Chris's Wiki :: blog/programming/GoUnionTypesAndZeroValues
One of the classical big reason to want union types in Go is so that one can implement the general pattern of an option type, in order to force people to deal explicitly with null values. Except this is not quite true on both sides. The compiler can enforce null value checks before use already, and union and option types by themselves don't fully protect you against null values. Much like people ignore error returns (and the Go compiler allows this), people can skip over that they can't extract an underlying value from their Result value and return a zero value from their 'get a result' function.
My view is that the power of option types is what they do in the rest of the language, but they can only do this if you can express their guarantees in the type system. The important thing you need for this is non-nullable types. This is what lets you guarantee that something is a proper value extracted from an error-free Result or whatever. If you can't express this in your types, everyone has to check, one way or another, or you risk a null sneaking in.
Go doesn't currently have a type concept for 'something that can't be null', or for that matter a concept that is exactly 'null'. The closest Go equivalent is the general idea of zero values, of which nil pointers (and nil interfaces) are a special case (but you can also have zero value maps and channels, which also have special semantics; the zero value of slices is more normal). If you want to make Result and similar types particularly useful in Go, I believe that you need to change this, somehow introducing types that don't have a zero value.
