Why compiler struggles with this code?

Hello. I have got some unclear error. With this function I want to combine repeated repeated into groups.

import Foundation

func countingValleys(s: String) -> Int {
    let combinedSteps = s.reduce(into: [(direction: Character, count: Int)]()) {
        if $0.last?.direction == $1 ?? false { $0[$0.endIndex - 1].count += 1 }
        $0.append(($1, 1))
    return combinedSteps.count
countingValleys(s: "udduddudduuuudddudud")

But there is error.
error: cannot convert value of type 'String.Element' (aka 'Character') to expected argument type '_?'
Why cant compiler infer type of $1, which should be Character? (right?)

I think you meant to do this

if $0.last?.direction == $1 {  
   $0[$0.endIndex - 1].count += 1 
} else {
    $0.append(($1, 1))

The problem with type checking is that comparing == returns Bool even in optional version. So you're doing Bool ?? false, not Bool? ?? false, which seems to be confusing the compiler.
if $0.last?.direction is nil, the comparison returns false anyway.

One other thing is that you probably don't wan't to append if you're grouping them.

1 Like

Thank you for solution. It works great.

Yeap, seems like my fault in fact. Gonna do more reading. Cheers.