Add Receiver.take_while and drop_while

[llucax]

What's needed?

Receiver.filter(predicate) is very ambiguous for people not used to functional programming, as it is never clear if the receiver should receive the messages for which predicate(message) returns True or False.

Proposed solution

Add 2 new methods to Receivers:

• take_while(): Alias for filter().
• drop_while(): Alias for filter(lambda m: not predicate(m))

We can also consider to deprecate filter() in the future as a way to promote more readable code.

Use cases

No response

Alternatives and workarounds

• Add a not predicate or filterfalse method: it is more complicated and/or more work.

Additional context

The take/drop while idiom is already very widely used in many languages in conjunction with iterators/streams, like Java, Rust, Ruby and even Python itself offer those in the itertools built-in module.

[llucax]

Oh, dammit, I just realized we misinterpreted what take_while and drop_while do.

• take_while stops the iteration as soon as one element doesn't fulfill the predicate.
• drop_while stops using the predicate after an element fulfills the predicate.

>>> print(list(takewhile(lambda x: x < 4, [1, 2, 3, 4, 5, 6])))
[1, 2, 3]
>>> print(list(dropwhile(lambda x: x < 4, [1, 2, 3, 4, 3, 2, 1])))
[4, 3, 2, 1]

So we need new names, or forget about it / add filterfalse().

[llucax]

Here is a summary of what's done in different popular programming languages.

Names for filter Across Languages

Language	Function Name	Description
C++	std::ranges::filter	Filters elements based on a predicate (C++20 and above).
Java	.filter()	Filters elements in a stream based on a predicate.
Dart	.where()	Filters elements in an iterable based on a predicate.
Rust	.filter()	Filters elements in an iterator based on a predicate.
Go	Custom Implementation	Requires manual implementation of filtering.
PHP	array_filter	Filters elements of an array based on a callback function.
JavaScript	.filter()	Creates a new array with elements that pass the predicate.
Python	filter()	Constructs an iterator with elements that satisfy the predicate.
Ruby	.select()	Filters elements of an array based on a block (alias: filter).
Kotlin	.filter()	Returns a list of elements satisfying the predicate.
Scala	.filter()	Returns a collection of elements satisfying the predicate.

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Names for "Not Filter" Across Languages

Language	Explicit Name	Example for Negated Filter Using Filter-like Function
C++	None	std::ranges::views::filter(range, [&](auto x) { return !predicate(x); });
Java	None	list.stream().filter(x -> !predicate(x)).collect(Collectors.toList());
Dart	None	iterable.where((x) => !predicate(x));
Rust	None	`iterator.filter(
Go	None	Custom implementation required (negate the predicate manually).
PHP	None	array_filter($array, fn($x) => !$predicate($x));
JavaScript	None	array.filter(x => !predicate(x));
Python	None	filter(lambda x: not predicate(x), iterable) or [x for x in iterable if not predicate(x)]
Ruby	.reject()	`array.reject {
Kotlin	.filterNot()	list.filterNot { predicate(it) }
Scala	.filterNot()	list.filterNot(predicate)

It seems like filter() is the most popular by far. I like where() better, but I'm not sure if it is worth a name change for such a small gain.

Also reject() sounds OK, but it is not popular at all and having a negative filter doesn't seem to be popular at all, and considering that we can't do type narrowing with negative filters, I'm also considering fully dropping the idea of a negative version of filter.

Conclusion

Considering all of the above, I would close this as WONTFIX 🤷