diff --git a/compiler/erg_compiler/codegen.rs b/compiler/erg_compiler/codegen.rs
index 17877638..97b18950 100644
--- a/compiler/erg_compiler/codegen.rs
+++ b/compiler/erg_compiler/codegen.rs
@@ -133,6 +133,7 @@ pub struct PyCodeGenerator {
     pub(crate) py_version: PythonVersion,
     str_cache: CacheSet<str>,
     prelude_loaded: bool,
+    mutate_op_loaded: bool,
     in_op_loaded: bool,
     record_type_loaded: bool,
     module_type_loaded: bool,
@@ -149,6 +150,7 @@ impl PyCodeGenerator {
             cfg,
             str_cache: CacheSet::new(),
             prelude_loaded: false,
+            mutate_op_loaded: false,
             in_op_loaded: false,
             record_type_loaded: false,
             module_type_loaded: false,
@@ -426,7 +428,8 @@ impl PyCodeGenerator {
     }
 
     fn emit_load_const<C: Into<ValueObj>>(&mut self, cons: C) {
-        let value = cons.into();
+        let value: ValueObj = cons.into();
+        let is_int = value.is_int();
         let is_nat = value.is_nat();
         let is_bool = value.is_bool();
         if !self.cfg.no_std {
@@ -436,6 +439,9 @@ impl PyCodeGenerator {
             } else if is_nat {
                 self.emit_push_null();
                 self.emit_load_name_instr(Identifier::public("Nat"));
+            } else if is_int {
+                self.emit_push_null();
+                self.emit_load_name_instr(Identifier::public("Int"));
             }
         }
         let idx = self
@@ -450,7 +456,8 @@ impl PyCodeGenerator {
         self.write_instr(LOAD_CONST);
         self.write_arg(idx);
         self.stack_inc();
-        if !self.cfg.no_std && is_nat {
+        if !self.cfg.no_std && is_int {
+            // is_int => is_nat and is_bool
             self.emit_call_instr(1, Name);
             self.stack_dec();
         }
@@ -1258,12 +1265,18 @@ impl PyCodeGenerator {
     fn emit_unaryop(&mut self, unary: UnaryOp) {
         log!(info "entered {} ({unary})", fn_name!());
         let tycode = TypeCode::from(unary.lhs_t());
-        self.emit_expr(*unary.expr);
         let instr = match &unary.op.kind {
             // TODO:
             TokenKind::PrePlus => UNARY_POSITIVE,
             TokenKind::PreMinus => UNARY_NEGATIVE,
-            TokenKind::Mutate => NOP, // ERG_MUTATE,
+            TokenKind::Mutate => {
+                if !self.mutate_op_loaded {
+                    self.load_mutate_op();
+                }
+                self.emit_push_null();
+                self.emit_load_name_instr(Identifier::private("#mutate_operator"));
+                NOP // ERG_MUTATE,
+            }
             _ => {
                 CompileError::feature_error(
                     self.cfg.input.clone(),
@@ -1275,8 +1288,14 @@ impl PyCodeGenerator {
                 NOT_IMPLEMENTED
             }
         };
-        self.write_instr(instr);
-        self.write_arg(tycode as usize);
+        self.emit_expr(*unary.expr);
+        if instr != NOP {
+            self.write_instr(instr);
+            self.write_arg(tycode as usize);
+        } else {
+            self.emit_precall_and_call(1);
+            self.stack_dec();
+        }
     }
 
     fn emit_binop(&mut self, bin: BinOp) {
@@ -1792,7 +1811,7 @@ impl PyCodeGenerator {
                     if i != 0 && i != len - 1 {
                         self.dup_top();
                     }
-                    self.emit_load_const(i as i32);
+                    self.emit_load_const(i);
                     self.write_instr(Opcode311::BINARY_SUBSCR);
                     self.write_arg(0);
                     self.stack_dec();
@@ -1806,7 +1825,7 @@ impl PyCodeGenerator {
                     if i != 0 && i != len - 1 {
                         self.dup_top();
                     }
-                    self.emit_load_const(i as i32);
+                    self.emit_load_const(i);
                     self.write_instr(Opcode311::BINARY_SUBSCR);
                     self.write_arg(0);
                     self.stack_dec();
@@ -2695,10 +2714,15 @@ impl PyCodeGenerator {
     }
 
     fn load_prelude(&mut self) {
+        // NOTE: Integers need to be used in IMPORT_NAME
+        // but `Int` are called before importing it, so they need to be no_std mode
+        let no_std = self.cfg.no_std;
+        self.cfg.no_std = true;
         self.load_record_type();
         self.load_prelude_py();
         self.prelude_loaded = true;
         self.record_type_loaded = true;
+        self.cfg.no_std = no_std;
     }
 
     fn load_in_op(&mut self) {
@@ -2717,6 +2741,22 @@ impl PyCodeGenerator {
         self.in_op_loaded = true;
     }
 
