diff --git a/src/nasal_codegen.cpp b/src/nasal_codegen.cpp
index 1b3c6d6..f2a8649 100644
--- a/src/nasal_codegen.cpp
+++ b/src/nasal_codegen.cpp
@@ -1181,13 +1181,15 @@ const error& codegen::compile(parse& parse, linker& import) {
     // check global variables size
     if (global.size()>=STACK_DEPTH/2) {
         err.err("code",
-            "too many global variables: " + std::to_string(global.size()));
+            "too many global variables: " + std::to_string(global.size())
+        );
     }
 
     // check generated code size
     if (code.size()>0xffffff) {
         err.err("code",
-            "bytecode size overflow: " + std::to_string(code.size()));
+            "bytecode size overflow: " + std::to_string(code.size())
+        );
     }
     return err;
 }
diff --git a/src/nasal_gc.cpp b/src/nasal_gc.cpp
index b1bec6a..b85986d 100644
--- a/src/nasal_gc.cpp
+++ b/src/nasal_gc.cpp
@@ -536,7 +536,9 @@ void gc::extend(u8 type) {
 }
 
 void gc::init(
-    const std::vector<std::string>& s, const std::vector<std::string>& argv) {
+    const std::vector<std::string>& constant_strings,
+    const std::vector<std::string>& argv
+) {
     // initialize counters
     worktime = 0;
     for(u8 i = 0; i<gc_type_size; ++i) {
@@ -547,16 +549,24 @@ void gc::init(
     cort = nullptr;
 
     // init constant strings
-    strs.resize(s.size());
+    strs.resize(constant_strings.size());
     for(u32 i = 0; i<strs.size(); ++i) {
-        strs[i]=var::gcobj(new nas_val(vm_str));
+        // incremental initialization, avoid memory leak in repl mode
+        if (strs[i].type==vm_str && strs[i].str()==constant_strings[i]) {
+            continue;
+        }
+        strs[i] = var::gcobj(new nas_val(vm_str));
         strs[i].val.gcobj->unmut = 1;
-        strs[i].str() = s[i];
+        strs[i].str() = constant_strings[i];
     }
 
     // record arguments
     env_argv.resize(argv.size());
     for(usize i = 0; i<argv.size(); ++i) {
+        // incremental initialization, avoid memory leak in repl mode
+        if (env_argv[i].type==vm_str && env_argv[i].str()==argv[i]) {
+            continue;
+        }
         env_argv[i] = var::gcobj(new nas_val(vm_str));
         env_argv[i].val.gcobj->unmut = 1;
         env_argv[i].str() = argv[i];
diff --git a/src/nasal_gc.h b/src/nasal_gc.h
index 4b5c85c..590d1ed 100644
--- a/src/nasal_gc.h
+++ b/src/nasal_gc.h
@@ -66,7 +66,7 @@ struct nas_val;   // nas_val includes gc-managed types
 
 struct var {
 public:
-    u8 type;
+    u8 type = vm_none;
     union {
         u32 ret;
         i64 cnt;
diff --git a/src/repl.cpp b/src/repl.cpp
index 52aaded..602bf81 100644
--- a/src/repl.cpp
+++ b/src/repl.cpp
@@ -10,6 +10,9 @@ namespace nasal {
 namespace repl {
 
 void repl::add_command_history(const std::string& history) {
+    if (command_history.size() && command_history.back()==history) {
+        return;
+    }
     command_history.push_back(history);
     if (command_history.size()>1000) {
         command_history.pop_front();
@@ -74,16 +77,12 @@ void repl::help() {
 }
 
 bool repl::run() {
-    using clk = std::chrono::high_resolution_clock;
-    const auto den = clk::duration::period::den;    
-
     auto nasal_lexer = std::unique_ptr<lexer>(new lexer);
     auto nasal_parser = std::unique_ptr<parse>(new parse);
     auto nasal_linker = std::unique_ptr<linker>(new linker);
     auto nasal_opt = std::unique_ptr<optimizer>(new optimizer);
     auto nasal_codegen = std::unique_ptr<codegen>(new codegen);
 
-    auto start = clk::now();
     update_temp_file();
     if (nasal_lexer->scan("<nasal-repl>").geterr()) {
         return false;
@@ -102,9 +101,6 @@ bool repl::run() {
         return false;
     }
 
-    auto end = clk::now();
-    std::clog << "[compile time: " << (end-start).count()*1000.0/den << " ms]\n";
-
     // TODO: gc init stage in this run may cause memory leak,
     // because constant strings will be generated again.
     // but we could not delete old strings, they maybe still on stack.