the-way-to-go_ZH_CN/eBook/examples/chapter_14/general_lazy_evalution1.go

// general_lazy_evalution1.go
package main

import (
	"fmt"
)

type Any interface{}
type EvalFunc func(Any) (Any, Any)

func main() {
	evenFunc := func(state Any) (Any, Any) {
		os := state.(int)
		ns := os + 2
		return os, ns
	}
	even := BuildLazyIntEvaluator(evenFunc, 0)

	for i := 0; i < 10; i++ {
		fmt.Printf("%vth even: %v\n", i, even())
	}
}

func BuildLazyEvaluator(evalFunc EvalFunc, initState Any) func() Any {
	retValChan := make(chan Any)
	loopFunc := func() {
		var actState Any = initState
		var retVal Any
		for {
			retVal, actState = evalFunc(actState)
			retValChan <- retVal
		}
	}
	retFunc := func() Any {
		return <-retValChan
	}
	go loopFunc()
	return retFunc
}

func BuildLazyIntEvaluator(evalFunc EvalFunc, initState Any) func() int {
	ef := BuildLazyEvaluator(evalFunc, initState)
	return func() int {
		return ef().(int)
	}
}

/* Output:
0th even: 0
1th even: 2
2th even: 4
3th even: 6
4th even: 8
5th even: 10
6th even: 12
7th even: 14
8th even: 16
9th even: 18
*/