diff --git a/examples/function.noja b/examples/function.noja
index 1eac397..4dc7e21 100644
--- a/examples/function.noja
+++ b/examples/function.noja
@@ -4,4 +4,4 @@ fun sayHello(name: String = "unnamed person")
 
 sayHello("Francesco");
 sayHello();
-sayHello(none);
\ No newline at end of file
+sayHello(none);
diff --git a/examples/json/json.noja b/examples/json/json.noja
index 45d4059..6e4009a 100644
--- a/examples/json/json.noja
+++ b/examples/json/json.noja
@@ -1,240 +1,320 @@
+Scanner = {src: String, i: int};
+
+fun newScanner(src: String) {
+    scan = {src: src, i: 0};
+    assert(istypeof(Scanner, scan));
+    return scan;
+}
+
+fun isDigit(char: ?String) {
+    if char == none:
+        return false;
+    return string.ord(char) >= string.ord('0')
+       and string.ord(char) <= string.ord('9');
+}
 
 fun isSpace(c: String)
-    return c == " " 
-        or c == "\t" 
-        or c == "\n";
+    return c == ' '
+        or c == '\t'
+        or c == '\n';
 
-fun isDigit(c: String) {
-    d = string.ord(c) 
-      - string.ord("0");
-    return d >= 0 
-       and d <= 9;
+fun min(x, y) {
+    if x < y:
+        return x;
+    return y;
 }
 
-ParsingContext = {str: String, cur: int};
-
-fun skipSpaces(context: ParsingContext) {
-    # Skip zero or more whitespace characters.
-    while context.cur < count(context.str) 
-      and isSpace(context.str[context.cur]):
-      context.cur = context.cur + 1;
-}
-
-fun parseNull(context: ParsingContext) {
-    
-    assert(context.cur < count(context.str) 
-       and context.str[context.cur] == 'n');
-    
-    if context.cur+3 > count(context.str)
-    or context.str[context.cur+1] != 'u' 
-    or context.str[context.cur+2] != 'l'
-    or context.str[context.cur+3] != 'l':
-        return none, "Unexpected token starting with \"n\"";
-    
-    context.cur = context.cur + count("null");
+fun hint(scan: Scanner, n: int = 1) {
+    k = scan.i + n;
+    if k < count(scan.src):
+        return scan.src[k];
     return none;
 }
 
-fun parseTrue(context: ParsingContext) {
-    
-    assert(context.cur < count(context.str) 
-       and context.str[context.cur] == 't');
-    
-    if context.cur+3 > count(context.str)
-    or context.str[context.cur+1] != 'r' 
-    or context.str[context.cur+2] != 'u'
-    or context.str[context.cur+3] != 'e':
-        return none, "Unexpected token starting with \"t\"";
-    
-    context.cur = context.cur + count("true");
-    return true;
+fun current(scan: Scanner)
+    return hint(scan, 0);
+
+fun consume(scan: Scanner, n: int = 1) {
+    assert(n > 0);
+    limit = count(scan.src);
+    scan.i = min(scan.i + n, limit);
 }
 
-fun parseFalse(context: ParsingContext) {
-    
-    assert(context.cur < count(context.str) 
-       and context.str[context.cur] == 'f');
-    
-    if context.cur+3 > count(context.str)
-    or context.str[context.cur+1] != 'a' 
-    or context.str[context.cur+2] != 'l'
-    or context.str[context.cur+3] != 's'
-    or context.str[context.cur+4] != 'e':
-        return none, "Unexpected token starting with \"f\"";
-    
-    context.cur = context.cur + count("false");
-    return false;
-}
+fun consumeSpaces(scan: Scanner)
+    while isSpace(current(scan)):
+        consume(scan);
 
-fun dummy() {}
-Func = type(dummy);
+fun parseString(scan: Scanner) {
 
-fun skip(context: ParsingContext, test_callback: Func) {
-    k = context.cur;
-    while k < count(context.str) 
-      and test_callback(context.str[k]):
-        k = k + 1;
-    return k;
-}
+    assert(current(scan) == '"');
+    consume(scan); # Consume the opening "
 
