diff --git a/src/circuit/command.rs b/src/circuit/command.rs
index 2a338c46..ac8b68c2 100644
--- a/src/circuit/command.rs
+++ b/src/circuit/command.rs
@@ -7,9 +7,9 @@ use std::iter::FusedIterator;
 
 use hugr::hugr::NodeType;
 use hugr::ops::{OpTag, OpTrait};
-use itertools::Itertools;
 use petgraph::visit as pv;
 
+use super::units::filter::FilteredUnits;
 use super::units::{filter, DefaultUnitLabeller, LinearUnit, UnitLabeller, Units};
 use super::Circuit;
 
@@ -59,8 +59,8 @@ impl<'circ, Circ: Circuit> Command<'circ, Circ> {
 
     /// Returns the linear units of this command in a given direction.
     #[inline]
-    pub fn linear_units(&self, direction: Direction) -> Units<&'_ Self> {
-        Units::new(self.circ, self.node, direction, self)
+    pub fn linear_units(&self, direction: Direction) -> FilteredUnits<filter::Linear, &Self> {
+        Units::new(self.circ, self.node, direction, self).filter_units::<filter::Linear>()
     }
 
     /// Returns the units and wires of this command in a given direction.
@@ -70,8 +70,7 @@ impl<'circ, Circ: Circuit> Command<'circ, Circ> {
         direction: Direction,
     ) -> impl IntoIterator<Item = (CircuitUnit, Wire)> + '_ {
         self.units(direction)
-            .filter_map(|(unit, port, _)| Some((unit, self.assign_wire(self.node, port)?)))
-            .collect_vec()
+            .filter_map(move |(unit, port, _)| Some((unit, self.assign_wire(self.node, port)?)))
     }
 
     /// Returns the output units of this command. See [`Command::units`].
@@ -82,27 +81,25 @@ impl<'circ, Circ: Circuit> Command<'circ, Circ> {
 
     /// Returns the linear output units of this command. See [`Command::linear_units`].
     #[inline]
-    pub fn linear_outputs(&self) -> Units<&'_ Self> {
+    pub fn linear_outputs(&self) -> FilteredUnits<filter::Linear, &Self> {
         self.linear_units(Direction::Outgoing)
     }
 
     /// Returns the output units and wires of this command. See [`Command::unit_wires`].
     #[inline]
     pub fn output_wires(&self) -> impl IntoIterator<Item = (CircuitUnit, Wire)> + '_ {
-        // Specialized version of `self.unit_wires()` that avoids collecting a
-        // `Vec` since it doesn't need to borrow `self`.
-        self.outputs()
-            .map(|(unit, port, _)| (unit, Wire::new(self.node, port)))
+        self.unit_wires(Direction::Outgoing)
     }
 
     /// Returns the output units of this command.
+    #[inline]
     pub fn inputs(&self) -> Units<&'_ Self> {
         self.units(Direction::Incoming)
     }
 
     /// Returns the linear input units of this command. See [`Command::linear_units`].
     #[inline]
-    pub fn linear_inputs(&self) -> Units<&'_ Self> {
+    pub fn linear_inputs(&self) -> FilteredUnits<filter::Linear, &Self> {
         self.linear_units(Direction::Incoming)
     }
 
@@ -199,12 +196,11 @@ where
     pub(super) fn new(circ: &'circ Circ) -> Self {
         // Initialize the map assigning linear units to the input's linear
         // ports.
+        //
+        // TODO: `with_wires` combinator for `Units`?
         let wire_unit = circ
-            .units()
-            .filter_map(|(unit, port, _)| match unit {
-                CircuitUnit::Linear(i) => Some((Wire::new(circ.input(), port), i)),
-                _ => None,
-            })
+            .linear_units()
+            .map(|(linear_unit, port, _)| (Wire::new(circ.input(), port), linear_unit))
             .collect();
 
         let nodes = pv::Topo::new(&circ.as_petgraph());