ios - 无法通过下标赋值 : subscript is get-only

Question

我正在使用 Expression创建一个非常先进的计算器，从用户那里获取字符串输入。我正在尝试添加函数来为计算器提供更多功能，但是我无法添加自定义或转换函数。您能否解释一下为什么会出现以下错误。

有效的代码

 public static let mathSymbols: [Symbol: SymbolEvaluator] = {
        var symbols: [Symbol: SymbolEvaluator] = [:]
        symbols[.function("atan", arity: 1)] = { atan($0[0]) }
        symbols[.function("abs", arity: 1)] = { abs($0[0]) }
        symbols[.function("log", arity: 1)] = { log2($0[0]) }
        return symbols
 }()

无效的输入

        symbols[.function("number", arity: 1)] = { Float($0[0]) }
        symbols[.function("number", arity: 1)] = { someFunction($0[0]) }

错误

Cannot assign through subscript: subscript is get-only

Answer 1

错误信息具有误导性。 Expression.swift定义

public typealias SymbolEvaluator = (_ args: [Double]) throws -> Double

这意味着闭包必须返回一个Double:

symbols[.function("number", arity: 1)] = { Double($0[0]) }