-fun parseInteger(context: ParsingContext) {
-    assert(context.cur < count(context.str) and isDigit(context.str[context.cur]));
-
-    buffer = 0;
-
-    do {
-        c = context.str[context.cur];
-        d = string.ord(c) - string.ord('0');
-        buffer = buffer * 10 + d;
-        context.cur = context.cur + 1;
-    } while context.cur < count(context.str) 
-        and isDigit(context.str[context.cur]);
-    
-    return buffer;
-}
-
-fun parseFloating(context: ParsingContext) {
-    assert(context.cur < count(context.str) 
-       and isDigit(context.str[context.cur]));
-
-    # It's ensured by the caller (parseIntegerOrFloating)
-    # that now the strings contains two sequences of
-    # digits with a dot in the middle.
-
-    buffer = 0.0;
-
-    # Scan the integer part.
-    do {
-        c = context.str[context.cur];
-        d = string.ord(c) - string.ord('0');
-        buffer = buffer * 10 + d;
-        context.cur = context.cur + 1;
-    } while context.str[context.cur] != ".";
-    
-    assert(isDigit(context.str[context.cur+1]));
-    
-    # Scan the decimal part.
-    q = 1;
-    do {
-        c = context.str[context.cur];
-        d = string.ord(c) - string.ord('0');
-        q = q / 10;
-        buffer = buffer + q * d;
-        context.cur = context.cur + 1;
-    } while context.cur < count(context.str)
-        and isDigit(context.str[context.cur+1]);
-
-    return buffer;
-}
-
-fun parseIntegerOrFloating(context: ParsingContext) {
-    assert(context.cur < count(context.str) 
-       and isDigit(context.str[context.cur]));
-
-    # Is the first digit sequence followed by a dot
-    # and then one or more digits?
-    
-    k = skip(context, isDigit);
-    if k+1 < count(context.str) and context.str[k] == "." and isDigit(context.str[k+1]):
-        # It's a floating point value, then!
-        return parseFloating(context);
-    else
-        # It's good old integer!
-        return parseInteger(context);
-}
-
-fun parseString(context: ParsingContext) {
-    
-    assert(context.cur < count(context.str) 
-       and context.str[context.cur] == '"');
-
-    context.cur = context.cur + 1;
-
-    buffer = "";
-
-    while context.cur < count(context.str) 
-      and context.str[context.cur] != '"': {
-        # TODO: Support special characters such as \n, \t ecc
-        buffer = string.cat(buffer, context.str[context.cur]);
-        context.cur = context.cur + 1;
+    temp = "";
+    char = current(scan);
+    while char != '"' and char != none: {
+        temp = string.cat(temp, char);
+        consume(scan);
+        char = current(scan);
     }
 
-    if context.cur == count(context.str):
-        return none, "Source string ended inside a string literal";
+    if char == none: {
+        val = none;
+        err = "Source ended inside a string literal";
+    } else {
+        assert(char == '"');
+        consume(scan);
+        val = temp;
+        err = none;
+    }
 
-    context.cur = context.cur + 1;
-    return buffer;
+    return val, err;
 }
 
-fun parseArray(context: ParsingContext) {
-    assert(context.cur < count(context.str) 
-       and context.str[context.cur] == '[');
+fun parseObject(scan: Scanner) {
+    
+    assert(current(scan) == '{');
+    consume(scan); 
 
-    context.cur = context.cur + 1;
+    obj = {};
+    consumeSpaces(scan);
+    if current(scan) != '}': do {
+        key, err = parseValue(scan);
+        if err != none: 
+            return none, err;
+        if type(key) != String:
+            return none, "Object key isn't a string";
 
-    skipSpaces(context);
+        consumeSpaces(scan);
+        if current(scan) != ':':
+            return none, "Missing ':' separator after key";
+        consume(scan);
 
