why does pants define its own wrapper around the stdlib

Enum

?

Apparently to support exhaustive match semantics. It looks like that code in our Enum subclass could just be extracted as a function to work with stdlib Enum; IOW: I'm not sure we need this and it probably is just an artifact of refactoring when our Enum class did more.

It was a compromise made when migrating away from

enum()

to avoid losing the functionality of

.resolve_for_enum_variant

. Originally we used a simple dictionary lookup, but several reviewers wanted the exhaustiveness check to be preserved Converting match() from a method to a function could make sense!

Yeah - looks like we could just kill our type and lift the match function:

$ python
Python 3.7.4 (default, Oct  4 2019, 06:57:26) 
[GCC 9.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from enum import Enum, auto
>>> from typing import Dict, TypeVar
>>> 
>>> 
>>> class Choices(Enum):
...   A = auto()
...   B = auto()
... 
>>> 
>>> E = TypeVar('E', bound=Enum)
>>> R = TypeVar('R')
>>> 
>>> 
>>> class EnumMatchError(ValueError):
...   """Issue when using match() on an enum."""
... 
>>> 
>>> class UnrecognizedMatchError(EnumMatchError):
...   """A value is used that is not a part of the enum."""
... 
>>> 
>>> class InexhaustiveMatchError(EnumMatchError):
...   """Not all values of the enum specified in the pattern match."""
... 
>>> 
>>> def match(enum_value: Enum, arms: Dict[E, R]):
...   enum_type = type(enum_value)
...   all_values = [e for e in enum_type]  
...   unrecognized_values = [value for value in arms if value not in all_values]
...   if unrecognized_values:
...     raise UnrecognizedMatchError(
...       f"Match includes values not defined in the enum. Unrecognized: {unrecognized_values}"
...     )
...   missing_values = [value for value in all_values if value not in arms]  
...   if missing_values:
...     raise InexhaustiveMatchError(
...       f"All enum values must be covered by the match. Missing: {missing_values}"
...     )
...   return arms[enum_value]
... 
>>> 
>>> match(Choices.A, {Choices.A: 42})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 12, in match
__main__.InexhaustiveMatchError: All enum values must be covered by the match. Missing: [<Choices.B: 2>]
>>> match(Choices.A, {Choices.A: 42.0, Choices.B: 1/137})
42.0
>>>

wow, i hadn’t considered that at all

Also does this belong in collections.py or in objects.py? I realized that after we remove datatype, we can get MyPy working with objects.py. Part of my original motivation for putting Enum in collections was because I thought there were more blocking issues in objects.py until realizing this that we only need to remove datatype()

meta.py

? i feel more comfortable putting stuff there if it's specifically to operate on stdlib types

Use

meta.py

for both? I’m not convinced either way and totally think it’s worth revisiting. I know it results in a bit more temporary churn, but hopefully these APIs will stabilize after one or two more weeks of tweaks like John&/ proposal

we might be able to kill

objects.py

after we kill

datatype

with

engine/objects.py

being that, but for the engine specifically somehow

About that file, going to take another look at Collection.of() this weekend after having to revert Collection[X]. I realized in the revert that there’s very little difference between Collection[X] and Tuple[X, ...]. We might be able to simplify by using the latter. Still testing if it will work

i think we should absolutely use

Tuple[X, ...]

mostly because i think i was conflating like "the specific

Collection.of()

factory" with "the ability to wrap types in other types" in my head the last time we discussed this

(Kind of related, my last day teaching is now Nov 1 so I am going to have much more time after two weeks to get our type coverage much higher once we finish these objects.py changes! Hoping to get us to majority using

partially_type_checked

)

your wish is my mind's demand: https://github.com/pantsbuild/pants/pull/8504 (move enum pattern matching into its own top-level function in

meta.py

)