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?

Answer 1

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.

Playground

Answer 2

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