2. Impure?
3. That's the point. It's to force people to stop using maps as if they're sorted.
4. Why would you want to rely on the arbitrary nature of a hash table for order?
5. That's the best type of boilerplate. It's short. It brings clarity. You might not need custom sort functions. The sort package supports basic types, which you'd use if you're keeping track of an index with []int for instance. https://golang.org/pkg/sort/
Disclaimer: I've never used this feature myself and I sympathize with the desire on the part of the Go designers to avoid accidental de facto stabilization of hash table iteration order--though it sounds like it could have been done without drawbacks by randomizing at process start.
Can you provide one? (I'm genuinely curious.)
But from a philosophical perspective, referential transparency is a big one. We should prefer to make functions referentially transparent, because it makes them easier to reason about. Sometimes we do not, because it would be too slow or too awkward. But in this case, the loss is gratuitous.
Or to put it concretely: in Python (and every other language), we have the fact that `dict.keys() == dict.keys()`. In Go, this is true sometimes. Doesn't that seem weird to you?