+    fn load_mutate_op(&mut self) {
+        let mod_name = if self.py_version.minor >= Some(10) {
+            Identifier::public("_erg_std_prelude")
+        } else {
+            Identifier::public("_erg_std_prelude_old")
+        };
+        self.emit_global_import_items(
+            mod_name,
+            vec![(
+                Identifier::public("mutate_operator"),
+                Some(Identifier::private("#mutate_operator")),
+            )],
+        );
+        self.mutate_op_loaded = true;
+    }
+
     fn load_control(&mut self) {
         let mod_name = Identifier::public("_erg_control");
         self.emit_import_all_instr(mod_name);
diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index 99804ddf..29e7b4fe 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -773,6 +773,8 @@ impl Context {
         // class("Rational"),
         // class("Integral"),
         int.register_builtin_impl("abs", fn0_met(Int, Nat), Immutable, Public);
+        int.register_builtin_py_impl("succ", fn0_met(Int, Int), Immutable, Public, Some("succ"));
+        int.register_builtin_py_impl("pred", fn0_met(Int, Int), Immutable, Public, Some("pred"));
         let mut int_ord = Self::builtin_methods(Some(mono("Ord")), 2);
         int_ord.register_builtin_impl(
             "__partial_cmp__",
@@ -1328,6 +1330,9 @@ impl Context {
         let mut int_mut = Self::builtin_mono_class("Int!", 2);
         int_mut.register_superclass(Int, &int);
         int_mut.register_superclass(mono("Float!"), &float_mut);
+        let t = pr_met(mono("Int!"), vec![], None, vec![kw("i", Int)], NoneType);
+        int_mut.register_builtin_py_impl("inc!", t.clone(), Immutable, Public, Some("inc"));
+        int_mut.register_builtin_py_impl("dec!", t, Immutable, Public, Some("dec"));
         let mut int_mut_mutable = Self::builtin_methods(Some(mono("Mutable")), 2);
         int_mut_mutable.register_builtin_const("ImmutType", Public, ValueObj::builtin_t(Int));
         let f_t = kw("func", func(vec![kw("old", Int)], None, vec![], Int));
diff --git a/compiler/erg_compiler/lib/std/_erg_int.py b/compiler/erg_compiler/lib/std/_erg_int.py
new file mode 100644
index 00000000..78b0e413
--- /dev/null
+++ b/compiler/erg_compiler/lib/std/_erg_int.py
@@ -0,0 +1,85 @@
+from _erg_result import Error
+
+class Int(int):
+    def try_new(i): # -> Result[Nat]
+        if isinstance(i, int):
+            Int(i)
+        else:
+            Error("not an integer")
+    def succ(self):
+        return Int(self + 1)
+    def pred(self):
+        return Int(self - 1)
+    def mutate(self):
+        return IntMut(self)
+
+class IntMut(): # inherits Int
+    value: Int
+
+    def __init__(self, i):
+        self.value = Int(i)
+    def __repr__(self):
+        return self.value.__repr__()
+    def __eq__(self, other):
+        if isinstance(other, Int):
+            return self.value == other
+        else:
+            return self.value == other.value
+    def __ne__(self, other):
+        if isinstance(other, Int):
+            return self.value != other
+        else:
+            return self.value != other.value
+    def __le__(self, other):
+        if isinstance(other, Int):
+            return self.value <= other
+        else:
+            return self.value <= other.value
+    def __ge__(self, other):
+        if isinstance(other, Int):
+            return self.value >= other
+        else:
+            return self.value >= other.value
+    def __lt__(self, other):
+        if isinstance(other, Int):
+            return self.value < other
+        else:
+            return self.value < other.value
+    def __gt__(self, other):
+        if isinstance(other, Int):
+            return self.value > other
+        else:
+            return self.value > other.value
+    def __add__(self, other):
+        if isinstance(other, Int):
+            return IntMut(self.value + other)
+        else:
+            return IntMut(self.value + other.value)
+    def __sub__(self, other):
+        if isinstance(other, Int):
+            return IntMut(self.value - other)
+        else:
+            return IntMut(self.value - other.value)
+    def __mul__(self, other):
+        if isinstance(other, Int):
+            return IntMut(self.value * other)
+        else:
+            return IntMut(self.value * other.value)
+    def __floordiv__(self, other):
+        if isinstance(other, Int):
+            return IntMut(self.value // other)
+        else:
+            return IntMut(self.value // other.value)
+    def __pow__(self, other):
+        if isinstance(other, Int):
+            return IntMut(self.value ** other)
+        else:
+            return IntMut(self.value ** other.value)
+    def inc(self, i=1):
+        self.value = Int(self.value + i)
+    def dec(self, i=1):
+        self.value = Int(self.value - i)
+    def succ(self):
+        return self.value.succ()
+    def pred(self):
+        return self.value.pred()
diff --git a/compiler/erg_compiler/lib/std/_erg_mutate_operator.py b/compiler/erg_compiler/lib/std/_erg_mutate_operator.py
new file mode 100644
index 00000000..e5753140
--- /dev/null
+++ b/compiler/erg_compiler/lib/std/_erg_mutate_operator.py
@@ -0,0 +1,5 @@
+def mutate_operator(x):
+    if hasattr(x, 'mutate'):
+        return x.mutate()
+    else:
+        return x
diff --git a/compiler/erg_compiler/lib/std/_erg_nat.py b/compiler/erg_compiler/lib/std/_erg_nat.py
index f13e0820..4cadd671 100644
--- a/compiler/erg_compiler/lib/std/_erg_nat.py
+++ b/compiler/erg_compiler/lib/std/_erg_nat.py
@@ -1,6 +1,8 @@
 from _erg_result import Error
+from _erg_int import Int
+from _erg_int import IntMut
 
