Optional parameters in Go hack

gistfile1.go

// A cute hack demonstrating one (weird?) way of doing optional
// parameters in Go.
//
// Please don't write code like this. It is fun code, though.
//
// Output: (see it live at golang.org)
//
//      I like cheese.
//      Do you like cheese too?
//      I DO!
//      ***** Cheese party! *****
//
// My apologies,
// <bradfitz@golang.org>

package main

import (
	"fmt"
	"strings"
	"sync"
)

func main() {
	s := Sentence("I like cheese")()
	fmt.Println(s)

	s = Sentence("Do you like cheese too").Question(true)()
	fmt.Println(s)

	s = Sentence("I do").Exclamation(true).AllCaps(true)()
	fmt.Println(s)

	s = Sentence("Cheese party").Exclamation(true).SurroundWith("*", 5)()
	fmt.Println(s)
}

type SentenceFunc func() string

func Sentence(base string) SentenceFunc {
	a := &sentenceArgs{base: base}
	var f SentenceFunc
	f = SentenceFunc(func() string {
		argsLock.Lock()
		defer argsLock.Unlock()
		args[f] = nil, false
		return a.result()
	})
	argsLock.Lock()
	defer argsLock.Unlock()
	args[f] = a
	return f
}

type sentenceArgs struct {
	base      string
	q, bang   bool
	caps      bool
	surround  string
	surroundN int
}

func (a *sentenceArgs) result() string {
	s := a.base
	if a.q { s += "?" }
	if a.bang { s += "!" }
	if !a.bang && !a.q { s += "." }
	if a.caps { s = strings.ToUpper(s) }
	if a.surround != "" {
		rp := strings.Repeat(a.surround, a.surroundN)
		s = fmt.Sprintf("%s %s %s", rp, s, rp)
	}
	return s
}

var argsLock sync.Mutex
var args = make(map[SentenceFunc]*sentenceArgs)

func (f SentenceFunc) args() *sentenceArgs {
	argsLock.Lock()
        defer argsLock.Unlock()
	return args[f]
}

func (f SentenceFunc) Exclamation(v bool) SentenceFunc {
	f.args().bang = v
	return f
}

func (f SentenceFunc) Question(v bool) SentenceFunc {
	f.args().q = v
	return f
}

func (f SentenceFunc) AllCaps(v bool) SentenceFunc {
	f.args().caps = v
	return f
}

func (f SentenceFunc) SurroundWith(s string, n int) SentenceFunc {
	args := f.args()
	args.surround, args.surroundN = s, n
	return f
}