feat(lattices): make unit () a point lattice (#793)

lattices/README.md

@@ -35,7 +35,8 @@ Take a look at the [`lattice` rustdocs](https://hydro-project.github.io/hydroflo
 * [`Seq<Lat>`] - growing `Vec` of nested lattices, like `MapUnion<<usize, Lat>>` but without missing entries.
 * [`DomPair<LatKey, LatVal>`]* - a versioned pair where the `LatKey` dominates the `LatVal`.
 * [`Conflict<T>`]* - adds a "conflict" top to domain `T`. Merging inequal `T`s results in top.
-* [`Point<T>`]* - a single "point lattice" value which cannot be merged with any inequal value.
+* [`Point<T, *>`]* - a single "point lattice" value which cannot be merged with any inequal value.
+* [`()`](https://doc.rust-lang.org/std/primitive.unit.html) - the "unit" lattice, a "point lattice" with unit `()` as the only value in the domain.
 
 *Special implementations which do not obey all lattice properties but are still useful under
 certain circumstances.

lattices/src/lib.rs

@@ -16,6 +16,7 @@ mod point;
 mod seq;
 pub mod set_union;
 pub mod test;
+mod unit;
 mod with_bot;
 mod with_top;
 

lattices/src/unit.rs

@@ -0,0 +1,27 @@
+use crate::{IsBot, IsTop, LatticeFrom, LatticeOrd, Merge};
+
+impl Merge<()> for () {
+    fn merge(&mut self, _other: ()) -> bool {
+        false
+    }
+}
+
+impl LatticeOrd for () {}
+
+impl LatticeFrom<()> for () {
+    fn lattice_from(other: ()) -> Self {
+        other
+    }
+}
+
+impl IsBot for () {
+    fn is_bot(&self) -> bool {
+        true
+    }
+}
+
+impl IsTop for () {
+    fn is_top(&self) -> bool {
+        true
+    }
+}