String to Expression in Swift

Question

I need Help I have a simple math expression parser:

public indirect enum Expr {
    case variable // could take name as associated type to support multiple vars
    case const(Decimal)
    case negate(Expr)
    case sum([Expr])
    case product([Expr])
    case quotient(Expr, Expr)
    case power(Expr, Decimal)
}

public extension Expr {
    static prefix func -(expr: Expr)    -> Expr { .negate(expr) }
    static func +(lhs: Expr, rhs: Expr) -> Expr { .sum([lhs, rhs]) }
    static func -(lhs: Expr, rhs: Expr) -> Expr { .sum([lhs, .negate(rhs)]) }
    static func *(lhs: Expr, rhs: Expr) -> Expr { .product([lhs, rhs]) }
    static func /(lhs: Expr, rhs: Expr) -> Expr { .quotient(lhs, rhs) }
    static func ^(lhs: Expr, rhs: Decimal) -> Expr { .power(lhs, rhs) }
}

extension Expr: ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral {
    public init(integerLiteral: Int64) {
        self = .const(Decimal(integerLiteral))
    }
    
    public init(floatLiteral: Double) {
        self = .const(Decimal(floatLiteral))
    }
}

It works great with expressions:

for example:

let x: Expr = .variable
let expr: Expr = 3*(x+5)
print(expr)

prints:

product([SymbolicDifferentiator_Sources.Expr.const(3), SymbolicDifferentiator_Sources.Expr.sum([SymbolicDifferentiator_Sources.Expr.variable, SymbolicDifferentiator_Sources.Expr.const(5)])])

But I want to parse expressions entered with Command Line (as I String):

I've tried

let stringWithMathematicalOperation: String = "5*5" // Example
let exp: NSExpression = NSExpression(format: stringWithMathematicalOperation)
let result: Expr = exp.expressionValue(with:nil, context: nil) as! Expr

But it gives me an error on the last string: Could not cast value of type '__NSCFNumber' (0x1fe417d18) to 'SymbolicDifferentiator_Sources.Expr' (0x1070e4510).

How can I convert String entered in Command Line to expression and cast it to my Expr indirect enum?