-    if context.cur == count(context.str):
-        return none, "Source string ended inside an array";
-
-    if context.str[context.cur] == ']':
-        return [];
-
-    list = [];
-
-    while true: {
-
-        item, err = parseValue(context);
+        val, err = parseValue(scan);
         if err != none:
             return none, err;
 
-        list[count(list)] = item;
+        if obj[key] != none:
+            return none, "Duplicate key in object";
+        obj[key] = val;
 
-        skipSpaces(context);
-
-        if context.cur == count(context.str):
-            return none, "Source string ended inside an array";
-
-        if context.str[context.cur] == ']':
-            break;
-
-        if context.str[context.cur] != ',':
-            return none, "Unexpected token inside an array, where either ',' or ']' were expected";
-
-        context.cur = context.cur + 1;
-
-        skipSpaces(context);
-    }
-    assert(context.cur < count(context.str)
-       and context.str[context.cur] == ']');
-
-    context.cur = context.cur + 1;
-    return list;
+        consumeSpaces(scan);
+        if current(scan) == '}':
+            done = true;
+        else {
+            if current(scan) != ',':
+                return none, "Invalid character where either ',' or '}' were expected";
+            consume(scan); # Consume the ","
+            done = false;
+        }
+    } while not done;
+    assert(current(scan) == '}');
+    consume(scan);
+    return obj;
 }
 
-fun parseValue(context: ParsingContext) {
+fun parseArray(scan: Scanner) {
     
-    if context.cur == count(context.str):
-        return none, "Source ended where a value was expected";
+    assert(current(scan) == '[');
+    consume(scan); # Consume the [
+    
+    array = [];
+    
+    consumeSpaces(scan);
+    if current(scan) != ']': do {
 
