diff --git a/README.md b/README.md
index da6bb20..5bd5d1f 100644
--- a/README.md
+++ b/README.md
@@ -31,8 +31,8 @@ ToDo List:
 - [ ] add inclusive operators like '!=', '>=', '<='
 - [ ] add support for priority operators
 - [X] add input_number() std function
-- [ ] add null type (dirty way to handle no returns and some errors)
-- [ ] add type() std function and others type checking functions
+- [X] add NULL type (dirty way to handle no returns and some errors)
+- [X] add type() std function and others type checking functions
 - [X] add ceil() and floor() std functions
 - [X] base of the CLI
 - [ ] evaluate expression from stdin
@@ -49,6 +49,7 @@ ToDo List:
 - [ ] add fully features strings support
 - [ ] add [classic problem solving](https://rosettacode.org) with code examples
 - [ ] add Web Assembly support and publish a demo website
+- [ ] refactor: use malloc for list
 - [ ] refactor: remove inconsistency on how functions returns error codes
 
 ## Installation
@@ -151,8 +152,7 @@ random_int(min, max)
 random_float(min, max)
 type(var) -> return the type of a var as int
 is_null(var)
-is_int(var)
-is_float(var)
+is_number(var)
 print_string(str)
 print_newline()
 print_ascii(nb)
diff --git a/src/evaluator.c b/src/evaluator.c
index 9ad577b..8cfc085 100644
--- a/src/evaluator.c
+++ b/src/evaluator.c
@@ -170,7 +170,7 @@ int evaluator_reduce_parenthesis_pattern(struct List* evalList) {
         if (
             ((i >= 1 && list_get_type(evalList, i-1) != TYPE_FUNC_NAME) || (i == 0)) &&
             list_get_type(evalList, i) == TYPE_OPEN_PARENTHESIS &&
-            is_type_number(list_get_type(evalList, i+1)) &&
+            is_type_literal(list_get_type(evalList, i+1)) &&
             list_get_type(evalList, i+2) == TYPE_CLOSE_PARENTHESIS
         ) {
             patternPos = i;
@@ -208,7 +208,7 @@ int evaluator_reduce_var(struct StateContainer* state, struct List* evalList) {
 
     byte type = var_store_get_type_from_key(state->varStore, varKey);
 
-    if (EVALUATOR_DEBUG_LEVEL >= 2) printf("Going to reduce var key %d at pos %d\n", patternPos);
+    if (EVALUATOR_DEBUG_LEVEL >= 2) printf("Going to reduce var key %d at pos %d\n", varKey, patternPos);
     
     byte varVal[get_size_of_type(type)];
     var_store_copy_from_key(state->varStore, varKey, &varVal);
@@ -288,7 +288,7 @@ int evaluator_reduce_function_call(struct List* evalList, int initialPosition) {
         }
         
         if (
-            is_type_number(list_get_type(evalList, pos)) &&
+            is_type_literal(list_get_type(evalList, pos)) &&
             list_get_type(evalList, pos+1) == TYPE_COMMA
         ) {
             // this is a new argument
@@ -297,7 +297,7 @@ int evaluator_reduce_function_call(struct List* evalList, int initialPosition) {
             continue;
         }
         if (
-            is_type_number(list_get_type(evalList, pos)) &&
+            is_type_literal(list_get_type(evalList, pos)) &&
             list_get_type(evalList, pos+1) == TYPE_CLOSE_PARENTHESIS
         ) {
             // we didn't match the last pattern so it's the last argument
@@ -572,7 +572,7 @@ int evaluate(struct StateContainer* state, char* inputStr, int* resultPtr, unsig
         printf("Now going to actually evaluate...\n");
     }
 
-    while (evalList.num_elements > 1 || !is_type_number(list_get_type(&evalList, 0))) {
+    while (evalList.num_elements > 1 || !is_type_literal(list_get_type(&evalList, 0))) {
         if (EVALUATOR_DEBUG_LEVEL >= 2) list_print(&evalList);
 
         // int reduceVarOpStat = evaluator_reduce_var(state, &evalList);
diff --git a/src/evaluator.h b/src/evaluator.h
index 40a8424..13eb35a 100644
--- a/src/evaluator.h
+++ b/src/evaluator.h
@@ -2,7 +2,7 @@
 #ifndef EVALUATOR_H_
 #define EVALUATOR_H_
 
