Go 2 Error Values

My main critique with Error Values is the Formatter interface.

I fear that nearly every format function will be written in the exact same fashion:

func (e *MyErr) Format(p errors.Printer) (next error) {
    p.Print(/* short error string */)
    if p.Detail() {
        p.Print(/* details */)
        p.Print(/* details */)
        p.Print(/* details */)
    }
    return /* caused by */
}

But we already have something to replace short error string with, and something to replace caused by with. The only thing that isn't provided is the details.

Instead of a Formatter, we should have a Detailer, and let the library printing the error handle the rest.

Detailer could look something like...

type Detailer interface {
    Detail() []string
}

Any advantage provided by using p.Printf can be equally achieved using fmt.Sprintf.

A Formatter was wanted to allow for localization. This is still allowed under using Detailer instead, as localization directives may be placed in in the string slice if you chose to do so, just as you got to choose if you wanted to put localization directives in your p.Print statements.

Then, the library printing the error (fmt, golang.org/x/text/message, etc) may format as needed. For the same effect as in the overview.

This is of course simplified quite a lot, I'm writing this at 3:30am and don't want to be thinking too hard!

func printErr(err error) {
    fmt.Println("--- %s", err.Error())

    if det, ok := err.(errors.Detailer); ok {
        for _, line := range det.Detail() {
            fmt.Println("    " + line)
        }
    }

    if wrap, ok := err.(errors.Wrapper); ok {
        causedBy := wrap.Unwrap()
        if causedBy != nil {
            printErr(causedBy)
        }
    }
}

Thank you for reading, I hope you consider this.

deanveloper/ErrorPrinting.md

Go 2 Error Values

mpvl commented Sep 25, 2018