-class Nat(int):
+class Nat(Int):
     def try_new(i): # -> Result[Nat]
         if i >= 0:
             return Nat(i)
@@ -16,3 +18,67 @@ class Nat(int):
             return self - other
         else:
             return 0
+    def mutate(self):
+        return NatMut(self)
+
+class NatMut(IntMut): # and Nat
+    value: Nat
+
+    def __init__(self, n):
+        self.value = n
+    def __repr__(self):
+        return self.value.__repr__()
+    def __eq__(self, other):
+        if isinstance(other, Int):
+            return self.value == other
+        else:
+            return self.value == other.value
+    def __ne__(self, other):
+        if isinstance(other, Int):
+            return self.value != other
+        else:
+            return self.value != other.value
+    def __le__(self, other):
+        if isinstance(other, Int):
+            return self.value <= other
+        else:
+            return self.value <= other.value
+    def __ge__(self, other):
+        if isinstance(other, Int):
+            return self.value >= other
+        else:
+            return self.value >= other.value
+    def __lt__(self, other):
+        if isinstance(other, Int):
+            return self.value < other
+        else:
+            return self.value < other.value
+    def __gt__(self, other):
+        if isinstance(other, Int):
+            return self.value > other
+        else:
+            return self.value > other.value
+    def __add__(self, other):
+        if isinstance(other, Nat):
+            return NatMut(self.value + other)
+        else:
+            return NatMut(self.value + other.value)
+    def __mul__(self, other):
+        if isinstance(other, Nat):
+            return NatMut(self.value * other)
+        else:
+            return NatMut(self.value * other.value)
+    def __pow__(self, other):
+        if isinstance(other, Nat):
+            return NatMut(self.value ** other)
+        else:
+            return NatMut(self.value ** other.value)
+    def try_new(i): # -> Result[Nat]
+        if i >= 0:
+            return NatMut(i)
+        else:
+            return Error("Nat can't be negative")
+
+    def times(self, f):
+        for _ in range(self.value):
+            f()
diff --git a/compiler/erg_compiler/lib/std/_erg_range.py b/compiler/erg_compiler/lib/std/_erg_range.py
index 6a0914d5..933958dd 100644
--- a/compiler/erg_compiler/lib/std/_erg_range.py
+++ b/compiler/erg_compiler/lib/std/_erg_range.py
@@ -69,7 +69,7 @@ class RangeIterator:
                 self.needle = chr(ord(self.needle) + 1)
         else:
             if not(self.needle in self.rng):
-                self.needle = self.needle.incremented()
+                self.needle = self.needle.succ()
 
