diff --git a/src/littlelambda.cpp b/src/littlelambda.cpp
index 7c2fa29..c94d266 100644
--- a/src/littlelambda.cpp
+++ b/src/littlelambda.cpp
@@ -50,15 +50,17 @@ static bool _try_parse_as(const char* start, const char* end, T& out, V... v) {
     return false;
 }
 
+lam_hooks::~lam_hooks() = default;
+
 struct lam_vm {
-    ugc_t gc;
-    lam_hooks hooks;
-    lam_env* root;
-    std::unordered_map<std::string, lam_value> imports;
+    ugc_t gc{};
+    lam_hooks* hooks{};
+    lam_env* root{};
+    std::unordered_map<std::string, lam_value> imports{};
     struct {
-        lam_u64 alloc_count;
-        lam_u64 free_count;
-        lam_u64 gc_iter_count;
+        lam_u64 alloc_count{};
+        lam_u64 free_count{};
+        lam_u64 gc_iter_count{};
     } gc_stats;
 };
 
@@ -66,7 +68,8 @@ struct lam_vm {
 // Register T with the garbage collector
 template <typename T>
 static T* callocPlus(lam_vm* vm, size_t extra) {
-    void* p = calloc(1, sizeof(T) + extra);
+    void* p = vm->hooks->mem_alloc(sizeof(T) + extra);
+    memset(p, 0, sizeof(T) + extra);
     auto o = reinterpret_cast<T*>(p);
     ugc_register(&vm->gc, &o->header);
     vm->gc_stats.alloc_count += 1;
@@ -351,7 +354,7 @@ struct lam_env_impl : lam_env {
 
 static lam_env* lam_new_env(lam_vm* vm, lam_env* parent, const char* name) {
     assert(parent == nullptr || vm == static_cast<lam_env_impl*>(parent)->vm);
-    auto* d = callocPlus<lam_env_impl>(vm, sizeof(lam_env_impl));
+    auto* d = callocPlus<lam_env_impl>(vm, 0);
     return new (d) lam_env_impl(vm, parent, name);
 }
 
@@ -744,7 +747,7 @@ static lam_env* lam_make_env_builtin(lam_vm* vm) {
             auto modname = a[0].as_symbol();
             auto it = env->vm->imports.find(modname->val());
             if (it == env->vm->imports.end()) {
-                lam_result result = env->vm->hooks.import(env->vm, modname->val());
+                lam_result result = env->vm->hooks->import(env->vm, modname->val());
                 if (result.code == 0) {
                     return result.value;
                 } else {
@@ -1221,15 +1224,17 @@ lam_result lam_vm_import(lam_vm* vm, const char* name, const void* data, size_t
 }
 
 lam_vm* lam_vm_new(lam_hooks* hooks) {
-    void* addr = hooks->alloc(sizeof(lam_vm));
+    hooks->init();
+    void* addr = hooks->mem_alloc(sizeof(lam_vm));
     lam_vm* vm = new (addr) lam_vm;
     ugc_init(&vm->gc, &lam_ugc_visit, &lam_ugc_free);
     vm->gc.userdata = vm;
-    memcpy(&vm->hooks, hooks, sizeof(lam_hooks));
+    vm->hooks = hooks;
     vm->root = lam_make_env_builtin(vm);
     return vm;
 }
 
 void lam_vm_delete(lam_vm* vm) {
+    vm->hooks->quit();
     // TODO
 }
diff --git a/src/littlelambda.h b/src/littlelambda.h
index 60c1a6a..e42811c 100644
--- a/src/littlelambda.h
+++ b/src/littlelambda.h
@@ -319,13 +319,12 @@ void lam_print(lam_value val, const char* end = nullptr);
 
 /// Functionality provided by external systems.
 struct lam_hooks {
-    using alloc_func = void*(size_t);
-    using free_func = void(void*);
-    using import_func = lam_result(lam_vm* vm, const char* modname);
-
-    alloc_func* alloc;
-    free_func* free;
-    import_func* import;
+    virtual ~lam_hooks();
+    virtual void* mem_alloc(size_t size) =0;
+    virtual void mem_free(void* addr) = 0;
+    virtual void init() = 0;
+    virtual void quit() = 0;
+    virtual lam_result import(lam_vm* vm, const char* modname) = 0;
 };
 
 /// Initialize a vm.
diff --git a/src/test.cpp b/src/test.cpp
index 5bd3676..f894203 100644
--- a/src/test.cpp
+++ b/src/test.cpp
@@ -1,9 +1,23 @@
 #define _CRT_SECURE_NO_WARNINGS
 #include <cstdio>
-#include <string>
 #include <format>
+#include <string>
 #include <unordered_map>
 #include <vector>
+#if __cpp_lib_stacktrace >= 202011L
+#include <stacktrace> // C++23 feature
+#else
+namespace std {
+struct stacktrace_entry {
+    std::string description() { return ""; }
+};
+struct stacktrace {
+    static stacktrace current() { return {}; }
+    struct stacktrace_entry* begin() { return nullptr; }
+    stacktrace_entry* end() { return nullptr; }
+};
+}  // namespace std
+#endif
 #include "littlelambda.h"
 
 static int slurp_file(const char* path, std::vector<char>& buf) {
@@ -63,10 +77,40 @@ static lam_result import_impl(lam_vm* vm, const char* modname) {
     return lam_result::fail(1, "Import failed");
 }
 
+struct DebugHooks : lam_hooks {
+    DebugHooks() {}
+
+    void* mem_alloc(size_t size) override {
+        void* p = malloc(size);
+        assert(allocs.find(p) == allocs.end());
+        allocs[p] = std::stacktrace::current();
+        return p;
+    }
+    void mem_free(void* addr) override {
+        auto it = allocs.find(addr);
+        assert(it != allocs.end());
+        allocs.erase(it);
+        free(addr);
+    }
+    void init() override {
+        //assert(allocs.empty());
+        allocs.clear();
+    }
+    void quit() override {
+        /*for (auto it : allocs) {
+            printf("%p\n", it.first);
+            for (auto p : it.second) {
+                printf("\t%s\n", p.description().c_str());
+            }
+        }*/
+    }
+    lam_result import(lam_vm* vm, const char* modname) override { return import_impl(vm, modname); }
 
+    std::unordered_map<void*, std::stacktrace> allocs;
+};
 
 int main() {
-    lam_hooks hooks = {&malloc, &free, &import_impl};
+    DebugHooks hooks;
     if (1) {
         lam_vm* vm = lam_vm_new(&hooks);
         read_and_eval(vm, "module.ll");