Swift protocol extension with specific Self type

Question

I added an extension for the UnsignedInteger protocol to add a hex method that represents the number in hex format. I also want for specific conforming structs to have default value for a parameter. What I wrote is the below.

extension UnsignedInteger {
    func hex(withFieldWidth fieldWidth: Int, andUseUppercase uppercase: Bool = true) -> String {
        return String(format: "%0\(fieldWidth)\(uppercase ? "X" : "x")", self as! CVarArg)
    }
}

extension UnsignedInteger where Self == UInt8 {
    func hex(withFieldWidth fieldWidth: Int = 2, andUseUppercase uppercase: Bool = true) -> String {
        // should call the UnsignedInteger implementation with the default parameters
        return hex(withFieldWidth: fieldWidth, andUseUppercase: uppercase)
    }
}

extension UnsignedInteger where Self == UInt16 {
    func hex(withFieldWidth fieldWidth: Int = 4, andUseUppercase uppercase: Bool = true) -> String {
        // should call the UnsignedInteger implementation with the default parameters
        return hex(withFieldWidth: fieldWidth, andUseUppercase: uppercase)
    }
}

However, for the UInt8 and UInt16 specific extensions, it seems to be calling itself and not the hex from the first extension block, as explained by the warning message I get for the UInt8 and UInt16 blocks: All paths through this function will call itself.

If I remove the fieldWidh from the UInt8 and UInt16 blocks, calling hex (with hardcoded values for fieldWidth) seems to compile fine, I believe this way it is calling the hex method from the first extension block. Below is the code that compiles fine.

extension UnsignedInteger {
    func hex(withFieldWidth fieldWidth: Int, andUseUppercase uppercase: Bool = true) -> String {
        return String(format: "%0\(fieldWidth)\(uppercase ? "X" : "x")", self as! CVarArg)
    }
}

extension UnsignedInteger where Self == UInt8 {
    func hex(andUseUppercase uppercase: Bool = true) -> String {
        // should call the UnsignedInteger implementation with the default parameters
        return hex(withFieldWidth: 2, andUseUppercase: uppercase)
    }
}

extension UnsignedInteger where Self == UInt16 {
    func hex(andUseUppercase uppercase: Bool = true) -> String {
        // should call the UnsignedInteger implementation with the default parameters
        return hex(withFieldWidth: 4, andUseUppercase: uppercase)
    }
}

Is there a way to specify a default value for the parameter for specific conforming structs when doing a protocol extension?

score 1 · Accepted Answer · answered Jul 09 '21 at 18:38

1

Is there a way to specify a default value for the parameter for specific conforming structs when doing a protocol extension?

You have highlighted the problems with this approach in the question already.

How can I solve it in a different way?

UnsignedInteger inherits from BinaryInteger that can provide you bitWidth information (UInt8 => 8, UInt16 => 16 and so on).

extension UnsignedInteger {
    func hex(uppercase: Bool = true) -> String {
        let fieldWidth = self.bitWidth / 4
        return String(format: "%0\(fieldWidth)\(uppercase ? "X" : "x")", self as! CVarArg)
    }
}

Above makes it work for UInt, UInt8, UInt16, UInt32 & UInt64.

Taking it one step further, you can do this with FixedWidthInteger & now it will work for all signed and unsigned integers.

answered Jul 09 '21 at 18:38

Tarun Tyagi

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Thanks @Tarun Tyagi, as you mentioned, I used bitWidth to have a default value for the parameter and it worked. But what if there was no bitWidth field, would what I was trying be in any way possible? – hakuna matata Jul 09 '21 at 18:56
1

@hakunamatata Swift compiler will rightfully choose a more-specialized implementation (`UInt8`, `UInt16` in your case) of a function rather than choosing a base implementation (`UnsignedInteger` in your case). So when you are calling the `hex(...` with exact same signature as the base implementation, it will always choose the current implementation rather than base implementation. That'll become an infinite loop in itself at runtime - so compiler warns you upfront about this issue. – Tarun Tyagi Jul 09 '21 at 19:31
1

@hakunamatata The only solution would be to change function signature in a meaningful way (not just add default value) like you did in 2nd version - which you don't want to do. You want to have same function signature. – Tarun Tyagi Jul 09 '21 at 19:32

score 1 · Answer 2 · answered Jul 10 '21 at 00:19

This has already been answered but it can be improved. There is no need to force cast as CVarArg you can simply add a constraining where Self: CVarArg. You can also pass the number of characters as a CVarArg of the String initializer and use * in your string format.

extension UnsignedInteger where Self: CVarArg {
    func hex(uppercase: Bool = true) -> String {
        .init(format: "%0*\(uppercase ? "X" : "x")", bitWidth / 4, self)
    }
}

UInt8(255).hex()   // "FF"
UInt8(255).hex(uppercase: false)   // "ff"
UInt16(255).hex()  // "00FF"
UInt16(255).hex(uppercase: false)  // "00ff"
UInt32(255).hex()  // "000000FF"
UInt32(255).hex(uppercase: false)  // "000000ff"

Yeah I didn't notice that the UInt types conform to it, this will let me avoid casting self. Thanks @Leo Dabus. — hakuna matata, Jul 10 '21 at 07:23

Swift protocol extension with specific Self type

2 Answers2