The idiomatic way to implement generators (yield) in Golang for recursive functions

Question

[ Note: I read Python-style generators in Go, this is not a duplicate of it. ]

In Python / Ruby / JavaScript / ECMAScript 6, generator functions could be written usi

Answer 1

I agree with icza's answer. To summarize, there are are two alternatives:

1. mapping function: Use a callback to iterate over a collection. func myIterationFn(yieldfunc (myType)) (stopIterating bool). This has the disadvantage of ceding the control flow to myGenerator function. myIterationFn is not a Pythonic generator because it doesn't return an iterable sequence.
2. channels: Use a channel and be wary of leaking goroutines. It's possible to transform myIterationFn into a function that returns an iterable sequence. The following code provides an example of such a transformation.

myMapper := func(yield func(int) bool) {
    for i := 0; i < 5; i++ {
        if done := yield(i); done {
            return
        }
    }
}
iter, cancel := mapperToIterator(myMapper)
defer cancel() // This line is very important - it prevents goroutine leaks.
for value, ok := iter(); ok; value, ok = iter() {
    fmt.Printf("value: %d\n", value)
}

Here's a complete program as an example. mapperToIterator does the transformation from a mapping function to a generator. Go's lack of generics requires casting from interface{} to int.

package main

import "fmt"

// yieldFn reports true if an iteration should continue. It is called on values
// of a collection.
type yieldFn func(interface{}) (stopIterating bool)

// mapperFn calls yieldFn for each member of a collection.
type mapperFn func(yieldFn)

// iteratorFn returns the next item in an iteration or the zero value. The
// second return value is true when iteration is complete.
type iteratorFn func() (value interface{}, done bool)

// cancelFn should be called to clean up the goroutine that would otherwise leak.
type cancelFn func()

// mapperToIterator returns an iteratorFn version of a mappingFn. The second
// return value must be called at the end of iteration, or the underlying
// goroutine will leak.
func mapperToIterator(m mapperFn) (iteratorFn, cancelFn) {
    generatedValues := make(chan interface{}, 1)
    stopCh := make(chan interface{}, 1)
    go func() {
        m(func(obj interface{}) bool {
            select {
            case <-stopCh:
                return false
            case generatedValues <- obj:
                return true
            }
        })
        close(generatedValues)
    }()
    iter := func() (value interface{}, notDone bool) {
        value, notDone = <-generatedValues
        return
    }
    return iter, func() {
        stopCh <- nil
    }
}

func main() {
    myMapper := func(yield yieldFn) {
        for i := 0; i < 5; i++ {
            if keepGoing := yield(i); !keepGoing {
                return
            }
        }
    }
    iter, cancel := mapperToIterator(myMapper)
    defer cancel()
    for value, notDone := iter(); notDone; value, notDone = iter() {
        fmt.Printf("value: %d\n", value.(int))
    }
}