On Monday, 18 August 2025 at 17:28:44 UTC+1 andreas graeper wrote: 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 ? interfaces are like boxes, containing a <concrete-type, ref-to-value> pair. A nil interface is effectively <nil, nil> - no type and therefore no value. A nil interface is not the same as a nil pointer. Hence, an interface containing any concrete value of type *foo (even if that value is nil) is not a nil interface value. https://go.dev/play/p/-RMIHaJmbno And if your function returned a *XError type, even if it returned a nil pointer, but the caller assigned it to a variable which holds an "error" type (which is an interface), this would show as a not-nil interface value. https://go.dev/play/p/kL8jYHuLBnA The normal rule in Go is "return concrete types, accept interfaces". This gives maximum flexibility and type safety. However, errors are a special case: conventionally you return a value of interface type "error". In that case, a nil (interface) value means "no error" and any other value means "some error". Since it's an interface, you know that any non-nil value implements the Error() method. Therefore, you did the right thing here: func f() (int,error) { return 0,XError{110} // bad-case return 110,nil // good-case } although a separate convention is that it's recommended that error values are pointers - this was discussed recently on this group. -- 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/0d62279e-a38e-4ebd-830f-028f14e2923dn%40googlegroups.com.
