struct Accumulator {
    var value = 0

    mutating func doIt(_ n: Int) {
        value += n
    }

    mutating func callAsFunction(_ n: Int) {
        doIt(n)
    }
}


let result1 = [1, 2, 3, 4].reduce(into: Accumulator()) {
    $0.doIt($1)     // 1) call mutating is fine here
}
.value

// this should work, but do not
let result2 = [1, 2, 3, 4].reduce(into: Accumulator()) {
    // 2)
    // in Xcode 12.5b2 playground:
    // error: cannot use mutating member on immutable value: '$0' is immutable
    // in iPad Playground:
    // Type of expression is ambiguous without more context
    $0($1)
}
.value


let result3 = [1, 2, 3, 4].reduce(into: Accumulator()) { (a: inout Accumulator, n: Int) in
    a(n)          // 3) this is fine
}
.value