-#define EVALUATOR_DEBUG_LEVEL 0
+#define EVALUATOR_DEBUG_LEVEL (G_DEBUG_LEVEL)
 
 int evaluate(struct StateContainer* state, char* inputStr, int* resultPtr, unsigned char* typePtr);
 
diff --git a/src/funcs.c b/src/funcs.c
index 92696d6..5fbf44c 100644
--- a/src/funcs.c
+++ b/src/funcs.c
@@ -5,6 +5,35 @@
 #include "./config.h"
 #include "./types.h"
 #include "./utils.h"
+#include "./number_parsing.h"
+
+// just return the type id of a variable
+int type_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
+{
+    UNUSED(args);
+    *resType = TYPE_INT;
+    *res = types[0];
+
+    return 0;
+}
+
+int is_null_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
+{
+    UNUSED(args);
+    *resType = TYPE_INT;
+    *res = types[0] == TYPE_NULL;
+
+    return 0;
+}
+
+int is_number_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
+{
+    UNUSED(args);
+    *resType = TYPE_INT;
+    *res = is_type_number(types[0]);
+
+    return 0;
+}
 
 int abs_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
 {
@@ -100,6 +129,9 @@ int print_ascii_impl(int* res, unsigned char* resType, unsigned char* types, int
 
 int print_newline_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
 {
+    UNUSED(types);
+    UNUSED(args);
+
     *resType = TYPE_INT;
     *res = 1;
     printf("\n");
@@ -109,6 +141,8 @@ int print_newline_impl(int* res, unsigned char* resType, unsigned char* types, i
 
 int input_number_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
 {
+    UNUSED(types);
+    UNUSED(args);
     printf("? ");
 
     char* line;
@@ -119,16 +153,16 @@ int input_number_impl(int* res, unsigned char* resType, unsigned char* types, in
 
     // printf("len=%d, lineSize=%d '%s' \n", len, lineSize, line);
 
-    char* toParse[lineSize+1];
-    str_extract(toParse, line, 0, lineSize-1);
+    char toParse[lineSize+1];
+    str_extract((char*) toParse, line, 0, lineSize-1);
 
-    int st = parse_int(toParse, res);
+    int st = parse_int((char*) &toParse, res);
     if (st == 0) {
         *resType = TYPE_INT;
         return 0;
     }
     if (st != 0) {
-        st = parse_float(toParse, (float*) res);
+        st = parse_float((char*) &toParse, (float*) res);
         if (st == 0) {
             *resType = TYPE_FLOAT;
             return 0;
@@ -248,6 +282,8 @@ int max_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
 
 int get_pi_impl(int* res, unsigned char* resType, unsigned char* types, int* args)
 {
+    UNUSED(args);
+    UNUSED(types);
     float val = CST_PI;
     *res = *(int *)(&val);
     *resType = TYPE_FLOAT;
@@ -294,6 +330,10 @@ struct FuncIntro {
 // void* are actually long!
 
 struct FuncIntro intros[] = {
+    {"type", &type_impl, 1},
+    {"is_null", &is_null_impl, 1},
+    {"is_number", &is_number_impl, 1},
+
     {"abs", &abs_impl, 1},
     
     {"sqrt", &sqrt_impl, 1},
diff --git a/src/line_processing.h b/src/line_processing.h
index 96e3e7b..ad262f8 100644
--- a/src/line_processing.h
+++ b/src/line_processing.h
@@ -2,7 +2,7 @@
 #ifndef LINE_PROCESSING_H_
 #define LINE_PROCESSING_H_
 
-#define LINE_PROCESSING_DEBUG_LEVEL 0
+#define LINE_PROCESSING_DEBUG_LEVEL (G_DEBUG_LEVEL)
 
