How to get elements of a ListKind variable's value?

[cueckoo]

Originally opened by @zzxwill in cuelang/cue#600

In test.cue, there are two variables cmd and rules. I'd like to get all elements of their values. For cmd, to get type string. For rules, to get element path, rewriteTarget and their types.

cmd: [...string]

rules: [...{
	path:          string
	rewriteTarget: *"" | string
}]

Here is my code, but I just could not even get the values, nor their elements.

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"

	"cuelang.org/go/cue"
)

func main() {
	path, _ := filepath.Abs("./test.cue")
	data, err := ioutil.ReadFile(path)

	if err != nil {
		fmt.Println(err)
		os.Exit(-1)
	}
	r := cue.Runtime{}
	template, _ := r.Compile("", string(data))
	stc, _ := template.Value().Struct()
	for i := 0; i < stc.Len(); i++ {
		val := stc.Field(i).Value
		switch val.IncompleteKind() {
		case cue.ListKind:
			iter, _ := val.List()
			for iter.Next() {
				fmt.Println(iter.Value())
			}

		}
	}
}

I also put the .cue file and the go file at https://github.com/zzxwill/try-cloudnative/blob/master/poc.go. Please help take a look at this question. Thanks.

[cueckoo]

Original reply by @mpvl in cuelang/cue#600 (comment)

In CUE, currently the only way to get these values are doing something like:

cmd: [...string]

type: (cmd & [_])[0]

Which results in string for type

In the API you can use Value.Elem() to get the same.

[cueckoo]

Original reply by @mpvl in cuelang/cue#600 (comment)

We are considering allowing ? to do a lookup op optional fields. So for the following struct

a: {
    opt?: string
}

one could write a.opt? to get the value string. Similarly, for a struct

a: [string]: <100
a: [>"e"]:  >10

for a.foo? one would get >10 & <100, that is the value that would be given when adding foo as a field.

Similarly then, and assuming we will support using a selector for numeric values, one could do cmd.0? to query what the first element would be if it were defined. So the get the same for the "next" undefined entry, one could query cmd[len(cmd)]?.

Anyway, that is just a thought. But this seems to come up often enough to warrant it. People can weigh in here if they think it is useful.