-    c = context.str[context.cur];
-    if c == "{":        val, err = parseObject(context);
-    else if c == "[":   val, err = parseArray(context);
-    else if c == '"':   val, err = parseString(context);
-    else if isDigit(c): val, err = parseIntegerOrFloating(context);
-    else if c == 'n':   val, err = parseNull(context);
-    else if c == 't':   val, err = parseTrue(context);
-    else if c == 'f':   val, err = parseFalse(context);
-    else {
-        val = none;
-        err = string.cat("Unexepected character \"", c, "\" where a value was expected");
+        val, err = parseValue(scan);
+        if err != none:
+            return none, err;
+        array[count(array)] = val;
+
+        consumeSpaces(scan);
+        if current(scan) == ']':
+            done = true;
+        else {
+            if current(scan) != ',':
+                return none, "Invalid character where either ',' or ']' were expected";
+            consume(scan); # Consume the ","
+            done = false;
+        }
+    } while not done;
+    assert(current(scan) == ']');
+    consume(scan);
+    return array;
+}
+
+fun parseFalse(scan: Scanner) {
+    
+    assert(current(scan) == 'f');
+    consume(scan);
+
+    if current(scan) != 'a':
+        return none, "Invalid character after 'f'";
+    consume(scan);
+
+    if current(scan) != 'l':
+        return none, "Invalid character after 'fa'";
+    consume(scan);
+
+    if current(scan) != 's':
+        return none, "Invalid character after 'fal'";
+    consume(scan);
+
+    if current(scan) != 'e':
+        return none, "Invalid character after 'fals'";
+    consume(scan);
+
+    return false;
+}
+
+fun parseTrue(scan: Scanner) {
+    
+    assert(current(scan) == 't');
+    consume(scan);
+
+    if current(scan) != 'r':
+        return none, "Invalid character after 't'";
+    consume(scan);
+
+    if current(scan) != 'u':
+        return none, "Invalid character after 'tr'";
+    consume(scan);
+
+    if current(scan) != 'e':
+        return none, "Invalid character after 'tru'";
+    consume(scan);
+
+    return true;
+}
+
+fun parseNull(scan: Scanner) {
+    
+    assert(current(scan) == 'n');
+    consume(scan);
+
+    if current(scan) != 'u':
+        return none, "Invalid character after 'n'";
+    consume(scan);
+
+    if current(scan) != 'l':
+        return none, "Invalid character after 'nu'";
+    consume(scan);
+
+    if current(scan) != 'l':
+        return none, "Invalid character after 'nul'";
+    consume(scan);
+
+    return none;
+}
+
+fun integerFromDigit(char: String) {
+    assert(count(char) == 1);
+    return string.ord(char) 
+         - string.ord('0');
+}
+
+fun parseNumber(scan: Scanner) {
+
+    char = current(scan);
+    
+    sign = 1;
+    if char == '+' or char == '-': {
+        
+        sign = {'+': 1, '-': -1}[char];
+        consume(scan);
+
+        char = current(scan);
+        if not isDigit(char):
+            return none, "Invalid character after +/-";
     }
+
+    temp = 0.0;
+    do {
+        assert(isDigit(char));
+
+        n = integerFromDigit(char);
+        temp = temp * 10 + n;
+
+        consume(scan);
+        char = current(scan);
+
+    } while isDigit(char);
+
+    if char == '.' and isDigit(hint(scan, 1)): {
+        
+        consume(scan); # Consume the dot
+        char = current(scan);
+        
+        q = 1.0;
+        do {
+            q = q / 10;
+            n = integerFromDigit(char);
+            temp = temp + q * n;
+            consume(scan);
+            char = current(scan);
+        } while isDigit(char);
+    }
+
+    return sign * temp;
+}
+
+fun parseValue(scan: Scanner) {
+
+    table = {
+        '"': parseString,
+        '{': parseObject,
+        '[': parseArray,
+        'f': parseFalse,
+        't': parseTrue,
+        'n': parseNull,
+        '+': parseNumber,
+        '-': parseNumber,
+        '0': parseNumber,
+        '1': parseNumber,
+        '2': parseNumber,
+        '3': parseNumber,
+        '4': parseNumber,
+        '5': parseNumber,
+        '6': parseNumber,
+        '7': parseNumber,
+        '8': parseNumber,
+        '9': parseNumber
+    };
+
+    consumeSpaces(scan);
+    char = current(scan);
+    if char == none: {
+        val = none;
+        err = "Source ended where a value was expected";
+    } else {
+        callback = table[char];
+        if callback == none: {
+            val = none;
+            err = "Invalid character where a value was expected";
+        } else
+            (val, err) = callback(scan);
+    }
+    return (val, err);
+}
+
+fun parse(src: String) {
+    scan = newScanner(src);
+    val, err = parseValue(scan);
     return val, err;
 }
 
-fun parse(str: String) {
-    context = {str: str, cur: 0};
-    skipSpaces(context);
-    val, err = parseValue(context);
-    return val, err;
+fun parseFile(file: String | File) {
+
+    if type(file) == String: {
+        file, err = files.open(file, files.READ);
+        if err != none:
+            return none, err;
+    }
+
+    src, err = files.read(file);
+    if err != none:
+        return none, err;
+    assert(type(src) == String);
+
+    val, err = parse(src);
+    return val, err; 
 }
\ No newline at end of file
diff --git a/examples/json/test.noja b/examples/json/test.noja
index d903ddc..d1dee91 100644
--- a/examples/json/test.noja
+++ b/examples/json/test.noja
@@ -1,11 +1,122 @@
-json  = import("json.noja");
-utils = import("../utils.noja");
+json = import("json.noja");
 
-assert(json.parse("null") == none);
-assert(json.parse("true") == true);
-assert(json.parse("false") == false);
-assert(json.parse('"Hello, world!"') == "Hello, world!");
-assert(utils.compareAny(json.parse('[]'), []));
-assert(utils.compareAny(json.parse('[1, 2, 3]'), [1, 2, 3]));
+Error = String;
+JSONValueType = (bool | float | String | Map | List | None);
+TestCase = [String, ?JSONValueType, ?Error];
 
