YieldGenerator - Python's "yield" generators for Swift

The YieldGenerator class and the associated yieldSequence allow you recode generators as closure containing a loop using a thread and semaphores internally to implement "inversion of control". For example, a loop to implement the first 20 numbers in the Fibanacci series can be expressed as follows:

    let generator = YieldGenerator<Int> {
        (yield : (Int) -> Bool) in
        var a = 0, b = 1
        for _ in 0..<20 {
            if !yield(b) {
                break
            }
            let sum = a + b
            a = b
            b = sum
        }
    }

    while let value = generator.next() {
        println("Value: \(value)")
    }

A generator can be used in a for loop by converting it into a sequence:

    for value in generator.sequence() {
        println("Value: \(value)")
    }

Or you can wrap the generator up by using the yieldSequence() function

    for value in yieldSequence( {
        (yield : (Int) -> Bool) in
        var a = 0, b = 1
        for _ in 0..<20 {
            if !yield(b) {
                break
            }
            let sum = a + b
            a = b
            b = sum
        }
    } ) {
        println("Value: \(value)")
    }

The generator loop is run in it's own thread calling yield() when it wants to make a value available to the generator. The boolean value returned by the yield closure is false if the loop consuming the generator's sequence has exited and the yield loop itself should exit.

I'd thought this was an original idea but the terrain was already surveyed by Kristopher Johnson just a month after Swift came out! Anyway, this is my take on it. I've included Kristopher's tests for sanity.

On OSX the following api also is available that can be used to the output to process output of a UNIX command, something like popen() of old..

    public func CommandSequence( command: String, workingDirectory: String = "/tmp",
        linesep: NSString = "\n", filter: String? = nil, filter2: String? = nil ) -> SequenceOf<NSString>

Finally, as NSTask() and fork()'s in general for Cocoa processes are quite expensive I've added the much faster:

    public func BashSequence( command: String, workingDirectory: String = "/tmp" ) -> SequenceOf<NSString>

This runs up a single bash process for all subcommands which are written to standard input rather than forking each time. Ouput is delimited by a token ___END___ $status so output of the subprocess must finish with a newline.

License MIT

Copyright (c) 2015 John Holdsworth

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