[im-a-baby]

Generally, returning a typed error in the function signature in Go is very annoying and dangerous. An example why:

    import "fmt"
    
    type MyError struct{}
    
    func (m *MyError) Error() string {
     return ""
    }

    func myFunc() *MyError {
     return nil
    }
    
    func main() {
     var err error
     err = myFunc()
     if err == nil {
      fmt.Println("No Error")
     } else {
      fmt.Println("Error") // This prints
     }
    }

[gregwebs]

I agree this is awful, but isn't it solved here by not using pointers? I don't remember coming across a case where a custom error type benefited from being a pointer. I am sure such a case exists, but then such a case should be used when giving example code.

[im-a-baby]

No not using pointers would not solve the issue. The issue is that the specific error implementation type is part of the method signature. If you returned type MyError instead of *MyError, then it would be impossible to return nil from the method because only pointers can be nil.

[gregwebs]

I get that. But why define the Error method on a pointed receiver? In my experience almost all errors can be immutable particularly once you start using wrapping.

[gregwebs]

Never mind- that doesn't actually make a difference. The solution in this case is to not re-use an existing error variable- but re-use of an error variable is common practice.

[im-a-baby]

Yea, the issue is that nil's in Go are typed. So `err == nil` is checking err against error's nil value, but the value stored in err is *MyError's nil value.

    package main
    
    import "fmt"
    
    type MyError struct{}
    
    func (m *MyError) Error() string {
        return ""
    }
    
    func main() {
        var err error = nil
        var myErr *MyError = nil

        fmt.Println(err == myErr) // this is false
    }

[gregwebs]

Comparing different types normally is a type error. But for interfaces the non-interface value will be converted to the type of the interface value. This conversion gives it a boxed dynamic type. This is different type than just assigning nil, which is the untyped nil, which sets the interface to its zero value.

Normally one would not compare two interface values, so this issue really comes up when comparing an interface to nil. If Go had proper option types then this issue would go away.

https://go101.org/article/nil.html

package main

import "fmt"

type Err1 struct{}

func (m Err1) Error() string { return "" }

type Err2 struct{}

func (m Err2) Error() string { return "" }

func main() {

  var err error = nil

  var myErr1 *Err1 = nil

  var myErr2 *Err2 = nil

  fmt.Println(err == myErr1)                      // this is false
  fmt.Println(err == myErr2)                      // this is false
  fmt.Println((error)(myErr1) == (error)(myErr2)) // this is false. requires the cast to do the comparison
 }

[svnpenn]

thats kind of misleading, as you could just do:

    func myFunc() error

[im-a-baby]

Yes you could do that, but then you’d lose type info on the error.