Iter function with no outputs in a path(...) throws errors

[wader]

Version: 9bf7af3

I noticed while debugging a path expression query and because in fq debug is implemented by some io functions that print and return empty iterator. So debug is defined something like this: def debug: (["DEBUG", .] | tojson | _write), .

Reproduction:

package main

import (
	"log"
	"os"

	"github.com/itchyny/gojq"
)

func main() {
	query, err := gojq.Parse(os.Args[1])
	if err != nil {
		panic(err)
	}

	code, err := gojq.Compile(query,
		gojq.WithIterFunction("a", 0, 0, func(i1 interface{}, i2 []interface{}) gojq.Iter {
			log.Println("in a")
			return gojq.NewIter()
		}),
	)
	if err != nil {
		panic(err)
	}

	iter := code.Run(nil)

	for {
		v, ok := iter.Next()
		if !ok {
			break
		}
		if err, ok := v.(error); ok {
			log.Printf("err: %#+v\n", err)
			break
		}
		log.Printf("v: %#+v\n", v)
	}
}

# this seems to come from the Iter case in execute.go
$ go run test.go 'try path(a) catch .'
2022/09/29 12:47:56 in a
2022/09/29 12:47:56 v: "invalid path on iterating against: gojq.Iter"

# gojq.emptyIter{} is probably because of the special 0 values case in gojq.NewIter()
$ go run test.go 'path(a)'
2022/09/29 12:47:59 in a
2022/09/29 12:47:59 err: &gojq.invalidPathIterError{v:gojq.emptyIter{}}

I would have expect it to work like empty. That is only do invalid path check if the Iter outputted some value.

$ go run test.go 'path(empty)'
<no output at all>

[itchyny]

Maybe I rejected against Iters just in case without deep considerations. Do we need to make gojq.NewIter(errors.New("hello")) works as error("hello") so that path(error(f)) should emit a hello error? Or what happens for gojq.WithIterFunction("f", 0, 0, func(v interface{}, _ []interface{}) gojq.Iter { return gojq.NewIter(v) }) which would be expected to work like def f: .;. Checking against emitted non-error values would be enough.

[wader]

Yes being able to output an error without causing invalid path error would be good i think. Being able to make it work like def f: .; i think would require to output a "suffix"-path for each value somehow i guess? feels like that could get complicated.

So would a sane behaviour for a WithIterFunction inside a path(...) be that it has to either emit no values at all or output an error that gets thrown and iteration stops? all other cases cause invalid path?

[itchyny]

Fixed the issue.

[wader]

Thanks!