diff --git a/oop/types.lua b/oop/types.lua
index 6fdd12a..ef36efd 100644
--- a/oop/types.lua
+++ b/oop/types.lua
@@ -2,9 +2,31 @@ local oop = require("oop")
 
 local Types = {}
 
+local function freeze_table(t)
+    return setmetatable({}, {
+        __index = t,
+        __newindex = function()
+            error("Tuple is immutable", 2)
+        end,
+        __len = function()
+            return #t
+        end,
+        __pairs = function()
+            return pairs(t)
+        end,
+        __ipairs = function()
+            return ipairs(t)
+        end
+    })
+end
+
 local function IntDef(cls)
+    local base_newindex = cls.__newindex
+
     function cls.__init(this, value)
+        rawset(this, "__frozen", false)
         this.value = math.floor(tonumber(value) or 0)
+        rawset(this, "__frozen", true)
     end
 
     function cls.__add(a, b)
@@ -36,11 +58,22 @@ local function IntDef(cls)
     function cls.to_number(this)
         return this.value
     end
+
+    function cls.__newindex(this, key, value)
+        if rawget(this, "__frozen") and key ~= "__fields" then
+            error("Int is immutable", 2)
+        end
+        return base_newindex(this, key, value)
+    end
 end
 
 local function FloatDef(cls)
+    local base_newindex = cls.__newindex
+
     function cls.__init(this, value)
+        rawset(this, "__frozen", false)
         this.value = tonumber(value) or 0.0
+        rawset(this, "__frozen", true)
     end
 
     function cls.__add(a, b)
@@ -72,6 +105,13 @@ local function FloatDef(cls)
     function cls.to_number(this)
         return this.value
     end
+
+    function cls.__newindex(this, key, value)
+        if rawget(this, "__frozen") and key ~= "__fields" then
+            error("Float is immutable", 2)
+        end
+        return base_newindex(this, key, value)
+    end
 end
 
 local function ListDef(cls)
@@ -161,10 +201,11 @@ local function TupleDef(cls)
     local base_newindex = cls.__newindex
 
     function cls.__init(this, items)
+        rawset(this, "__frozen", false)
         if type(items) == "table" then
-            this.items = { table.unpack(items) }
+            this.items = freeze_table({ table.unpack(items) })
         else
-            this.items = {}
+            this.items = freeze_table({})
         end
         rawset(this, "__frozen", true)
     end
@@ -372,8 +413,12 @@ local function SetDef(cls)
 end
 
 local function BoolDef(cls)
+    local base_newindex = cls.__newindex
+
     function cls.__init(this, value)
+        rawset(this, "__frozen", false)
         this.value = not not value
+        rawset(this, "__frozen", true)
     end
 
     function cls.__tostring(this)
@@ -383,11 +428,22 @@ local function BoolDef(cls)
     function cls.__eq(a, b)
         return a.value == b.value
     end
+
+    function cls.__newindex(this, key, value)
+        if rawget(this, "__frozen") and key ~= "__fields" then
+            error("Bool is immutable", 2)
+        end
+        return base_newindex(this, key, value)
+    end
 end
 
 local function StrDef(cls)
+    local base_newindex = cls.__newindex
+
     function cls.__init(this, value)
+        rawset(this, "__frozen", false)
         this.value = tostring(value or "")
+        rawset(this, "__frozen", true)
     end
 
     function cls.__len(this)
@@ -401,6 +457,13 @@ local function StrDef(cls)
     function cls.__tostring(this)
         return this.value
     end
+
+    function cls.__newindex(this, key, value)
+        if rawget(this, "__frozen") and key ~= "__fields" then
+            error("Str is immutable", 2)
+        end
+        return base_newindex(this, key, value)
+    end
 end
 
 Types.Int = oop.class(IntDef)