Swift Restart Skip Block

Swift Restart Skip Block.swift

// Let's start with an example

var c = 0
// Inside a jump block, calling the restart function starts over
// and calling the skip function exists the block
jump { restart, skip in
    for i in 1...4 {
        println("c: \(c); i: \(i)")
        if c == 4 { skip() }
        else if i == 3 { c++; restart() }
    }
}

// We will use the jump function to implement a while loop!

func myWhile(test: () -> Bool, loop: () -> ()) {
    jump { restart, skip in
        if test() {
            loop()
            restart()
        }
    }
}

var x = 0
myWhile({ x < 3 }){
    println(x++)    // -> 0 --> 1 --> 2
}

// Note that this is equivalent to having a start: goto before our block
// and a skip: goto after our block in a lanaguage that supports gotos

// Note that this is also equivalent to the following
// Also note that this simpler alternative cannot be implemented with closures
// because continue and break cannot be used in closures even if called by a loop
var repeat = true
while repeat {
  // Insert code goes here
  // restart() --> continue
  // skip() --> repeat = false; break
  repeat = false
}

// Implementation, it's pretty cool
// We use C's setjmp and longjmp

typealias JumpType = UnsafeMutablePointer<Int32>
let jumpSize = UInt(sizeof(jmp_buf))

func jump(f: (() -> (), () -> ()) -> ()) {
    
    // Malloc
    let skip    = JumpType(malloc(jumpSize))
    let restart = JumpType(malloc(jumpSize))

    // Declare restart jump
    let didRestart = setjmp(restart)
    
    // Wait for setup, and then call
    if (didRestart != 0) {
        f({ longjmp(restart, Int32(-1)) }, { longjmp(skip, Int32(-1)) })
    }
    
    let didSkip = setjmp(skip)
    // If we haven't yet run the function, skip back to the start
    if (didSkip == 0) {
        longjmp(restart, Int32(-1))
    }
    
    // Avoid memory leakage
    free(skip)
    free(restart)
}