What would be the equivalent in Go to a std::set? Note that only uniqueness is important, I don't care about ordering.
I've considered using dummy type, such as map[string]bool (where bool is the dummy), however often I find in Go I need to provide a type where one is not required, such as a channel used as a semaphore, and this case. Am I missing something idiomatic to Go?
Using a map with dummy values as a set is common practice in languages like Perl, which do not have sets. I think it is an acceptable way to get sets in Go, unless you want to implement it yourself or use some third-party implementation. Of course, your datatype has to be one that is allowed as the key in a map, i.e. not struct, array, or slice.
Using map[string]bool is perfectly fine.
There are also some more fancy libraries to handle sets, see for example: https://github.com/pwil3058/gosets
But I would still stick with a simple map, it is more idiomatic and simpler, which is always good.