diff --git a/src/autorouter/autorouter.rs b/src/autorouter/autorouter.rs
index c3b50d3..c8c97b1 100644
--- a/src/autorouter/autorouter.rs
+++ b/src/autorouter/autorouter.rs
@@ -14,7 +14,7 @@ use crate::{
 use super::{
     autoroute::AutorouteExecutionStepper,
     compare_detours::CompareDetoursExecutionStepper,
-    measure_length::MeasureLengthExecutionStepper,
+    measure_length::MeasureLength,
     place_via::PlaceViaExecutionStepper,
     ratsnest::{Ratsnest, RatvertexIndex},
     remove_bands::RemoveBandsExecutionStepper,
@@ -138,11 +138,8 @@ impl<M: AccessMesadata> Autorouter<M> {
         CompareDetoursExecutionStepper::new(self, ratline1, ratline2, options)
     }
 
-    pub fn measure_length(
-        &mut self,
-        selection: &BandSelection,
-    ) -> Result<MeasureLengthExecutionStepper, AutorouterError> {
-        MeasureLengthExecutionStepper::new(selection)
+    pub fn measure_length(&mut self, selection: &BandSelection) -> MeasureLength {
+        MeasureLength(selection.clone())
     }
 
     pub fn ratline_endpoints(
diff --git a/src/autorouter/execution.rs b/src/autorouter/execution.rs
index f5b6fe5..e4b2c31 100644
--- a/src/autorouter/execution.rs
+++ b/src/autorouter/execution.rs
@@ -5,14 +5,14 @@ use serde::{Deserialize, Serialize};
 use crate::{
     board::mesadata::AccessMesadata,
     layout::via::ViaWeight,
-    stepper::{PollStep, Step, StepError},
+    stepper::{EvalImmut, PollStep, Step, StepError},
 };
 
 use super::{
     autoroute::{AutorouteExecutionStepper, AutorouteStatus},
     compare_detours::CompareDetoursExecutionStepper,
     invoker::{Invoker, InvokerError},
-    measure_length::MeasureLengthExecutionStepper,
+    measure_length::MeasureLength,
     place_via::PlaceViaExecutionStepper,
     remove_bands::RemoveBandsExecutionStepper,
     selection::{BandSelection, PinSelection},
@@ -36,7 +36,7 @@ pub enum ExecutionStepper {
     PlaceVia(PlaceViaExecutionStepper),
     RemoveBands(RemoveBandsExecutionStepper),
     CompareDetours(CompareDetoursExecutionStepper),
-    MeasureLength(MeasureLengthExecutionStepper),
+    MeasureLength(MeasureLength),
 }
 
 impl ExecutionStepper {
@@ -70,7 +70,7 @@ impl ExecutionStepper {
                 });
             }
             ExecutionStepper::MeasureLength(measure_length) => {
-                let length = measure_length.doit(autorouter)?;
+                let length = measure_length.eval(autorouter)?;
                 Poll::Ready(format!("Total length of selected bands: {}", length))
             }
         }
diff --git a/src/autorouter/invoker.rs b/src/autorouter/invoker.rs
index 0f70a77..31a39f2 100644
--- a/src/autorouter/invoker.rs
+++ b/src/autorouter/invoker.rs
@@ -18,7 +18,7 @@ use super::{
     compare_detours::CompareDetoursExecutionStepper,
     execution::{Command, ExecutionStepper},
     history::{History, HistoryError},
-    measure_length::MeasureLengthExecutionStepper,
+    measure_length::MeasureLength,
     place_via::PlaceViaExecutionStepper,
     remove_bands::RemoveBandsExecutionStepper,
     Autorouter, AutorouterError,
@@ -26,22 +26,30 @@ use super::{
 
 #[enum_dispatch]
 pub trait GetMaybeNavmesh {
-    fn maybe_navmesh(&self) -> Option<&Navmesh>;
+    fn maybe_navmesh(&self) -> Option<&Navmesh> {
+        None
+    }
 }
 
 #[enum_dispatch]
 pub trait GetMaybeTrace {
-    fn maybe_trace(&self) -> Option<&TraceStepper>;
+    fn maybe_trace(&self) -> Option<&TraceStepper> {
+        None
+    }
 }
 
 #[enum_dispatch]
 pub trait GetGhosts {
-    fn ghosts(&self) -> &[PrimitiveShape];
+    fn ghosts(&self) -> &[PrimitiveShape] {
+        &[]
+    }
 }
 
 #[enum_dispatch]
 pub trait GetObstacles {
-    fn obstacles(&self) -> &[PrimitiveIndex];
+    fn obstacles(&self) -> &[PrimitiveIndex] {
+        &[]
+    }
 }
 
