类型化地图不支持索引

Question

I want to extend go-validator to return a better type:

type Error map[string][]error

// Will output the first error when stringified (e.g. for json response).
func (err Error) Error() string {
    for k, errs := range err {
        return fmt.Sprintf("%s value had %s", k, errs[0].Error())
    }

    return "no error"
}

func Validate(v interface{}) error {
    if ok, errs := DefaultValidator.Validate(v); !ok {
        return Error(errs)
    }

    return nil
}

As I am now writing tests for this I ran into the problem that it the typed map Error seems to have lost it indexing capabilities:

err = Validate(Value{
    Foo:      "bar",
    Email:    "[e-mail]",
    Required: "1234",
})
c.Check(err, check.IsNil)

err, ok := Validate(Value{}).(Error)
c.Check(ok, check.Equals, true)
c.Check(err["Foo"], check.DeepEquals, []error{validator.ErrMin})
c.Check(err["Email"], check.DeepEquals, []error{validator.ErrInvalid})
c.Check(err["Required"], check.DeepEquals, []error{validator.ErrZeroValue})

Returns:

model/validation/validation_test.go:42: invalid operation: err["Foo"] (type error does not support indexing)
model/validation/validation_test.go:43: invalid operation: err["Email"] (type error does not support indexing)
model/validation/validation_test.go:44: invalid operation: err["Required"] (type error does not support indexing)

I also tried to cast to type map[string][]error but got "impossible type assertion".

What is wrong with my approach? How can I get indexing back to work?

Answer 1

It seems like your err variable is initialised with error type. When you do

err, ok := Validate(Value{}).(Error)

you merely check if err is actually an Error. If you change err there to say errs, it should work.

Playground example: http://play.golang.org/p/ljNroPzVbd