Just started using Golang. I think that it is idiomatic to declare an error variable and use it in your error structure to determine what went wrong, as is done in strconv.go. There, ErrRange and ErrSyntax is declared, and when appropriate, references to those are stored in NumError structs when they return. I think that the reason is because then the address of the reference to the error stored in NumError can be compared with the ErrRange and ErrSyntax variables to determine which type of error was returned.
Are there "standard" such declared error types? In Java, for example, you have things like java.lang.IllegalArgumentException. Is there, for instance, ErrArgument or ErrUnsupportedOperation that I can use in my own code instead of creating new error variables that mean the same thing every time?