 #define BLOCK_IF 2
 #define BLOCK_WHILE 3
diff --git a/src/list.c b/src/list.c
index 0b84b23..09112b5 100644
--- a/src/list.c
+++ b/src/list.c
@@ -188,48 +188,12 @@ void list_print(struct List* list)
     printf("=== LIST REPORT === \n");
     printf("num of elements: %d \n", list->num_elements);
     for (int i = 0; i < list->num_elements; i++) {
-        printf("- i: %d, ", i);
+        
         int type = list_get_type(list, i);
-        if (type == TYPE_INT) {
-            int d = 0;
-            list_get(list, i, &d);
-
-            printf("type: INT, val: %d", d);
-        }
-        if (type == TYPE_FLOAT) {
-            float d = 0;
-            list_get(list, i, &d);
-
-            printf("type: FLOAT, val: %f", d);
-        }
-        if (type == TYPE_OPERATOR) {
-            char d = 0;
-            list_get(list, i, &d);
-
-            printf("type: OPERATOR, val: %d '%c'", d, d);
-        }
-        if (type == TYPE_FUNC_NAME) {
-            short d = 0;
-            list_get(list, i, &d);
-
-            printf("type: FUNC_NAME, val: %d", d);
-        }
-        if (type == TYPE_VAR_NAME) {
-            short d = 0;
-            list_get(list, i, &d);
-
-            printf("type: VAR_NAME, val: %d", d);
-        }
-        if (type == TYPE_OPEN_PARENTHESIS) {
-            printf("type: OPEN_PARENTHESIS");
-        }
-        if (type == TYPE_COMMA) {
-            printf("type: COMMA");
-        }
-        if (type == TYPE_CLOSE_PARENTHESIS) {
-            printf("type: CLOSE_PARENTHESIS");
-        }
-        printf("\n");
+        int data = 0;
+        list_get(list, i, &data);
+        
+        printf("- i: %d, %s \n", i, get_repr(type, (void*) &data));
     }
     printf("=== END === \n");
 }
diff --git a/src/types.c b/src/types.c
index dcc9096..3e79a33 100644
--- a/src/types.c
+++ b/src/types.c
@@ -3,6 +3,11 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+int is_type_literal(byte type)
+{
+    return is_type_number(type) || type == TYPE_NULL;
+}
+
 int is_type_number(byte type)
 {
     return type == TYPE_INT || type == TYPE_FLOAT;
@@ -25,6 +30,7 @@ int get_size_of_type(byte type)
         return 1;
     }
     if (
+        t == TYPE_NULL ||
         t == TYPE_OPEN_PARENTHESIS ||
         t == TYPE_CLOSE_PARENTHESIS ||
         t == TYPE_COMMA
@@ -36,25 +42,36 @@ int get_size_of_type(byte type)
 
 char* get_repr(byte type, void* valPtr)
 {
-    char* res= (char*) malloc(sizeof(char)*32);
+    char* res = (char*) malloc(sizeof(char)*32);
+    if (res == 0) return 0;
     if (type == 0) {
+        sprintf(res, "NO_TYPE");
+    } else if (type == TYPE_NULL) {
         sprintf(res, "NULL");
-        return res;
-    }
-    if (type == TYPE_INT) {
+    } else if (type == TYPE_INT) {
         sprintf(res, "INT(%d)", *((int*) valPtr));
-        return res;
-    }
-    if (type == TYPE_FLOAT) {
+    } else if (type == TYPE_FLOAT) {
         sprintf(res, "FLOAT(%f)", *((float*) valPtr));
-        return res;
+    } else if (type == TYPE_OPEN_PARENTHESIS) {
+        sprintf(res, "OPEN_PARENTHESIS");
+    } else if (type == TYPE_CLOSE_PARENTHESIS) {
+        sprintf(res, "CLOSE_PARENTHESIS");
+    } else if (type == TYPE_COMMA) {
+        sprintf(res, "COMMA");
+    } else if (type == TYPE_OPERATOR) {
+        sprintf(res, "OPERATOR(%c)", *((char*) valPtr));
+    } else if (type == TYPE_VAR_NAME) {
+        sprintf(res, "VAR_NAME(ref: %d)", *((int*) valPtr));
+    } else if (type == TYPE_FUNC_NAME) {
+        sprintf(res, "FUNC_NAME(ref: %d)", *((int*) valPtr));
+    } else {
+        sprintf(res, "UNKNOWN type=%d", type);
     }
-
-    sprintf(res, "UNKNOWN type=%d", type);
     return res;
 }
 
 int convert_to_int(byte type, int repr) {
+    if (type == TYPE_NULL) return 0;
     if (type == TYPE_FLOAT) return (int) *((int*) &repr);
     return repr;
 }
diff --git a/src/types.h b/src/types.h
index f6236f5..2bfd4d0 100644
--- a/src/types.h
+++ b/src/types.h
@@ -2,6 +2,7 @@
 #ifndef TYPES_H_
 #define TYPES_H_
 
