main.go

package main

import (
	"fmt"

	"example.org/int/typ"
)

func main() {
	for item := range items {
		if item.StringValue() != nil {
			fmt.Println("Found a string value:", *item.StringValue())
			if *item.StringValue() == "bat" {
				break
			}
		}
	}
}

func items(push func(typ.Variant) bool) {
	for item := range integers {
		formattedItem := typ.String(string(item))
		if !push(formattedItem) {
			return
		}
	}

	for item := range words {
		formattedItem := typ.String(item)
		if !push(formattedItem) {
			return
		}
	}
}

func integers(push func(int32) bool) {
	if !push(1) ||
		!push(2) ||
		!push(3_000) ||
		!push(4) ||
		!push(5) {
		return
	}
}

func words(push func(string) bool) {
	if !push("hat") ||
		!push("bat") ||
		!push("cat") ||
		!push("mat") {
		return
	}
}

variant.go

package typ

type Variant interface {
	StringValue() *string
	Int32Value() *int32
}

type variant struct {
	stringValue *string
	int32Value  *int32
}

func String(value string) Variant {
	return variant{stringValue: &value}
}

func Int32(value int32) Variant {
	return variant{int32Value: &value}
}

func (t variant) StringValue() *string {
	return t.stringValue
}

func (t variant) Int32Value() *int32 {
	return t.int32Value
}