Wrap error from go/format as it is scanner.ErrorList

[veggiemonk]

Hi,

Using jen a lot lately (great project, btw). It was difficult to debug as the error is just a string with the resulting code which can be quite hard to figure out without doing properly formatting the resulting error.

Since go/format returns a scanner.ErrorList error type, it would be nice to be able to errors.As to interact with the errors. This allow the code to be more like:

err := f.Render(customJenCode)
if err != nil {
    var se scanner.ErrorList
    if errors.As(err, &se) {
	for _, e := range se {
		fmt.Printf("position: %+v", e.Pos)
		fmt.Println("message:", e.Msg)
	}
    }
    t.Fatal(err)
}

Currently, I'm using my fork and this is the best I could come up with:

func jenDebug(err error) {
	es := err.Error()
	var se scanner.ErrorList
	if errors.As(err, &se) {
		for _, e := range se {
			// fmt.Println("line:", e.Pos.Line)
			// fmt.Println("column:", e.Pos.Column)
			// fmt.Println("message:", e.Msg)
			_, a, found := strings.Cut(es, e.Msg)
			if found {
				ss := strings.Split(a, "\n")
				for i, l := range ss {
					if i == e.Pos.Line {
						fmt.Printf("%d: %s  <---- ERROR: %s\n", i, l, e.Msg)
					} else {
						fmt.Printf("%d: %s\n", i, l)
					}
				}
				// fmt.Println("after:", a)
			}
		}
	}
}

This would help the debugging experience, not perfect by any means.
Let me know what you think.

Not an expert on the matter so I would appreciate to know how others debug generated code 😄