Firstly used Swift's Array Initializer with Access to Uninitialized Storage; Read about Curry Function in Swift

First time used Swift 5.1. feature Array Initializer with Access to Uninitialized Storage refactoring this:

extension Data {
    var sha1: [UInt8] {
        var digest = [UInt8](repeating: 0, count: Int(CC_SHA1_DIGEST_LENGTH))
        withUnsafeBytes { unsafeBytes in
            _ = CC_SHA1(unsafeBytes.baseAddress, CC_LONG(count), &digest)
        }
        return digest
    }
}

into this:

extension Data {
    var sha1: [UInt8] {
        let dLength = Int(CC_SHA1_DIGEST_LENGTH)
        return withUnsafeBytes { bytes in
            return [UInt8](unsafeUninitializedCapacity: dLength) { buf, initedCount in
                _ = CC_SHA1(bytes.baseAddress, CC_LONG(count), buf.baseAddress)
                initedCount = dLength
            }
        }
    }
}

Functional Swift: Curry Function. First time I got which this feature called this way. Oh, no! This feature was called after Haskel Curry.

Why I started to read about curried functions? I have this:

sha1.map { String(format: "%02hhx", $0) }.joined()

The idea is to get rid of String(format: "%02hhx", $0) inside map and to get something similar to:

let hexformat: (CVarArg...) -> String = format("%02hhx")
sha1.map(hexformat).joined()
// or even shorter - that's idea!
sha1.map(format("%02hhx")).joined()

Implementation of curried format function is very short:

func format(_ fmt: String) -> (CVarArg...) -> String {
    return { String(format: fmt, arguments: $0) } // (1)
}

let s1 = format("%02hhx")(255)
let s2 = format("%02hhx-%02hhx")(255, 255)
let s3 = format("%02hhx-%02hhx-%@")(255, 255, "string")

Unfortunately it doesn’t compile with error at (1):

Cannot convert return expression of type '([CVarArg]) -> String' to return type '(CVarArg...) -> String'

The problem is because parameter CVarArg... is visible inside function having type [CVarArg]. But closure has to be converted back to CVarArg.... Or I can’t find how to do this conversion. Or this doesn’t exist.

It works like charm for single value formatting:

func singleValueFormat(_ fmt: String) -> (CVarArg) -> String {
    return { String(format: fmt, arguments: [$0]) }
}
[1,2,3,4,5].map(singleValueFormat("%02hhx")).joined() // "0102030405"

Swift meetup question: how to cast [CVarArg] to CVarArg...?