Go — handling multiple errors elegantly?

Question

Is there a way to clean up this (IMO) horrific-looking code?

    aJson, err1 := json.Marshal(a)
bJson, err2 := json.Marshal(b)
cJson, err3 := json.Marshal(c)
dJson, err4 := json.Marshal(d)
eJson, err5 := json.Marshal(e)
fJson, err6 := json.Marshal(f)
gJson, err4 := json.Marshal(g)
if err1 != nil {
    return err1
} else if err2 != nil {
    return err2
} else if err3 != nil {
    return err3
} else if err4 != nil {
    return err4
} else if err5 != nil {
    return err5
} else if err5 != nil {
    return err5
} else if err6 != nil {
    return err6
} 

Specifically, I'm talking about the error handling. It would be nice to be able to handle all the errors in one go.

Answer 1

var err error
f := func(dest *D, src S) bool {
    *dest, err = json.Marshal(src)
    return err == nil
} // EDIT: removed ()

f(&aJson, a) &&
    f(&bJson, b) &&
    f(&cJson, c) &&
    f(&dJson, d) &&
    f(&eJson, e) &&
    f(&fJson, f) &&
    f(&gJson, g)
return err

Answer 2

Put the result in a slice instead of variables, put the intial values in another slice to iterate and return during the iteration if there's an error.

var result [][]byte
for _, item := range []interface{}{a, b, c, d, e, f, g} {
    res, err := json.Marshal(item)
    if err != nil {
        return err
    }
    result = append(result, res)
}

---

You could even reuse an array instead of having two slices.

var values, err = [...]interface{}{a, b, c, d, e, f, g}, error(nil)
for i, item := range values {
    if values[i], err = json.Marshal(item); err != nil {
        return err
    }
}

Of course, this'll require a type assertion to use the results.

Answer 3

define a function.

func marshalMany(vals ...interface{}) ([][]byte, error) {
    out := make([][]byte, 0, len(vals))
    for i := range vals {
        b, err := json.Marshal(vals[i])
        if err != nil {
            return nil, err
        }
        out = append(out, b)
    }
    return out, nil
}

you didn't say anything about how you'd like your error handling to work. Fail one, fail all? First to fail? Collect successes or toss them?

Answer 4

I believe the other answers here are correct for your specific problem, but more generally, panic can be used to shorten error handling while still being a well-behaving library. (i.e., not panicing across package boundaries.)

Consider:

func mustMarshal(v interface{}) []byte {
    bs, err := json.Marshal(v)
    if err != nil {
        panic(err)
    }
    return bs
}

func encodeAll() (err error) {
    defer func() {
        if r := recover(); r != nil {
            var ok bool
            if err, ok = r.(error); ok {
                return
            }
            panic(r)
        }
    }()

    ea := mustMarshal(a)    
    eb := mustMarshal(b)
    ec := mustMarshal(c)

    return nil
}

This code uses mustMarshal to panic whenever there is a problem marshaling a value. But the encodeAll function will recover from the panic and return it as a normal error value. The client in this case is never exposed to the panic.

But this comes with a warning: using this approach everywhere is not idiomatic. It can also be worse since it doesn't lend itself well to handling each individual error specially, but more or less treating each error the same. But it has its uses when there are tons of errors to handle. As an example, I use this kind of approach in a web application, where a top-level handler can catch different kinds of errors and display them appropriately to the user (or a log file) depending on the kind of error.

It makes for terser code when there is a lot of error handling, but at the loss of idiomatic Go and handling each error specially. Another down-side is that it could prevent something that should panic from actually panicing. (But this can be trivially solved by using your own error type.)

Answer 5

You can create a reusable method, then catch error only one if condition. This implementation will only show last error though.

func hasError(errs ...error) error {
    for i, _ := range errs {
        if errs[i] != nil {
            return errs[i]
        }
    }
    return nil
}

aJson, err := json.Marshal(a)
bJson, err2 := json.Marshal(b)
cJson, err3 := json.Marshal(c)
dJson, err4 := json.Marshal(d)
eJson, err5 := json.Marshal(e)
fJson, err6 := json.Marshal(f)
gJson, err7 := json.Marshal(g)

if error := util.hasError(err, err1, err2, err3, err4, err5, err6, err7); error != nil {
    return error
}

Answer 6

You can use go-multierror by Hashicorp.

var result error

if err := step1(); err != nil {
    result = multierror.Append(result, err)
}
if err := step2(); err != nil {
    result = multierror.Append(result, err)
}

return result