-print("No assertions fired!\n");
+fun compareLists(a: List, b: List) {
+    len = count(a);
+    if len != count(b):
+        return false;
+    i = 0;
+    while i < len: {
+        if a[i] != b[i]:
+            return false;
+        i = i+1;
+    }
+    return true;
+}
+
+fun compareMaps(a: Map, b: Map) {
+    keys = keysof(a);
+    len  = count(keys);
+    if len != count(b):
+        return false;
+    i = 0;
+    while i < len: {
+        k = keys[i];
+        if a[k] != b[k]:
+            return false;
+        i = i+1;
+    }
+    return true;
+}
+
+fun compareAny(a, b) {
+    Ta = type(a);
+    Tb = type(b);
+
+    if Ta != Tb:
+        return false;
+    if Ta == List:
+        return compareLists(a, b);
+    if Ta == Map:
+        return compareMaps(a, b);
+    return a == b;
+}
+
+fun runTestCase(test_case: TestCase) {
+    res, err = json.parse(test_case[0]);
+    return compareAny(res, test_case[1])
+       and compareAny(err, test_case[2]);
+}
+
+fun runAllTestCases(test_cases: List) {
+    total  = count(test_cases);
+    passed = 0;
+    evaluated = 0;
+    while evaluated < total: {
+        
+        test_case = test_cases[evaluated];
+        input  = test_case[0];
+        expres = test_case[1]; # Expected result
+        experr = test_case[2]; # Expected error
+
+        res, err = json.parse(input);
+        
+        #print("res=", res, ", err=", err, "\n");
+        #print("expres=", expres, ", experr=", experr, "\n");
+        
+        result = compareAny(res, expres)
+             and compareAny(err, experr);
+
+        if result: {
+            print("Test ", evaluated, ": PASSED\n");
+            passed = passed+1;
+        } else {
+
+            print("Test ", evaluated, ": FAILED\n");
+            print("\tInput:\n");
+            print("\t\t", input, "\n");
+
+            if experr == none: {
+                if err == none:
+                    print("\tGot result ", res, " but was expected ", expres, "\n");
+                else
+                    print("\tFailed unexpectedly [", err, "]\n");
+            } else if err == none:
+                print("\tSucceded unexpectedly. Was expected error [", experr, "]\n");
+            else
+                print("\tWas expected error [", experr, "], but got [", err, "]\n");
+        }
+        evaluated = evaluated+1;
+    }
+    return passed, (total-passed), total;
+}
+
+fun newPassingTestCase(src: String, res: JSONValueType)
+    return [src, res, none];
+
+fun newFailingTestCase(src: String, err: String)
+    return [src, none, err];
+
+test_cases = [
+    newPassingTestCase("null", none),
+    newPassingTestCase("true", true),
+    newPassingTestCase("false", false),
+    newPassingTestCase("\"Hello, world!\"", "Hello, world!"),
+    newPassingTestCase("1", 1.0),
+    newPassingTestCase("2.3", 2.3),
+    newPassingTestCase("[]", []),
+    newPassingTestCase("[1]", [1.0]),
+    newPassingTestCase("[1, 2]", [1.0, 2.0]),
+    newPassingTestCase("[1, 2, 3]", [1.0, 2.0, 3.0]),
+    newPassingTestCase("{}", {}),
+    newPassingTestCase("{\"k1\": true}", {k1: true}),
+    newPassingTestCase("{\"k1\": true, \"k2\": false}", {k1: true, k2: false})
+];
+
+passed, failed, total = runAllTestCases(test_cases);
+print("passed: ", passed, "\n");
+print("failed: ", failed, "\n");
+print(" total: ", total,  "\n");
diff --git a/src/lib/builtins/basic.c b/src/lib/builtins/basic.c
index de0115b..708676d 100644
--- a/src/lib/builtins/basic.c
+++ b/src/lib/builtins/basic.c
@@ -317,6 +317,20 @@ static int bin_istypeof(Runtime *runtime, Object **argv, unsigned int argc, Obje
 	return 1;
 }
 
