in midst of refactor

orchid-base/src/join.rs

@@ -0,0 +1,34 @@
+//! Join hashmaps with a callback for merging or failing on conflicting keys.
+
+use std::hash::Hash;
+
+use hashbrown::HashMap;
+use never::Never;
+
+/// Combine two hashmaps via an infallible value merger. See also
+/// [try_join_maps]
+pub fn join_maps<K: Eq + Hash, V>(
+  left: HashMap<K, V>,
+  right: HashMap<K, V>,
+  mut merge: impl FnMut(&K, V, V) -> V,
+) -> HashMap<K, V> {
+  try_join_maps(left, right, |k, l, r| Ok(merge(k, l, r))).unwrap_or_else(|e: Never| match e {})
+}
+
+/// Combine two hashmaps via a fallible value merger. See also [join_maps]
+pub fn try_join_maps<K: Eq + Hash, V, E>(
+  left: HashMap<K, V>,
+  mut right: HashMap<K, V>,
+  mut merge: impl FnMut(&K, V, V) -> Result<V, E>,
+) -> Result<HashMap<K, V>, E> {
+  let mut mixed = HashMap::with_capacity(left.len() + right.len());
+  for (key, lval) in left {
+    let val = match right.remove(&key) {
+      None => lval,
+      Some(rval) => merge(&key, lval, rval)?,
+    };
+    mixed.insert(key, val);
+  }
+  mixed.extend(right);
+  Ok(mixed)
+}