular code or a match clause, given that semantics are quite
different.
Instead, it seems like the latter would be more explicitly spelled out
as, e.g.:
case instanceof(bytes):
print("it's a bytes object")
* The acronym "ADT" is never defined.
* """If there are more positional items than the length of
__match_args__, an ImpossibleMatchError is raised."""
What if there are less positional items than ``len(__match_args__)``?
Can the match succeed rather than raise ImpossibleMatchError? This
seems potentially error-prone.
Overall, my main concern with this PEP is that the matching semantics
and pragmatics are different from everything else in the language.
When reading and understanding a match clause, there's a cognitive
overhead because suddently `Point(x, 0)` means something entirely
different (it doesn't call Point.__new__, it doesn't lookup `x` in the
locals or globals...). Obviously, there are cases where this is
worthwhile, but still.
It may be useful to think about different notations for these new
things, rather than re-use the object construction notation.
For example:
case Point with (x, y):
print(f"Got a point with x={x}, y={y}")
or:
case Point @ (x, y):
print(f"Got a point with x={x}, y={y}")
(yes, "@" is the matrix multiplication operator... but it's probably
much less likely to appear in common code and especially with a class
object at the left)
Or:
case Point as (x, y):
print(f"Got a point with x={x}, y={y}")
perhaps as 'as' is already used for binding.
The disadvantage there is with nested patterns:
case Coordinate(Point(x1, y1), Point(x2, y2)):
where you're matching a coordinate and binding to x1, y1, x2 and y2.
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