     def __iter__(self):
         return self
@@ -88,7 +88,7 @@ class RangeIterator:
         else:
             if self.needle in self.rng:
                 result = self.needle
-                self.needle = self.needle.incremented()
+                self.needle = self.needle.succ()
                 return result
         raise StopIteration
 
diff --git a/compiler/erg_compiler/lib/std/_erg_std_prelude.py b/compiler/erg_compiler/lib/std/_erg_std_prelude.py
index c7664816..47c65d1c 100644
--- a/compiler/erg_compiler/lib/std/_erg_std_prelude.py
+++ b/compiler/erg_compiler/lib/std/_erg_std_prelude.py
@@ -1,7 +1,9 @@
 from _erg_range import Range, LeftOpenRange, RightOpenRange, OpenRange, ClosedRange, RangeIterator
 from _erg_result import Result, Error, is_ok
-from _erg_nat import Nat
+from _erg_int import Int, IntMut
+from _erg_nat import Nat, NatMut
 from _erg_bool import Bool
-from _erg_str import Str
+from _erg_str import Str, StrMut
 from _erg_array import Array
 from _erg_in_operator import in_operator
+from _erg_mutate_operator import mutate_operator
diff --git a/compiler/erg_compiler/lib/std/_erg_std_prelude_old.py b/compiler/erg_compiler/lib/std/_erg_std_prelude_old.py
index ec67f54a..b8a17aff 100644
--- a/compiler/erg_compiler/lib/std/_erg_std_prelude_old.py
+++ b/compiler/erg_compiler/lib/std/_erg_std_prelude_old.py
@@ -54,7 +54,6 @@ class Bool(Nat):
 
 class Str(str):
     def __instancecheck__(cls, obj):
-        print(cls, obj)
         return obj == Str or obj == str
 
     def try_new(s: str): # -> Result[Nat]
diff --git a/compiler/erg_compiler/lib/std/_erg_str.py b/compiler/erg_compiler/lib/std/_erg_str.py
index 6d278376..183015eb 100644
--- a/compiler/erg_compiler/lib/std/_erg_str.py
+++ b/compiler/erg_compiler/lib/std/_erg_str.py
@@ -13,27 +13,47 @@ class Str(str):
             return Str(self[i])
         else:
             return None
+    def mutate(self):
+        return StrMut(self)
 
-class StrMut(Str):
+class StrMut(): # Inherits Str
+    value: Str
+
+    def __init__(self, s: str):
+        self.value = s
+    def __repr__(self):
+        return self.value.__repr__()
+    def __eq__(self, other):
+        if isinstance(other, Str):
+            return self.value == other
+        else:
+            return self.value == other.value
+    def __ne__(self, other):
+        if isinstance(other, Str):
+            return self.value != other
+        else:
+            return self.value != other.value
     def try_new(s: str):
         if isinstance(s, str):
-            return StrMut(s)
+            self = StrMut()
+            self.value = s
+            return self
         else:
             return Error("Str! can't be other than str")
     def clear(self):
-        self = ""
+        self.value = ""
     def pop(self):
-        if len(self) > 0:
-            last = self[-1]
-            self = self[:-1]
+        if len(self.value) > 0:
+            last = self.value[-1]
+            self.value = self.value[:-1]
             return last
         else:
             return Error("Can't pop from empty `Str!`")
     def push(self, c: str):
-        self += c
+        self.value += c
     def remove(self, idx: int):
-        char = self[idx]
-        self = self[:idx] + self[idx+1:]
+        char = self.value[idx]
+        self.value = self.value[:idx] + self.value[idx+1:]
         return char
     def insert(self, idx: int, c: str):
-        self = self[:idx] + c + self[idx:]
+        self.value = self.value[:idx] + c + self.value[idx:]
diff --git a/compiler/erg_compiler/transpile.rs b/compiler/erg_compiler/transpile.rs
index a3255ea6..a104414a 100644
--- a/compiler/erg_compiler/transpile.rs
+++ b/compiler/erg_compiler/transpile.rs
@@ -185,6 +185,7 @@ pub struct ScriptGenerator {
     level: usize,
     fresh_var_n: usize,
     namedtuple_loaded: bool,
+    mutate_op_loaded: bool,
     in_op_loaded: bool,
     range_ops_loaded: bool,
     builtin_types_loaded: bool,
@@ -197,6 +198,7 @@ impl ScriptGenerator {
             level: 0,
             fresh_var_n: 0,
             namedtuple_loaded: false,
+            mutate_op_loaded: false,
             in_op_loaded: false,
             range_ops_loaded: false,
             builtin_types_loaded: false,
@@ -220,12 +222,14 @@ impl ScriptGenerator {
     // TODO: more smart way
     fn replace_import(src: &str) -> String {
         src.replace("from _erg_nat import Nat", "")
-            .replace("from _erg_result import Error", "")
-            .replace("from _erg_result import is_ok", "")
+            .replace("from _erg_int import IntMut", "")
+            .replace("from _erg_int import Int", "")
             .replace("from _erg_bool import Bool", "")
             .replace("from _erg_str import Str", "")
             .replace("from _erg_array import Array", "")
             .replace("from _erg_range import Range", "")
+            .replace("from _erg_result import Error", "")
+            .replace("from _erg_result import is_ok", "")
     }
 
