While loop

[japanoise]

It'd be rad to have a while loop construct. I've had to hack it in myself, using this:

func truthyResult(env *zygo.Zlisp, fun *zygo.SexpFunction) (bool, error) {
	res, err := env.Apply(fun, []zygo.Sexp{})
	if err != nil {
		return false, err
	}
	switch rt := res.(type) {
	case *zygo.SexpBool:
		return rt.Val, nil
	case *zygo.SexpSentinel:
		return false, nil
	default:
		return true, nil
	}
}

// While hacks a while loop into zygomys
func While(env *zygo.Zlisp, name string, args []zygo.Sexp) (zygo.Sexp, error) {
	if len(args) != 2 {
		return zygo.SexpNull, errors.New("Syntax: (while cond body)")
	}
	var cond, body *zygo.SexpFunction
	switch st := args[0].(type) {
	case *zygo.SexpFunction:
		cond = st
	default:
		return zygo.SexpNull, errors.New("While loop takes two functions")
	}
	switch st := args[1].(type) {
	case *zygo.SexpFunction:
		body = st
	default:
		return zygo.SexpNull, errors.New("While loop takes two functions")
	}
	result, err := truthyResult(env, cond)
	if err != nil {
		return zygo.SexpNull, err
	}
	for result {
		_, err := env.Apply(body, []zygo.Sexp{})
		result, err = truthyResult(env, cond)
		if err != nil {
			return zygo.SexpNull, err
		}
	}
	return zygo.SexpNull, err
}

Is there a better way? Or would this require extending the language?

I'd like to be able to do:

(while (== true cond)
  (some)
  (body))

[glycerine]

Hi @japanoise that would be pretty cool. Just add a couple of correctness tests to the test suite and file a pull request, and we'll put it in.

[japanoise]

Well, currently the API is too clumsy. I wouldn't put it in as-is.

You have to do e.g.

(while (fn [] (==cond true) (fn [] (body goes here))

I was wondering if you had a macro hack or something to make it nicer.

[glycerine]

The proper way would be to add it to the built in codegen, alongside the for builtin. You could probably just copy and modify the for Generator.GenerateForLoop code in generator.go:817 to be like Go and only need the middle test.