Set — unordered unique collection

A built-in, no-import type. See the Language reference and examples/sets.qn.

A Set is written [|T|]. Set is a built-in parametric collection — like []T, not a user-defined generic — written with the same pipe fence [| … |]; the fence is what keeps a set literal distinct from an array ([1, 2, 3] is an array, [|1, 2, 3|] is a set).

primes :: [|Num|] = [|2, 3, 5, 7|]                        ~ a Set
none   :: [|Num|] = [||]                                  ~ empty set

Elements may be Num, Text (hashed by content, consistent with ==), Bool, or a user type that opts in — a record or sum defining both a % hash hook (% = () -> Num => …, it the value) and an == member; both are required, and %/== must agree (see Map). Duplicates collapse. A set is immutable / persistent: every mutator (add, the set operators) returns a new set and never touches the receiver.

Iteration order is UNSPECIFIED — conceptually a set is unordered, so never rely on the order of items/each. (It is not insertion order. It may look stable run-to-run; that is not a contract.)

A set carries a built-in .size field (element count, like an array’s .size); everything else is a reserved method (resolved ahead of any same-named user overload when the receiver is a Set):

Set method	Result	Notes
has(x)	Bool	membership
add(x)	new [|T|]	a fresh set with x added (persistent)
remove(x)	new [|T|]	a fresh set without x (persistent); removing an absent element is a no-op
items()	[]T	the elements as an array (order unspecified)
each(x => …)	the receiver set	runs the body per element for effect, then returns the set (chains)

Set algebra (each builds a new set of the same element type):

[|1, 2, 3|] + [|3, 4, 5|]    ~ union        → {1, 2, 3, 4, 5}
[|1, 2, 3|] - [|3, 4, 5|]    ~ difference   → {1, 2}
[|1, 2, 3|] +- [|3, 4, 5|]   ~ intersection → {3}   (`+-` and `-+` are the same operator)

Like the empty array [] (which is []Num), an empty set literal defaults to Num element type and cannot yet be annotated to another type.

(See examples/sets.qn.)