+static int bin_keysof(Runtime *runtime, Object **argv, unsigned int argc, Object *rets[static MAX_RETS], Error *error)
+{
+	UNUSED(argc);
+	UNUSED(rets);
+	UNUSED(runtime);
+	ASSERT(argc == 1);
+	Heap   *heap = Runtime_GetHeap(runtime);
+	Object *keys = Object_KeysOf(argv[0], heap, error);
+	if (keys == NULL)
+		return -1;
+	rets[0] = keys;
+	return 1;
+}
+
 void bins_basic_init(StaticMapSlot slots[])
 {
 	slots[0].as_type = Object_GetTypeType();
@@ -361,5 +375,6 @@ StaticMapSlot bins_basic[] = {
 	{ "error",  SM_FUNCT, .as_funct = bin_error, .argc = 1 },
 	{ "assert", SM_FUNCT, .as_funct = bin_assert, .argc = -1 },
 	{ "istypeof", SM_FUNCT, .as_funct = bin_istypeof, .argc = 2, },
+	{ "keysof", SM_FUNCT, .as_funct = bin_keysof, .argc = 1, },
 	{ NULL, SM_END, {}, {} },
 };
diff --git a/src/lib/objects/o_buffer.c b/src/lib/objects/o_buffer.c
index 9cc43cc..845c80c 100644
--- a/src/lib/objects/o_buffer.c
+++ b/src/lib/objects/o_buffer.c
@@ -274,4 +274,14 @@ static void buffer_print(Object *self, FILE *fp)
 {
 	BufferObject *buffer = (BufferObject*) self;
 	print_bytes(fp, buffer->payload->body + buffer->offset, buffer->length);
+}
+
+static Object*
+keysof(Object *self, 
+	   Heap   *heap, 
+	   Error  *error)
+{
+	BufferObject *buffer = (BufferObject*) self;
+	int count = buffer->length;
+	return Object_NewListOfConsecutiveIntegers(0, count-1, heap, error);
 }
\ No newline at end of file
diff --git a/src/lib/objects/o_list.c b/src/lib/objects/o_list.c
index f0d4ef8..1ea3c32 100644
--- a/src/lib/objects/o_list.c
+++ b/src/lib/objects/o_list.c
@@ -48,6 +48,7 @@ static void    walkexts(Object *self, void (*callback)(void   **referer, unsigne
 static Object *copy(Object *self, Heap *heap, Error *err);
 static int hash(Object *self);
 static bool istypeof(Object *self, Object *other, Heap *heap, Error *error);
+static Object* keysof(Object *self, Heap *heap, Error  *error);
 
 static TypeObject t_list = {
 	.base = (Object) { .type = &t_type, .flags = Object_STATIC },
@@ -59,6 +60,7 @@ static TypeObject t_list = {
 	.insert = insert,
 	.count = count,
 	.print = print,
+	.keysof = keysof,
 	.istypeof = istypeof,
 	.walk = walk,
 	.walkexts = walkexts,
@@ -92,6 +94,16 @@ istypeof(Object *self,
     return true;
 }
 
