实例成员不能在Swift中使用类型

Question

我的代码会产生错误：实例成员＆＃39;累加器＆＃39;不能用于类型＆＃39; CalculatorBrain＆＃39;

以下是代码：

//
//  CalculatorBrain.swift
//  Calculator
//
//  Created by Tony Hudson on 11/11/2017.
//  Copyright © 2017 Tony Hudson. All rights reserved.
//

import Foundation

func changeSign(operand: Double) -> Double{
    return -operand
}

func multiply(op1: Double, op2: Double) -> Double {
    return op1 * op2
}

struct CalculatorBrain {
    private var accumulator: Double?

    private enum Operation {
        case constant(Double)
        case unaryOperation((Double) -> Double)
        case binaryOperation((Double, Double) -> Double)
        case equals
    }

    private var operations: Dictionary<String,Operation> = [
        "π" : Operation.constant(Double.pi),
        "e" : Operation.constant(M_E),
        "√" : Operation.unaryOperation(sqrt),
        "cos" : Operation.unaryOperation(cos),
        "±" : Operation.unaryOperation({-$0}),
        "×" : Operation.binaryOperation({ $0 * $1}),
        "÷" : Operation.binaryOperation({ $0 / $1}),
        "+" : Operation.binaryOperation(){ $0 + $1},
        "−" : Operation.binaryOperation(){ $0 - $1},
        "=" : Operation.equals
    ]

    mutating func performOperation(_ symbol: String) {
        if let operation = operations[symbol] {
            switch operation {
            case .constant(let value):
                accumulator = value
            case .unaryOperation(let function):
                if accumulator != nil {
                    accumulator = function(accumulator!)
                }
            case .binaryOperation(let function):
                if accumulator != nil {
                    pendingBinaryOperation = PendingBinaryOperation(function: function, firstOperand: accumulator!)
                    accumulator = nil
                }
            case .equals:
                performPendingBinaryOperation()

            }

        }
    }

    private mutating func performPendingBinaryOperation() {
        if pendingBinaryOperation != nil && accumulator != nil {
            accumulator = pendingBinaryOperation!.perform(with: accumulator!)
            pendingBinaryOperation = nil
        }
    }

    private var pendingBinaryOperation: PendingBinaryOperation?

    private struct PendingBinaryOperation {
        let function: (Double, Double) -> Double
        let firstOperand: Double

        func perform(with secondOperand: Double) -> Double { //Error here
            return function(firstOperand, secondOperand)
        }
    }
    mutating func setOperand(_ operand: Double) { //Error here
        accumulator = operand
    }
    var result: Double? {
        get {
            return accumulator
        }
    }
}

我确定它的东西很傻，但我无法发现它！