Allowing pattern bindings in `for` loop `where` clauses

[ole (Ole Begemann)]

I came across a few interesting related threads:

In 2018, @broadway_lamb pitched "Allow optional unwrapping in where clause":

Allow optional unwrapping in where clause

The where clause in for loops currently allows us to test a certain condition while iterating a sequence to filter out the elements that don't satisfy that condition, which is cool because it lets us avoid nesting, i. e. instead of let numbers: [Int] = [1, 4, 5, 2, 9, 13, 7, 14, 40, 11] for number in numbers { if isPrime(number) { print("\(number) is prime") } } we can write let numbers: [Int] = [1, 4, 5, 2, 9, 13, 7, 14, 40, 11] for number in numbers where isPrime(number) { …

In 2016, @Joe_Groff provided some context why the if case and for case syntax exists:

@Joe_Groff

This was in fact our original design for the pattern-matching-in-conditions feature. We went with the alternative 'if case' design for a couple of reasons:

• As you noted, for loops are weird. for case makes it clearer that filtering is happening, whereas something like 'for x as T in sequence' is more ambiguous.
• The way 'if let x = y' ends up a no-op is a big pitfall
• Optional unwrapping is by far the most common use case for these conditions, and we got a huge amount of pushback from our users in Apple complaining about the extra syntax all over their code. 'if let' for better or worse has also become one of Swift's trademarks.

And another 2018 discussion whether the various pattern matching syntaxes can be unified:

Could Swift's patterns be more uniform?

I'm starting this thread since there seems to be a lot to discuss related to Swift's patterns. The subject has come up in at least three recent threads: Allow optional unwrapping in where clause Unwrapping and value binding patterns in the condition of ternary expressions SE-0231 — Optional iteration It would be interesting to see if Swift's patterns (in general, not just optional patterns) could be made more uniform, and thus easier to use/understand/remember/learn/teach, while at the same…