+static Object*
+keysof(Object *self, 
+	   Heap   *heap, 
+	   Error  *error)
+{
+	ListObject *list = (ListObject*) self;
+	int count = list->count;
+	return Object_NewListOfConsecutiveIntegers(0, count-1, heap, error);
+}
+
 static int hash(Object *self)
 {
 	ASSERT(self != NULL);
@@ -303,4 +315,33 @@ static void print(Object *self, FILE *fp)
 			fprintf(fp, ", ");
 	}
 	fprintf(fp, "]");
+}
+
+Object *Object_NewListOfConsecutiveIntegers(int first, int last, Heap *heap, Error *error)
+{
+	int count = last - first + 1;
+	
+	Object *list = Object_NewList(count, heap, error);
+	if (list == NULL)
+		return NULL;
+
+	for (int i = 0; i < count; i++) {
+
+		Object *key = Object_FromInt(i, heap, error);
+		if (key == NULL)
+			return NULL;
+
+		Object *val;
+		if (first == 0)
+			val = key;
+		else {
+			val = Object_FromInt(first+i, heap, error);
+			if (val == NULL)
+				return NULL;
+		}
+
+		if (!Object_Insert(list, key, val, heap, error))
+			return NULL;
+	}
+	return list;
 }
\ No newline at end of file
diff --git a/src/lib/objects/o_map.c b/src/lib/objects/o_map.c
index 9cd8d8c..e7c0793 100644
--- a/src/lib/objects/o_map.c
+++ b/src/lib/objects/o_map.c
@@ -48,7 +48,8 @@ static void walk(Object *self, void (*callback)(Object **referer, void *userp),
 static void walkexts(Object *self, void (*callback)(void **referer, unsigned int size, void *userp), void *userp);
 static Object *copy(Object *self, Heap *heap, Error *err);
 static int hash(Object *self);
-static bool istypeof(Object *self, Object *other, Heap   *heap, Error  *error);
+static bool istypeof(Object *self, Object *other, Heap *heap, Error *error);
+static Object *keysof(Object *self, Heap *heap, Error *error);
 
 static TypeObject t_map = {
 	.base = (Object) { .type = &t_type, .flags = Object_STATIC },
@@ -60,11 +61,21 @@ static TypeObject t_map = {
 	.insert = insert,
 	.count = count,
 	.print = print,
+	.keysof = keysof,
 	.istypeof = istypeof,
 	.walk = walk,
 	.walkexts = walkexts,
 };
 
+static Object*
+keysof(Object *self, 
+	   Heap   *heap, 
+	   Error  *error)
+{
+	MapObject *map = (MapObject*) self;
+	return Object_NewList2(map->count, map->keys, heap, error);
+}
+
 static bool 
 istypeof(Object *self, 
          Object *other,
diff --git a/src/lib/objects/o_string.c b/src/lib/objects/o_string.c
index 6924ce2..d861a2d 100644
--- a/src/lib/objects/o_string.c
+++ b/src/lib/objects/o_string.c
@@ -49,6 +49,7 @@ static void print(Object *obj, FILE *fp);
 static _Bool op_eql(Object *self, Object *other);
 static void walkexts(Object *self, void (*callback)(void **referer, unsigned int size, void *userp), void *userp);
 static Object *select_(Object *self, Object *key, Heap *heap, Error *error);
+static Object *keysof(Object *self, Heap *heap, Error *error);
 
 static TypeObject t_string = {
 	.base = (Object) { .type = &t_type, .flags = Object_STATIC },
@@ -60,6 +61,7 @@ static TypeObject t_string = {
 	.print = print,
 	.select = select_,
 	.op_eql = op_eql,
+	.keysof = keysof,
 	.walkexts = walkexts,
 };
 
@@ -231,4 +233,14 @@ static void walkexts(Object *self, void (*callback)(void **referer, unsigned int
 	StringObject *str = (StringObject*) self;
 	
 	callback((void**) &str->body, str->bytes+1, userp);
-}
\ No newline at end of file
+}
+
+static Object*
+keysof(Object *self, 
+	   Heap   *heap, 
+	   Error  *error)
+{
+	StringObject *str = (StringObject*) self;
+	int count = str->count;
+	return Object_NewListOfConsecutiveIntegers(0, count-1, heap, error);
+}
diff --git a/src/lib/objects/o_sum.c b/src/lib/objects/o_sum.c
index cca70d6..13f6a55 100644
--- a/src/lib/objects/o_sum.c
+++ b/src/lib/objects/o_sum.c
@@ -2,6 +2,7 @@
 #include "objects.h"
 
 static void walk(Object *self, void (*callback)(Object **referer, void *userp), void *userp);
+static void print(Object *self, FILE *fp);
 static bool istypeof(Object *self, Object *other, Heap *heap, Error *error);
 
 typedef struct {
@@ -18,7 +19,7 @@ static TypeObject t_sum = {
     .hash = NULL,
     .copy = NULL,
     .call = NULL,
-    .print = NULL,
+    .print = print,
 
