docs: improve multiset_delta() docs

hydroflow/tests/surface_multiset_delta.rs

@@ -4,8 +4,8 @@ use multiplatform_test::multiplatform_test;
 
 #[multiplatform_test]
 pub fn test_multiset_delta() {
-    let (input_send, input_recv) = hydroflow::util::unbounded_channel::<u32>();
-    let (result_send, mut result_recv) = hydroflow::util::unbounded_channel::<u32>();
+    let (input_send, input_recv) = hydroflow::util::unbounded_channel::<char>();
+    let (result_send, mut result_recv) = hydroflow::util::unbounded_channel::<char>();
 
     let mut flow = hydroflow_syntax! {
         source_stream(input_recv)
@@ -14,19 +14,31 @@ pub fn test_multiset_delta() {
     };
     assert_graphvis_snapshots!(flow);
 
-    input_send.send(3).unwrap();
-    input_send.send(4).unwrap();
-    input_send.send(3).unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('b').unwrap();
+    input_send.send('a').unwrap();
     flow.run_tick();
-    assert_eq!(&[3, 4, 3], &*collect_ready::<Vec<_>, _>(&mut result_recv));
+    // 'a', 'b', 'a'
+    assert_eq!(&['a', 'b', 'a'], &*collect_ready::<Vec<_>, _>(&mut result_recv));
 
-    input_send.send(3).unwrap();
-    input_send.send(5).unwrap();
-    input_send.send(3).unwrap();
-    input_send.send(3).unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('c').unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('a').unwrap();
+    flow.run_tick();
+    // 'c', 'a'
+    // First two 'a's are removed due to previous tick.
+    assert_eq!(&['c', 'a'], &*collect_ready::<Vec<_>, _>(&mut result_recv));
+
+    input_send.send('b').unwrap();
+    input_send.send('c').unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('a').unwrap();
+    input_send.send('a').unwrap();
     flow.run_tick();
-    // First two "3"s are removed due to previous tick.
-    assert_eq!(&[5, 3], &*collect_ready::<Vec<_>, _>(&mut result_recv));
+    // 3 'a's and the 'c' are removed due to previous tick.
+    assert_eq!(&['b', 'a'], &*collect_ready::<Vec<_>, _>(&mut result_recv));
 }
 
 #[multiplatform_test]

hydroflow_lang/src/graph/ops/multiset_delta.rs

@@ -5,30 +5,54 @@ use super::{
     WriteContextArgs, RANGE_0, RANGE_1,
 };
 
-// TODO(mingwei): more doc
-/// Multiset delta from the previous tick.
+/// The multiset inverse of [`persist()`](#persist).
+///
+/// > 1 input stream of `T`, 1 output stream of `T`, where `T: Eq + Hash`
+///
+/// For set semantics, [`unique()`](#unique) can be thought of as a "delta" operator, the inverse
+/// of [`persist()`](#persist). In `persist`, new items come in, and all items are repeatedly
+/// released out. Conversely, `unique` take repeated items in, and only releases the new ones out.
+///
+/// This operator does a similar inversion but for multiset semantics, with some caveats. When it
+/// receives duplicate items, instead of ignoring them, it "subtracts" them from the items received
+/// in the previous tick: i.e. if we received `k` copies of an item in the previous tick, and we
+/// receive `l > k` copies in the current tick, we output `l - k` copies of the item.
+/// However unlike `unique`, this count is only maintained for the previous tick, not over all time.
+///
+/// In the example below, in the second tick two 'a's are removed because two 'a's were received in
+/// the previous tick. The third 'a' is released though.
 ///
 /// ```rustbook
-/// let (input_send, input_recv) = hydroflow::util::unbounded_channel::<u32>();
+/// let (input_send, input_recv) = hydroflow::util::unbounded_channel::<char>();
 /// let mut flow = hydroflow::hydroflow_syntax! {
 ///     source_stream(input_recv)
 ///         -> multiset_delta()
 ///         -> for_each(|n| println!("{}", n));
 /// };
 ///
-/// input_send.send(3).unwrap();
-/// input_send.send(4).unwrap();
-/// input_send.send(3).unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('b').unwrap();
+/// input_send.send('a').unwrap();
+/// flow.run_tick();
+/// // 'a', 'b', 'a'
+///
+/// input_send.send('a').unwrap();
+/// input_send.send('c').unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('a').unwrap();
 /// flow.run_tick();
-/// // 3, 4,
+/// // 'c', 'a'
+/// // First two 'a's are removed due to previous tick.
 ///
-/// input_send.send(3).unwrap();
-/// input_send.send(5).unwrap();
-/// input_send.send(3).unwrap();
-/// input_send.send(3).unwrap();
+/// input_send.send('b').unwrap();
+/// input_send.send('c').unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('a').unwrap();
+/// input_send.send('a').unwrap();
 /// flow.run_tick();
-/// // 5, 3
-/// // First two "3"s are removed due to previous tick.
+/// // 'b', 'a'
+/// // 3 'a's and the 'c' are removed due to previous tick.
 /// ```
 pub const MULTISET_DELTA: OperatorConstraints = OperatorConstraints {
     name: "multiset_delta",

hydroflow_lang/src/graph/ops/persist.rs

@@ -7,7 +7,9 @@ use super::{
 use crate::diagnostic::{Diagnostic, Level};
 use crate::graph::{FlowProps, LatticeFlowType};
 
-/// Stores each item as it passes through, and replays all item every tick.
+/// Stores each item as it passes through, and replays all items every tick.
+///
+/// > 1 input stream, 1 output stream
 ///
 /// ```hydroflow
 /// // Normally `source_iter(...)` only emits once, but `persist()` will replay the `"hello"`