4 ms·

Can you elaborate on how and why err==nil changes in your second case? A link to the docs would be helpful. Thanks!

• TheDong 6y ago
  It's a combination of two things. First, type inference from "short variable declaration" (see the spec here: https://golang.org/ref/spec#Short_variable_declarations https://golang.org/ref/spec#Short_variable_declarations ) and secondly, nil interfaces vs typed nils stored in an interface type (see this FAQ entry https://golang.org/doc/faq#nil_error https://golang.org/doc/faq#nil_error ). Another way to explain the difference is the following: // case 1 is the same as var i int // inferred type of *MyErr because that's what returnsNilError returns var err *MyErr i, err = 0, (*MyErr)(nil) // err is of type (nil)(*MyErr) // case 2 is the same as // inferred type of error because os.Open returns that var err error err = os.Open("/tmp/file") var i int i err = 0, (error)((*MyErr)(nil)) // err is of type (error)((*MyErr)(nil)) Note well that "err" is a different type in those two cases. Type inference, plus the fact that short variable decleration re-uses an existing variable if the type is compatible, can result in previous code changing what type a variable is. It's well known that an interface with a type is != nil (i.e. "var nilErr *MyErr = nil; var errIface error = nilErr" results in an 'errIface' that is not equal to an untyped nil). I hope that explanation made sense to you.
  • jdsalaro 6y ago
    Thank you!