2.4 Sets
When {…} is used with elements that do not have : to separate a key and value, then {…} creates a set. (If a set-element expression uses :, then it will need to be in parentheses to avoid being parsed as a key–value pair.) A set is indexable with […], where the set’s elements act as indices to values of #true, while using any other value as an index produces #false. There’s a Set constructor that’s analogous to Map, but Set accepts just values to include in the set. The ++ operator effectively unions sets.
> new_friends["david"]
#true
> friends["david"]
#false
Using Set explicitly before {…} disables the special treatment of : to indicate a map, and each element within {…} is simply an expression. The Set constructor can also be used like a function with (…) instead of {…}.
Set.of and MutableSet work as you’d expect. When […] with := is used to modify a mutable set, the index is removed from the set if the assigned value is #false, otherwise the index is added to the set.
Within a set construction using {…}, a & form splice a set into the constructed set, analogous to the way & works for list constructions.
> {"dennis", & friends}
{"alice", "bob", "carol", "dennis"}
Also similar to maps, a repetition can be used to construct a set.
> {elem, ...}
{"a", "b", "c"}
Set forms work as bindings, too, analogous to map binding forms.
> [other_friend, ...]
["bob", "alice"]