The new go.dev website makes the language Spec hard to find, which is unfortunate.
The spec is here -- it is one of the easiest language specs to read. https://go.dev/ref/spec The section on the built-in (pre-declared) error interface is here https://go.dev/ref/spec#Errors Although struct values are allowed, they are very difficult to use correctly. I even mess them up sometimes. So until you get your feet with Go, I strongly recommend that you stick to only defining methods on pointers to structs, and passing around pointers to structs. Otherwise you will likely shoot yourself in the foot by trying to update a value that cannot be updated, and will wonder why your updates were lost. To create a struct that implements the error interface, you need to define all the methods in the error interface. Fortunately, there is only one. type error interface { Error() string } So your XError method could look look like this: func (x *XError) Error() string { return fmt.Sprintf("XError: i = %v", x.i) } Then have methods return &XError{i: 99} or whatever you need. func getAnError() error { return &XError{i: 99} } On Monday, August 18, 2025 at 5:28:44 PM UTC+1 andreas graeper wrote: type XError struct { i int } func f() (int,error) { return 0,XError{110} // bad-case return 110,nil // good-case } if r,e := f() ; e!=nil { println(e.i) // does not work var xe XError if errors.As(e,&xe) { println(xe.i) } } fun f() (int,XError) { return 0,XError{110} // seems to work return 123,nil // nil for XError is not possible } if r,e = f() ; e!=nil { println( e.i ) } // ok fun f() (int,*XError) { return 123,nil // this works } type-interface (error) can be nil type-struct (XError) cannot be nil, i need to replace with *XError ? is interface s.t like reference-type and struct a value-type ? thanks++, Andi -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/466c13eb-6881-4b7f-affe-84484793e297n%40googlegroups.com.