 #[derive(Debug, Clone)]
@@ -128,7 +136,7 @@ impl<M: AccessMesadata> Invoker<M> {
                 self.autorouter.compare_detours(selection, *options)?,
             ),
             Command::MeasureLength(selection) => {
-                ExecutionStepper::MeasureLength(self.autorouter.measure_length(selection)?)
+                ExecutionStepper::MeasureLength(self.autorouter.measure_length(selection))
             }
         })
     }
diff --git a/src/autorouter/measure_length.rs b/src/autorouter/measure_length.rs
index c20e70a..12c1366 100644
--- a/src/autorouter/measure_length.rs
+++ b/src/autorouter/measure_length.rs
@@ -3,11 +3,8 @@
 //! the length of specified band selections.
 
 use crate::{
-    board::mesadata::AccessMesadata,
-    drawing::graph::PrimitiveIndex,
-    geometry::{primitive::PrimitiveShape, shape::MeasureLength as MeasureLengthTrait},
-    graph::MakeRef,
-    router::{navmesh::Navmesh, trace::TraceStepper},
+    board::mesadata::AccessMesadata, geometry::shape::MeasureLength as MeasureLengthTrait,
+    graph::MakeRef, stepper::EvalImmut,
 };
 
 use super::{
@@ -16,65 +13,28 @@ use super::{
     Autorouter, AutorouterError,
 };
 
-pub struct MeasureLengthExecutionStepper {
-    selection: BandSelection,
-    maybe_length: Option<f64>,
-}
+pub struct MeasureLength(pub BandSelection);
 
-impl MeasureLengthExecutionStepper {
-    pub fn new(selection: &BandSelection) -> Result<Self, AutorouterError> {
-        Ok(Self {
-            selection: selection.clone(),
-            maybe_length: None,
-        })
-    }
+impl<M: AccessMesadata> EvalImmut<Autorouter<M>, f64> for MeasureLength {
+    type Error = AutorouterError;
 
-    pub fn doit(
-        &mut self,
-        autorouter: &mut Autorouter<impl AccessMesadata>,
-    ) -> Result<f64, AutorouterError> {
-        let length = if let Some(length) = self.maybe_length {
-            length
-        } else {
-            let mut length = 0.0;
+    fn eval(&self, autorouter: &Autorouter<M>) -> Result<f64, AutorouterError> {
+        let mut length = 0.0;
 
-            for selector in self.selection.selectors() {
-                let band = autorouter
-                    .board
-                    .bandname_band(selector.band.clone())
-                    .unwrap()
-                    .0;
-                length += band.ref_(autorouter.board.layout().drawing()).length();
-            }
-
-            self.maybe_length = Some(length);
-            length
-        };
+        for selector in self.0.selectors() {
+            let band = autorouter
+                .board
+                .bandname_band(selector.band.clone())
+                .unwrap()
+                .0;
+            length += band.ref_(autorouter.board.layout().drawing()).length();
+        }
 
         Ok(length)
     }
 }
 
-impl GetMaybeNavmesh for MeasureLengthExecutionStepper {
-    fn maybe_navmesh(&self) -> Option<&Navmesh> {
-        None
-    }
-}
-
-impl GetMaybeTrace for MeasureLengthExecutionStepper {
-    fn maybe_trace(&self) -> Option<&TraceStepper> {
-        None
-    }
-}
-
-impl GetGhosts for MeasureLengthExecutionStepper {
-    fn ghosts(&self) -> &[PrimitiveShape] {
-        &[]
-    }
-}
-
-impl GetObstacles for MeasureLengthExecutionStepper {
-    fn obstacles(&self) -> &[PrimitiveIndex] {
-        &[]
-    }
-}
+impl GetMaybeNavmesh for MeasureLength {}
+impl GetMaybeTrace for MeasureLength {}
+impl GetGhosts for MeasureLength {}
+impl GetObstacles for MeasureLength {}
diff --git a/src/stepper.rs b/src/stepper.rs
index 5e15753..db9e367 100644
--- a/src/stepper.rs
+++ b/src/stepper.rs
@@ -47,3 +47,11 @@ pub trait StepBack<C, S>: StepError {
 pub trait Abort<C> {
     fn abort(&mut self, context: &mut C);
 }
+
+/// Evaluations which don't modify the context
+/// (basically calculating derived values)
+pub trait EvalImmut<C, S> {
+    type Error;
+
+    fn eval(&self, context: &C) -> Result<S, Self::Error>;
+}