Custom error message

[1379]

The parameter of the RegisterValidation function is a validation function that returns a Boolean value. If we want to customize the error message, we have to register a translation function, which is too much trouble.
Our verification function may return an error for many reasons, so the error message should also be different, but only based on the returned boolean value, we cannot determine what caused the verification failure, so the error message of the translation function is only One kind.
Why can't you register a check function that returns error directly?
Just like the following.

type Func=func(fl FieldLevel) error
func (v *Validate) RegisterValidation(tag string, fn Func, callValidationEvenIfNull ...bool) error {

}

[1379]

Only based on the boolean value returned by the validation function, we cannot determine what caused the validation failure, so customizing the error message will be very troublesome

[deankarn]

@1379 sounds like a great enhancement for the next version of the lib, but unfortunately right now is a breaking change.

[1379]

@deankarn
I simply made some changes to the "validator", which is currently supported and used in my own project.
My implementation is like this:
Provides two RegisterValidation functions, the first parameter type is still "type Func=func(fl FieldLevel) boolean",
The parameter type of the second function becomes "type Func=func(fl FieldLevel) error".
When verifying, you only need to determine which type of verification function is.

[pwli0755]

Same need

[kke]

The custom validation error messages from sl.ReportError() are a bit silly too:

// sl.ReportError(field interface{}, fieldName string, structFieldName string, tag string, param string)
sl.ReportError(hosts, "hosts", "this does nothing", "some error message here", "this does nothing")

==>

 Key: 'ClusterConfig.Spec.hosts' Error:Field validation for 'hosts' failed on the 'some error message here' tag

If it was not from a tag, but a custom StructValidation , it should say something like:

 Key: 'ClusterConfig.Spec.hosts' Error:Field validation for 'hosts' failed: some error message here

[kevin2027]

here is a way to fix it #837

[pbikki]

same need. especially for sl.ReportError() on struct validation to include custom error messages to user

[joostvdhoff]

I think it's possible to implement this without a breaking change, though it'll be ugly with some bits of code purely dedicated to backwards compatibility. We'd have to put a new registration mechanism with new names next to the current one, and turn the current one into a proxy:

type FuncWithCustomError=func(fl FieldLevel) error
func (v *Validate) RegisterValidationWithCustomError(tag string, fn FuncWithCustomError, callValidationEvenIfNull ...bool) error {
    // Current implementation but for FuncWithCustomError instead of Func.
}

type Func=func(fl FieldLevel) bool
func (v *Validate) RegisterValidation(tag string, fn Func, callValidationEvenIfNull ...bool) error {
    fnWithCustomError := func(fl FieldLevel) error {
        if fn(fl) {
            return nil
        } else {
            return errors.New("")
        }
    }
    return RegisterValidationWithCustomError(tag, fnWithCustomError , callValidationEvenIfNull...)
}

Of course, it would still be a pretty big change to the deeper architecture of the library: all baked-in validators would need to be wrapped, all code which calls the validators needs to call them by the new signature, and I don't know the codebase well enough to tell if more things than that need to be changed.

Would the maintainer(s) of this project be on board with this solution? If yes, would you appreciate me familiarising myself with the codebase and cooking up a PR for this, or would you prefer to implement this yourself?