In my earlier I post I talked about not being very sure that a method I wrote for one of the Stanford iOS Programming course assignments was really embracing Swift as a language. I then posted about using Swift’s unit test framework that would make any refactoring easier and better.

And so on to the refactoring…

Well, with a fresh day came a fresh eye, and there was not too much I could really do to make the method more Swift than C/C++. I ended up mainly folding some let statements into inline expressions. Not exactly hard, but when all was completed I think the code is more or less as tight as it could be and still easily readable – opinions to the contrary are very welcome.

Here’s the revised method (see one of articles linked to above for the original code):

 
    private func formatDescription(ops: [Op]) -> (desc: String?, remainingOps: [Op]) {
        if !ops.isEmpty {
            var remainingOps = ops
            let op = remainingOps.removeLast()
            switch op {
            case .Operand(let operand):
                return ("\(operand)", remainingOps)
            case .UnaryOperation(let operation,_):
                let (op1Text, remainingOps1) = formatDescription(remainingOps)
                let returnText = "\(operation)(\(op1Text ?? "?"))"
                return (returnText, remainingOps1)
            case .BinaryOperation(let operation,_):
                let (op1Text, remainingOps1) = formatDescription(remainingOps)
                let (op2Text, remainingOps2) = formatDescription(remainingOps1)
                let returnText = "\(op2Text ?? "?") \(operation) \(op1Text ?? "?")"
                return (returnText, remainingOps2)
            case .Variable(let variable):
                return (variable, remainingOps)
            case .Constant(let constant):
                return (constant, remainingOps)
            }
        }
        return (nil, ops)
    }

Because of the unit tests written before, it was trivial to ensure the before and after are functionally equivalent.

I did end up adding a handful of extra unit tests to ensure cases where an operand was missing was correctly handled:

 

    func testDescriptionMissingOperand() {
        brain.performOperation("cos")
        XCTAssert(brain.description == "cos(?)")
    }

    func testDescriptionMissingOperand2() {
        brain.pushOperand(4)
        brain.performOperation("+")
        XCTAssert(brain.description == "? + 4.0")
    }

Note that the code as presented does not yet implement the whole assignment. The main piece missing is adding the formatting of expressions correctly with brackets by considering operator precedence.