diff --git a/ml.cpp b/ml.cpp
index 7419c84..084ec26 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -104,7 +104,7 @@ std::string read_file_contents(const std::string &filename) {
 #define to_string(x) static_cast<std::ostringstream>((std::ostringstream() << std::dec << x )).str()
 
 // Replace a substring with a replacement string in a source string
-void replace_substring(std::string &src, std::string substr, std::string replacement) {
+void replace_substring(std::string &src, const std::string& substr, const std::string &replacement) {
     size_t i = 0;
     for (i = src.find(substr, i); i != std::string::npos; i = src.find(substr, i)) {
         src.replace(i, substr.size(), replacement);
@@ -148,7 +148,7 @@ MlValue::MlValue(double f) : type(FLOAT) { stack_data.f = f; }
 
 MlValue::MlValue(const std::vector<MlValue> &list) : type(LIST), list(list) {}
 
-MlValue MlValue::quote(MlValue quoted) {
+MlValue MlValue::quote(const MlValue &quoted) {
     MlValue result;
     result.type = QUOTE;
 
diff --git a/ml.h b/ml.h
index 1e7f842..e5d8036 100644
--- a/ml.h
+++ b/ml.h
@@ -89,7 +89,7 @@ typedef MlValue (*Builtin)(std::vector<MlValue>, MlEnvironment &);
      MlValue(const std::vector<MlValue> &list);
 
      // Construct a quoted value
-     static MlValue quote(MlValue quoted);
+     static MlValue quote(const MlValue& quoted);
 
      // Construct an atom
      static MlValue atom(const std::string &s);