Best practice for unions in Go

Question

Go has no unions. But unions are necessary in many places. XML makes excessive use of unions or choice types. I tried to find out, which is the preferred way to work around

Answer 1

TL;DR You don't need a union, interface{} solves this better.

Unions in C are used to access special memory/hardware. They also subvert the type system. Go does not have the language primitives access special memory/hardware, it also shunned volatile and bit-fields for the same reason.

In C/C++ unions can also be used for really low level optimization / bit packing. The trade off: sacrifice the type system and increase complexity in favor of saving some bits. This of course comes with all the warnings about optimizations.

Imagine Go had a native union type. How would the code be better? Rewrite the code with this:

// pretend this struct was a union
type MiscUnion struct {
  c *Comment
  pi *ProcessingInstruction
  ws *WhiteSpace
}

Even with a builtin union accessing the members of MiscUnion requires a runtime check of some kind. So using an interface is no worse off. Arguably the interface is superior as the runtime type checking is builtin (impossible to get wrong) and has really nice syntax for dealing with it.

One advantage of a union type is static type check to make sure only proper concrete types where put in a Misc. The Go way of solving this is "New..." functions, e.g. MiscComment, MiscProcessingInstruction, MiscWhiteSpace.

Here is a cleaned up example using interface{} and New* functions: http://play.golang.org/p/d5bC8mZAB_