+#define TYPE_NULL 1
 #define TYPE_INT 2
 #define TYPE_FLOAT 3
 #define TYPE_OPERATOR 32
@@ -11,10 +12,15 @@
 #define TYPE_OPEN_PARENTHESIS 64
 #define TYPE_CLOSE_PARENTHESIS 65
 
+
+int is_type_literal(byte type);
+
 int is_type_number(byte type);
 
 int get_size_of_type(byte type);
 
+// get short string representation of a type
+// usefull when printing a report
 char* get_repr(byte type, void* data);
 
 byte convert_to_bool(byte type, int repr);
diff --git a/src/utils.h b/src/utils.h
index 10cd57b..6cd8d25 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -3,6 +3,9 @@
 #ifndef UTILS_H_
 #define UTILS_H_
 
+// useful to silent warning of gcc of uunused parameters in functions
+#define UNUSED(x) (void)(x)
+
 #define CST_PI 3.1415926535
 
 // define custom type (unsigned char)
diff --git a/src/var_store.c b/src/var_store.c
index 0bed8fc..8892dd9 100644
--- a/src/var_store.c
+++ b/src/var_store.c
@@ -4,6 +4,8 @@
 #include <stdio.h>
 #include <stdlib.h>
 
+#define MAX_INTEGER 47797852
+
 struct VariableStore* var_store_init()
 {
     struct VariableStore* store = (struct VariableStore*) malloc(sizeof(struct VariableStore));
@@ -21,10 +23,16 @@ struct VariableStore* var_store_init()
     for (int i = 0; i < store->allocatedLength; i++) {
         store->container[i].type = 0;
     }
+    var_store_add_constants(store);
+
     return store;
 }
 
-#define MAX_INTEGER 47797852
+void var_store_add_constants(struct VariableStore* store)
+{
+    // NULL constant
+    var_store_set(store, "NULL", TYPE_NULL, 0);
+}
 
 int var_store_hash_name(struct VariableStore* store, char* varName)
 {
@@ -42,7 +50,7 @@ int var_store_hash_name(struct VariableStore* store, char* varName)
 byte var_store_set(struct VariableStore* store, char* varName, byte type, void* valuePtr)
 {
     //printf("set variable at var name: '%s' \n", varName);
-    if (!(type == TYPE_FLOAT || type == TYPE_INT)) {
+    if (!(type == TYPE_FLOAT || type == TYPE_INT || type == TYPE_NULL)) {
         printf("[ERR] VARSTORE: unsupported type, cannot store type %d\n", type);
         return 0;
     }
diff --git a/src/var_store.h b/src/var_store.h
index 9140390..ac9e305 100644
--- a/src/var_store.h
+++ b/src/var_store.h
@@ -32,6 +32,9 @@ struct VariableStore {
 
 struct VariableStore* var_store_init();
 
+// add to the store some useful constants (like NULL constructor)
+void var_store_add_constants(struct VariableStore* store);
+
 int var_store_hash_name(struct VariableStore* store, char* varName);
 
 // get a pointer to the area of memory where is store the variable
diff --git a/tests/test_evaluation.c b/tests/test_evaluation.c
index b9ab7e6..eefe447 100644
--- a/tests/test_evaluation.c
+++ b/tests/test_evaluation.c
@@ -175,4 +175,20 @@ void test_evaluation()
     assert(resType == TYPE_INT);
     printf("actually got: %d \n", resVal);
     assert(7 == resVal);
+
+    evaluate(state, "is_number(123)", &resVal, &resType);
+    assert(resType == TYPE_INT);
+    assert(resVal);
+
+    evaluate(state, "is_number(-25.5)", &resVal, &resType);
+    assert(resType == TYPE_INT);
+    assert(resVal);
+
+    evaluate(state, "is_number(NULL)", &resVal, &resType);
+    assert(resType == TYPE_INT);
+    assert(!resVal);
+
+    evaluate(state, "is_null(NULL)", &resVal, &resType);
+    assert(resType == TYPE_INT);
+    assert(resVal);
 }