Covered in the FAQ, but really I keep forgetting all the time. https://golang.org/doc/faq#nil_error https://golang.org/doc/faq#nil_error Under the covers, inte

Covered in the FAQ, but really I keep forgetting all the time. https://golang.org/doc/faq#nil_error https://golang.org/doc/faq#nil_error Under the covers, interfaces are implemented as two elements, a type and a value. The value, called the interface's dynamic value, is an arbitrary concrete value and the type is that of the value. For the int value 3, an interface value contains, schematically, (int, 3). An interface value is nil only if the inner value and type are both unset, (nil, nil). In particular, a nil interface will always hold a nil type. If we store a nil pointer of type * int inside an interface value, the inner type will be * int regardless of the value of the pointer: (* int, nil). Such an interface value will therefore be non-nil even when the pointer inside is nil.