     .select = NULL,
     .delete = NULL,
@@ -64,3 +65,11 @@ static void walk(Object *self, void (*callback)(Object **referer, void *userp),
     callback(sum->items + 0, userp);
     callback(sum->items + 1, userp);
 }
+
+static void print(Object *self, FILE *fp)
+{
+    SumObject *sum = (SumObject*) self;
+    Object_Print(sum->items[0], fp);
+    fprintf(fp, " | ");
+    Object_Print(sum->items[1], fp);
+}
diff --git a/src/lib/objects/objects.c b/src/lib/objects/objects.c
index 2c77dfc..3c274a6 100644
--- a/src/lib/objects/objects.c
+++ b/src/lib/objects/objects.c
@@ -88,6 +88,16 @@ const TypeObject *Object_GetType(const Object *obj)
 	return obj->type;
 }
 
+Object*
+Object_KeysOf(Object *self, 
+			  Heap *heap, 
+			  Error *error)
+{
+	if (self->type->keysof == NULL)
+		return Object_NewNone(heap, error);
+	return self->type->keysof(self, heap, error);
+}
+
 bool Object_IsTypeOf(Object *typ, Object *obj, Heap *heap, Error *error)
 {
 	if (typ->type->istypeof == NULL)
diff --git a/src/lib/objects/objects.h b/src/lib/objects/objects.h
index 45d3844..778c777 100644
--- a/src/lib/objects/objects.h
+++ b/src/lib/objects/objects.h
@@ -71,6 +71,7 @@ struct TypeObject {
 	Object *(*delete)(Object *self, Object *key, Heap *heap, Error *err);
 	bool    (*insert)(Object *self, Object *key, Object *val, Heap *heap, Error *err);
 	int 	(*count)(Object *self);
+	Object *(*keysof)(Object *self, Heap *heap, Error *error);
 
 	// Types.
 	bool (*istypeof)(Object *self, Object *other, Heap *heap, Error *error);
@@ -104,6 +105,7 @@ int 		 Object_Hash (Object *obj);
 Object*		 Object_Copy (Object *obj, Heap *heap, Error *err);
 int          Object_Call (Object *obj, Object **argv, unsigned int argc, Object *rets[static MAX_RETS], Heap *heap, Error *err);
 void 		 Object_Print(Object *obj, FILE *fp);
+Object* 	 Object_KeysOf(Object *self, Heap *heap, Error *error);
 bool 		 Object_IsTypeOf(Object *typ, Object *obj, Heap *heap, Error *error);
 Object*		 Object_Select(Object *coll, Object *key, Heap *heap, Error *err);
 Object*		 Object_Delete(Object *coll, Object *key, Heap *heap, Error *err);
@@ -115,6 +117,7 @@ void 		 Object_WalkExtensions(Object *parent, void (*callback)(void   **referer,
 Object*		 Object_NewMap(int num, Heap *heap, Error *error);
 Object*		 Object_NewList(int capacity, Heap *heap, Error *error);
 Object*		 Object_NewList2(int num, Object **items, Heap *heap, Error *error);
+Object*		 Object_NewListOfConsecutiveIntegers(int first, int last, Heap *heap, Error *error);
 Object*		 Object_NewNone(Heap *heap, Error *error);
 Object*      Object_NewBuffer(size_t size, Heap *heap, Error *error);
 Object*		 Object_NewClosure(Object *parent, Object *new_map, Heap *heap, Error *error);