Golang type assertion

Question

I have created a type Role based off string, and I am now trying to get it to work with the database driver by implementing the Valuer and Scanner interfaces

type Role string

func (r *Role) Scan(value interface{}) error {
    r = (*Role)(value.(string))

    return nil
}

func (r *Role) Value(value driver.Value, err error) {
    if err != nil {
        value = string(r)
    }
}

I keep getting the error:

The Go code app/entities/user.go does not compile: cannot convert value.(string) (type string) to type *Role

What am I doing wrong here?

Dave C · Accepted Answer · 2015-05-15 01:25:18Z

24

Here is working code for the first function:

func (r *Role) Scan(value interface{}) error {
    *r = Role(value.(string))
    return nil
}

Although you may wish to use s, ok := value.(string) and return an error for !ok instead of panic-ing.

The signature for the a driver.Valuer is not what you gave but:

func (r Role) Value() (driver.Value, error) {
    return string(r), nil
}

Note this doesn't handle or produce NULL values.

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edited May 15, 2015 at 1:25

Dave C

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answered Jan 5, 2014 at 15:43

Max

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7 Comments

Lee Over a year ago

Aha, it was the pointer dereference that was catching me out.

Lee Over a year ago

Solution for the second one is func (r *Role) Value(value driver.Value, err error) { if err != nil { value = string(*r) } }

Günter Zöchbauer Over a year ago

That's interesting. I never saw Go code where the method receiver was modified and wouln't have considered doing this. I also was sure converting from an interface to a concrete type needs a type assertion.

Lee Over a year ago

There is a type assertion, value.(string)

Günter Zöchbauer Over a year ago

I see, just ignoring the second return value.

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Günter Zöchbauer · Accepted Answer · 2014-01-05 15:26:55Z

4

I don't think it's a good idea to modify the receiver of your method (r) in the Scan method.

You need a type assertion to convert value interface{} to string.
You are trying to convert a string to a pointer to Role.

func (r *Role) Scan(value interface{}) (retVal Role, err error) {
    var s string;

    if v,ok := value.(string); ok {
      s = v;
    }
    var rx Role
    rx = Role(s)

    var rx2 *Role
    rx2 = &rx
    _ = rx // just to silence the compiler for this demonstration
    _ = rx2 // just to silence the compiler for this demonstration
    return rx, nil
}

should work

edited Jan 5, 2014 at 15:26

answered Jan 5, 2014 at 15:03

Günter Zöchbauer

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7 Comments

Lee Over a year ago

If we are not modifying the receiver or returning a value how does the driver no what to do?

Günter Zöchbauer Over a year ago

The receiver r is like the this in objects. You can return more than one value (value and error) in Go. I don't now what requirements you have for your Scan method.

Lee Over a year ago

Yes, but the job of scan is to convert the string sent back from the database to a role, this is being assigned to rx, but not to r and not being returned only the local variable gets the result. I am missing something.

Lee Over a year ago

The Scanner interface defines scan as Scan(src interface{}) error you are returning multiple results

Günter Zöchbauer Over a year ago

Then you can't return anything else than an error. Can you provide a link to the API docs of your Scanner interface so I can look up what it is about?

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