Added set.Find (#18)

* Added set.Exists and set.Forall methods

* Replaced `Exists`/`Forall` with new `Find` method in `set`

set/set.go

@@ -67,6 +67,29 @@ func (s Of[T]) Equal(other Of[T]) bool {
 	return true
 }
 
+// Find returns the first value for which a function applied to the value
+// returns true. If the function does not return true for any value
+// in the set, Find returns false and the zero value of T.
+//
+// if several set elements would match, the first match will be chosen
+// arbitrarily because the iteration order is indeterminate.
+//
+// Find can also be used for two special cases:
+//   - To test whether any value exists that matches the predicate,
+//     a true boolean result is all that matters.
+//   - To test whether any value exists that does not match the predicate,
+//     in this case the inverse function should be supplied and
+//     a false result is all that matters.
+func (s Of[T]) Find(f func(T) bool) (T, bool) {
+	for val := range s {
+		if f(val) {
+			return val, true
+		}
+	}
+	var zero T
+	return zero, false
+}
+
 // Each calls a function on each element of the set in an indeterminate order.
 // It is safe to add and remove items during a call to Each,
 // but that can affect the sequence of values seen later during the same Each call.

set/set_test.go

@@ -24,6 +24,19 @@ func TestSet(t *testing.T) {
 		t.Errorf("got %v, want [1 2 3 4 5 6]", got)
 	}
 
+	v1, exists1 := s.Find(func(val int) bool { return val > 3 })
+	if !exists1 {
+		t.Errorf("Find(val > 3) failed")
+	}
+	if v1 <= 3 {
+		t.Errorf("Find(val > 3) failed with %d", v1)
+	}
+
+	_, exists2 := s.Find(func(val int) bool { return val > 10 })
+	if exists2 {
+		t.Errorf("Find(val > 10) failed")
+	}
+
 	s2 := New[int](5, 6, 7, 8)
 	i := Intersect(s, s2)
 	if !reflect.DeepEqual(i, Of[int](map[int]struct{}{5: {}, 6: {}})) {