How Can I Reliably Check for Nil Interfaces and Nil Values in Go?

Checking for Nil and Nil Interfaces in Go

When working with interfaces in Go, it's often necessary to check whether an interface is nil or contains a nil value. However, the standard approach using reflection can be verbose and error-prone.

For example, consider the following helper function:

func isNil(a interface{}) bool {
  defer func() { recover() }()
  return a == nil || reflect.ValueOf(a).IsNil()
}

This function uses reflection to check if the value is nil, but it requires a deferred recover() to handle panics when the value's type is not supported.

A More Direct Approach

As the answer to the original question suggests, a simpler and more reliable approach is to use direct comparison for nil interfaces and always allow assignment of nil to untyped interfaces. Here's how:

func isNil(a interface{}) bool {
  return a == nil
}

This function will correctly check for both nil interfaces and nil values within interfaces.

When Assignment of (*T)(nil) Is Allowed

The approach outlined above assumes that you will never assign (*T)(nil) to an interface. If this is not the case, you may need to use reflection to check for nil values:

func isNil(a interface{}) bool {
  if a == nil {
    return true
  }
  v := reflect.ValueOf(a)
  if v.Kind() == reflect.Ptr && v.IsNil() {
    return true
  }
  return false
}

However, it's generally recommended to avoid assigning (*T)(nil) to interfaces, as it can lead to subtle bugs. By enforcing the rule of never storing (*T)(nil) in interfaces, you can simplify your code and increase its reliability.

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