[Pitch] Introduce `let-else` syntax as alternative for single expression `guard-let-else`

Evolution Pitches
10

I find the use of guard very helpful both when I'm writing and reading code and think I would find the cognitive cost to be higher if any let or var assignment could potentially be an early exit.

I realize that indentation, spaces between lines, putting else clauses on the same or different lines is a matter of personal preference. But I think that some approaches make guard statements read with more continuity than others.

In the example provided, I would probably group the guard statements together, so that all of the things that need to be successful to proceed happen in one conceptual run of code.

guard let store = sharedNotesStore else { throw Error.noSharedNotesStoreAvailable }
guard let data = section.propertyDictionary.json else { throw Error.serializationFailure }
guard let section = section(withIdentifier: noteIdentifier, data: data) else { throw Error.noSectionFoundForNote }

let blobs: [SharedNote.Blob] = [.init("ABC")]
let key = UUID().uuidString
let sharedNote = SharedNote(store: store, section: section, key: key, blobs: blobs)

In the midst of a method, I also tend to add a line of whitespace before a guard statement, so that it stands out as a precondition for proceeding.

For me, one of the benefits of guard is that I know immediately that this is code that will exit early in some way. Even if the else clause is on the same line as shown in the example code, you are certain that you will find it at end of the line by seeing that initial guard keyword.

With the proposed change, for longer lines, you need to scan pretty far to the right to discover that this line of code can have bigger consequences than simply assignment. For each and every let or var declaration, you never know if maybe there's an early exit without scanning almost the entire line.

let section = section(withIdentifier: noteIdentifier, data: data) else { throw Error.noSectionFoundForNote }

I also think that if let and guard let both communicate that optional binding is occurring and that what is happening is not simply an assignment.

I can absolutely see the lack of flow in the code example provided, but I think this proposed change would reduce the clarity of all variable and constant declarations by making them ambiguous until the developer scanned the entire line. So, I am -1 for this proposal.

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