add StatefulInferer::replace_inferer (#65)

This is a bit ergonomic when there's only a &mut and not a value.

Author: tgolsson

CHANGELOG.md

@@ -9,6 +9,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 <!-- next-header -->
 ## [Unreleased] - ReleaseDate
+
+- Add `StatefulInferer::replace_inferer` which works with a `&mut
+  StatefulInferer`, at the cost of requiring the inferer to be of the
+  same type.
+
 ## [0.9.0] - 2025-09-04
 
 - Breaking: `Inferer.begin_agent` and `Inferer.end_agent` now take

crates/cervo-core/src/wrapper.rs

@@ -110,6 +110,8 @@ pub struct StatefulInferer<WrapStack: InfererWrapper, Inf: Inferer> {
 }
 
 impl<WrapStack: InfererWrapper, Inf: Inferer> StatefulInferer<WrapStack, Inf> {
+    /// Construct a new [`StatefulInferer`] by wrapping the given
+    /// inferer with the given wrapper stack.
     pub fn new(wrapper_stack: WrapStack, inferer: Inf) -> Self {
         Self {
             wrapper_stack,
@@ -121,8 +123,8 @@ impl<WrapStack: InfererWrapper, Inf: Inferer> StatefulInferer<WrapStack, Inf> {
     /// any state in wrappers.
     ///
     /// Requires that the shapes of the policies are compatible, but
-    /// they may be different concrete inferer implementations. If
-    /// this check fails, will return self unchanged.
+    /// they may be different inferer types. If this check fails, will
+    /// return self unchanged.
     pub fn with_new_inferer<NewInf: Inferer>(
         self,
         new_inferer: NewInf,
@@ -136,6 +138,22 @@ impl<WrapStack: InfererWrapper, Inf: Inferer> StatefulInferer<WrapStack, Inf> {
         })
     }
 
+    /// Replace the inner inferer with a new inferer while maintaining
+    /// any state in wrappers.
+    ///
+    /// Requires that the shapes of the policies are compatible If
+    /// this check fails, will not change self. Compared to
+    /// [`with_new_inferer`], also requires that the new inferer has
+    /// the same type as the old one.
+    pub fn replace_inferer(&mut self, new_inferer: Inf) -> anyhow::Result<()> {
+        if let Err(e) = Self::check_compatible_shapes(&self.inferer, &new_inferer) {
+            Err(e)
+        } else {
+            self.inferer = new_inferer;
+            Ok(())
+        }
+    }
+
     /// Validate that [`Old`] and [`New`] are compatible with each
     /// other.
     pub fn check_compatible_shapes<Old: Inferer, New: Inferer>(