Introduce an error for invalid enum values to distinguish 400 and 422 responses during JSON parsing

[aklinkert]

Hello there! 👋🏻

When passing an invalid enum value to a JSON field using the enum type an untyped / unwrapped error is thrown:

enumer/enumer.go

Line 17 in 750eb57

return 0, fmt.Errorf("%%s does not belong to %[1]s values", s)

This makes distinguishing harder between an actual 400 response (body is not parsable) and a 422 (the data you passed are wrong) response.

Would you be up to introducing a typed error, that can be identified using errors.Is() and eventually accepting a PR for that?

As of right now, as a workaround, I am parsing the error string while setting the response status code as follows:

status := 400
enumerErrorStringMatcher := regexp.MustCompile(`^"\w+ does not belong to \w+ values"$`)

if enumerErrorStringMatcher.MatchString(err.Error()) {
	status = http.StatusUnprocessableEntity
}

Cheers,
Alex

[bincyber]

Hi @aklinkert, I was also in need of this functionality, so I've gone ahead and opened a PR 😄

[asmsh]

Hi @aklinkert and @bincyber, I was also in need of a way to recognize errors produced from parsing enum values. However, as I wanna use that functionality in request validation, I can't depend on a specific error value, since my request might contain multiple enum types, and it's not practical to keep updating the logic every time I add a new enum to the request.

So, IMO, if we are gonna add support for exposing the parsing error, let's add more functionality to it, so that it's easier to investigate and constructing useful error messages, or even building the business logic to gracefully handle parsing errors to enum types.

What I have in mind is an API that looks like this (full code):
To investigate err from: day, err := DayString("Noday")

if _, ok := err.(interface{ InvalidEnumValueError() }); ok {
	fmt.Println("Error is InvalidEnumValueError")
}

// investigate specific error.
var invErr InvalidDayError
if errors.As(err, &invErr) {
	fmt.Println("Error is InvalidDayError")
	fmt.Println("InvalidDayError.Error:", invErr.Error()) // "Noday does not belong to Day values"
	fmt.Println("InvalidDayError.InvalidValue:", invErr.InvalidValue()) // "Noday"
	fmt.Println("InvalidDayError.EnumType:", invErr.EnumType()) // "Day"
	fmt.Println("InvalidDayError.ValueSource:", invErr.ValueSource()) // "" (one of "sql", "json", "text", ..., or "empty")
}

// or, investigate any enum error.
type EnumErr interface {
	error

	// InvalidEnumValueError is the identity method.
	InvalidEnumValueError()

	InvalidValue() string
	EnumType() string
	ValueSource() string
}
var invEnumErr EnumErr
if errors.As(err, &invEnumErr) {
	fmt.Println("Error is EnumErr")
	fmt.Println("EnumErr.Error:", invEnumErr.Error()) // "Noday does not belong to Day values"
	fmt.Println("EnumErr.InvalidValue:", invEnumErr.InvalidValue()) // "Noday"
	fmt.Println("EnumErr.EnumType:", invEnumErr.EnumType()) // "Day"
	fmt.Println("EnumErr.ValueSource:", invEnumErr.ValueSource()) // "" (one of "sql", "json", "text", ..., or "empty")
}

cc: @samiam2013

[samiam2013]

@asmsh feel free to open a PR of your own.

[samiam2013]

handled in #112