diff --git a/ml.cpp b/ml.cpp
index 01ab178..696fa3e 100644
--- a/ml.cpp
+++ b/ml.cpp
@@ -35,7 +35,11 @@
 #define INVALID_LAMBDA "invalid lambda"
 #define INVALID_BIN_OP "invalid binary operation"
 #define INVALID_ORDER "cannot order expression"
-#define BAD_CAST "cannot cast"
+#define BAD_CAST_INT "cannot cast as int"
+#define BAD_CAST_FLOAT "cannot cast as float"
+#define BAD_CAST_STRING "cannot cast as string"
+#define BAD_CAST_ATOM "cannot cast as atom"
+#define BAD_CAST_LIST "cannot cast as list"
 #define ATOM_NOT_DEFINED "atom not defined"
 #define INTERNAL_ERROR "internal virtual machine error"
 #define INDEX_OUT_OF_RANGE "index out of range"
@@ -181,42 +185,33 @@ bool MlValue::is_list() const {
 	return type == LIST;
 }
 
-// Get the "truthy" boolean value of this value.
 bool MlValue::as_bool() const {
 	return type != NIL && *this != MlValue(0l);        // TODO remove 0 as false
 }
 
-// Get this item's integer value
 long MlValue::as_int() const {
 	return cast_to_int().stack_data.i;
 }
 
-// Get this item's floating point value
 double MlValue::as_float() const {
 	return cast_to_float().stack_data.f;
 }
 
-// Get this item's string value
 std::string MlValue::as_string() const {
-	// If this item is not a string, throw a cast error.
 	if (type != STRING)
-		throw MlError(*this, MlEnvironment(), BAD_CAST);
+		throw MlError(*this, MlEnvironment(), BAD_CAST_STRING);
 	return str;
 }
 
-// Get this item's atom value
 std::string MlValue::as_atom() const {
-	// If this item is not an atom, throw a cast error.
 	if (type != ATOM)
-		throw MlError(*this, MlEnvironment(), BAD_CAST);
+		throw MlError(*this, MlEnvironment(), BAD_CAST_ATOM);
 	return str;
 }
 
-// Get this item's list value
 std::vector<MlValue> MlValue::as_list() const {
-	// If this item is not a list, throw a cast error.
 	if (type != LIST)
-		throw MlError(*this, MlEnvironment(), BAD_CAST);
+		throw MlError(*this, MlEnvironment(), BAD_CAST_LIST);
 	return list;
 }
 
@@ -254,7 +249,7 @@ MlValue MlValue::cast_to_int() const {
 		case STRING:
 			return MlValue(std::stol(str));
 		default:
-			throw MlError(*this, MlEnvironment(), BAD_CAST);
+			throw MlError(*this, MlEnvironment(), BAD_CAST_INT);
 	}
 }
 
@@ -268,7 +263,7 @@ MlValue MlValue::cast_to_float() const {
 		case STRING:
 			return MlValue(std::stod(str));
 		default:
-			throw MlError(*this, MlEnvironment(), BAD_CAST);
+			throw MlError(*this, MlEnvironment(), BAD_CAST_FLOAT);
 	}
 }
 
@@ -282,7 +277,7 @@ MlValue MlValue::cast_to_string() const {
 		case STRING:
 			return *this;
 		default:
-			throw MlError(*this, MlEnvironment(), BAD_CAST);
+			throw MlError(*this, MlEnvironment(), BAD_CAST_STRING);
 	}
 }