XXX Dirty but the idea is there

immer/extra/archive/box/archive.hpp

@@ -10,6 +10,8 @@
 
 #include <cereal/cereal.hpp>
 
+#include <boost/hana/functional/id.hpp>
+
 namespace immer::archive::box {
 
 template <typename T, typename MemoryPolicy>
@@ -115,25 +117,48 @@ save_to_archive(immer::box<T, MemoryPolicy> box,
     return {std::move(archive), id};
 }
 
-template <typename T, typename MemoryPolicy>
+template <typename T,
+          typename MemoryPolicy,
+          typename Archive    = archive_load<T, MemoryPolicy>,
+          typename TransformF = boost::hana::id_t>
 class loader
 {
 public:
-    explicit loader(archive_load<T, MemoryPolicy> ar)
+    explicit loader(Archive ar)
+        requires std::is_same_v<TransformF, boost::hana::id_t>
+        : ar_{std::move(ar)}
+    {
+    }
+
+    explicit loader(Archive ar, TransformF transform)
         : ar_{std::move(ar)}
+        , transform_{std::move(transform)}
     {
     }
 
-    immer::box<T, MemoryPolicy> load(container_id id) const
+    immer::box<T, MemoryPolicy> load(container_id id)
     {
         if (id.value >= ar_.boxes.size()) {
             throw invalid_container_id{id};
         }
-        return ar_.boxes[id.value];
+        if constexpr (std::is_same_v<TransformF, boost::hana::id_t>) {
+            return ar_.boxes[id.value];
+        } else {
+            if (auto* b = boxes.find(id)) {
+                return *b;
+            }
+            const auto& old_box = ar_.boxes[id.value];
+            auto new_box =
+                immer::box<T, MemoryPolicy>{transform_(old_box.get())};
+            boxes = std::move(boxes).set(id, new_box);
+            return new_box;
+        }
     }
 
 private:
-    const archive_load<T, MemoryPolicy> ar_;
+    const Archive ar_;
+    const TransformF transform_;
+    immer::map<container_id, immer::box<T, MemoryPolicy>> boxes;
 };
 
 template <typename T, typename MemoryPolicy>
@@ -166,8 +191,12 @@ struct container_traits<immer::box<T, MemoryPolicy>>
 {
     using save_archive_t = box::archive_save<T, MemoryPolicy>;
     using load_archive_t = box::archive_load<T, MemoryPolicy>;
-    using loader_t       = box::loader<T, MemoryPolicy>;
-    using container_id   = immer::archive::container_id;
+
+    template <typename Archive    = load_archive_t,
+              typename TransformF = boost::hana::id_t>
+    using loader_t = box::loader<T, MemoryPolicy, Archive, TransformF>;
+
+    using container_id = immer::archive::container_id;
 
     template <class F>
     static auto transform(F&& func)

immer/extra/archive/cereal/immer_box.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include <cereal/cereal.hpp>
+#include <immer/box.hpp>
+
+namespace cereal {
+
+template <typename Archive, typename T, typename MemoryPolicy>
+void CEREAL_LOAD_FUNCTION_NAME(Archive& ar, immer::box<T, MemoryPolicy>& m)
+{
+    T x;
+    ar(x);
+    m = x;
+}
+
+template <typename Archive, typename T, typename MemoryPolicy>
+void CEREAL_SAVE_FUNCTION_NAME(Archive& ar,
+                               const immer::box<T, MemoryPolicy>& m)
+{
+    ar(m.get());
+}
+
+} // namespace cereal

immer/extra/archive/champ/champ.hpp

@@ -44,7 +44,9 @@ class hash_validation_failed_exception : public archive_exception
     }
 };
 
-template <class Container>
+template <class Container,
+          typename Archive    = container_archive_load<Container>,
+          typename TransformF = boost::hana::id_t>
 class container_loader
 {
     using champ_t = std::decay_t<decltype(std::declval<Container>().impl())>;
@@ -61,12 +63,19 @@ class container_loader
     };
 
 public:
-    explicit container_loader(container_archive_load<Container> archive)
+    explicit container_loader(Archive archive)
+        requires std::is_same_v<TransformF, boost::hana::id_t>
         : archive_{std::move(archive)}
         , nodes_{archive_.nodes}
     {
     }
 
+    explicit container_loader(Archive archive, TransformF transform)
+        : archive_{std::move(archive)}
+        , nodes_{archive_.nodes, std::move(transform)}
+    {
+    }
+
     Container load(node_id root_id)
     {
         if (root_id.value >= archive_.nodes.size()) {
@@ -106,15 +115,20 @@ class container_loader
     }
 
 private:
-    const container_archive_load<Container> archive_;
+    const Archive archive_;
     nodes_loader<typename node_t::value_t,
                  typename traits::Hash,
                  typename traits::Equal,
                  typename traits::MemoryPolicy,
-                 traits::bits>
+                 traits::bits,
+                 TransformF>
         nodes_;
 };
 
+template <class Container>
+container_loader(container_archive_load<Container> archive)
+    -> container_loader<Container>;
+
 template <class Container>
 std::pair<container_archive_save<Container>, node_id>
 save_to_archive(Container container, container_archive_save<Container> archive)

immer/extra/archive/champ/load.hpp

@@ -6,7 +6,9 @@
 
 #include <immer/flex_vector.hpp>
 
+#include <boost/hana/functional/id.hpp>
 #include <boost/range/adaptor/indexed.hpp>
+
 #include <spdlog/spdlog.h>
 
 namespace immer::archive {
@@ -54,7 +56,8 @@ template <class T,
           typename Hash                  = std::hash<T>,
           typename Equal                 = std::equal_to<T>,
           typename MemoryPolicy          = immer::default_memory_policy,
-          immer::detail::hamts::bits_t B = immer::default_bits>
+          immer::detail::hamts::bits_t B = immer::default_bits,
+          typename TransformF            = boost::hana::id_t>
 class nodes_loader
 {
 public:
@@ -66,7 +69,14 @@ class nodes_loader
     using values_t = immer::flex_vector<immer::array<T>>;
 
     explicit nodes_loader(nodes_load<T, B> archive)
+        requires std::is_same_v<TransformF, boost::hana::id_t>
+        : archive_{std::move(archive)}
+    {
+    }
+
+    explicit nodes_loader(nodes_load<T, B> archive, TransformF transform)
         : archive_{std::move(archive)}
+        , transform_{std::move(transform)}
     {
     }
 
@@ -219,6 +229,7 @@ class nodes_loader
 
 private:
     const nodes_load<T, B> archive_;
+    const TransformF transform_;
     immer::map<node_id, std::pair<node_ptr, values_t>> collisions_;
     immer::map<node_id, std::pair<node_ptr, values_t>> inners_;
 };

immer/extra/archive/champ/traits.hpp

@@ -4,6 +4,8 @@
 #include <immer/extra/archive/champ/champ.hpp>
 #include <immer/extra/archive/traits.hpp>
 
+#include <boost/hana/functional/id.hpp>
+
 namespace immer::archive {
 
 template <class Container>
@@ -13,8 +15,12 @@ struct champ_traits
         immer::archive::champ::container_archive_save<Container>;
     using load_archive_t =
         immer::archive::champ::container_archive_load<Container>;
-    using loader_t     = immer::archive::champ::container_loader<Container>;
     using container_id = immer::archive::node_id;
+
+    template <typename Archive    = load_archive_t,
+              typename TransformF = boost::hana::id_t>
+    using loader_t =
+        immer::archive::champ::container_loader<Container, Archive, TransformF>;
 };
 
 template <typename K,