Yes, and when used in an immutable way, the GoLang approach is very powerful and efficient, because it allows one to pass large (parts of) immutable data-structures without copying.

How do I declare an immutable data structure in Go?

An unexported value type provided through an interface is immutable. The "context" object uses this. But it isn't really a solution for immutable value programming.

Its immutable everywhere, or just outside the package that declares it?

Outside the package that declares it, though if you have it as an interface value you'd have to do work to penetrate it.

Since it's not like I'm trying to sell this as a solution to all immutable data requirements, I didn't worry too much about that detail. In the end it's all about effort, anyhow; with the right invocations in Haskell I can mutate "immutable" data just fine. So even an interface in the package that declared it is still nontrivial protection against accidentally mutating it.

I don't think thats really true about Haskell, unless you are talking about C code you've invoked through FFI in which case all bets are off in every language.

Isn't it a lot simpler to just say that Go has no feature support for immutable data? The fact that you can hide accessors in a package is small comfort if I have to read all the source code in that package to know the actual semantics.

What about lists? Eg, an immutable list of packets.

AFAIAA, the only immutable core type in go is string.