     fn load_namedtuple(&mut self) {
@@ -235,6 +239,7 @@ impl ScriptGenerator {
     // TODO: name escaping
     fn load_range_ops(&mut self) {
         self.prelude += &Self::replace_import(include_str!("lib/std/_erg_result.py"));
+        self.prelude += &Self::replace_import(include_str!("lib/std/_erg_int.py"));
         self.prelude += &Self::replace_import(include_str!("lib/std/_erg_nat.py"));
         self.prelude += &Self::replace_import(include_str!("lib/std/_erg_str.py"));
         self.prelude += &Self::replace_import(include_str!("lib/std/_erg_range.py"));
@@ -246,16 +251,22 @@ impl ScriptGenerator {
         self.prelude += &Self::replace_import(include_str!("lib/std/_erg_in_operator.py"));
     }
 
+    fn load_mutate_op(&mut self) {
+        self.prelude += &Self::replace_import(include_str!("lib/std/_erg_mutate_operator.py"));
+    }
+
     fn load_builtin_types(&mut self) {
         if self.range_ops_loaded {
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_array.py"));
         } else if self.in_op_loaded {
+            self.prelude += &Self::replace_import(include_str!("lib/std/_erg_int.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_nat.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_bool.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_str.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_array.py"));
         } else {
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_result.py"));
+            self.prelude += &Self::replace_import(include_str!("lib/std/_erg_int.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_nat.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_bool.py"));
             self.prelude += &Self::replace_import(include_str!("lib/std/_erg_str.py"));
@@ -265,7 +276,20 @@ impl ScriptGenerator {
 
     fn transpile_expr(&mut self, expr: Expr) -> String {
         match expr {
-            Expr::Lit(lit) => lit.token.content.to_string(),
+            Expr::Lit(lit) => {
+                if matches!(
+                    &lit.value,
+                    ValueObj::Bool(_) | ValueObj::Int(_) | ValueObj::Nat(_) | ValueObj::Str(_)
+                ) {
+                    if !self.builtin_types_loaded {
+                        self.load_builtin_types();
+                        self.builtin_types_loaded = true;
+                    }
+                    format!("{}({})", lit.value.class(), lit.token.content)
+                } else {
+                    lit.token.content.to_string()
+                }
+            }
             Expr::Call(call) => self.transpile_call(call),
             Expr::BinOp(bin) => match bin.op.kind {
                 TokenKind::Closed
@@ -314,8 +338,15 @@ impl ScriptGenerator {
                 }
             },
             Expr::UnaryOp(unary) => {
-                let mut code = "(".to_string();
-                if unary.op.kind != TokenKind::Mutate {
+                let mut code = "".to_string();
+                if unary.op.kind == TokenKind::Mutate {
+                    if !self.mutate_op_loaded {
+                        self.load_mutate_op();
+                        self.mutate_op_loaded = true;
+                    }
+                    code += "mutate_operator(";
+                } else {
+                    code += "(";
                     code += &unary.op.content;
                 }
                 code += &self.transpile_expr(*unary.expr);
diff --git a/compiler/erg_compiler/ty/value.rs b/compiler/erg_compiler/ty/value.rs
index 3e8f8c2b..22bba80c 100644
--- a/compiler/erg_compiler/ty/value.rs
+++ b/compiler/erg_compiler/ty/value.rs
@@ -624,6 +624,14 @@ impl ValueObj {
         }
     }
 
+    pub fn is_int(&self) -> bool {
+        match self {
+            Self::Int(_) | Self::Nat(_) | Self::Bool(_) => true,
+            Self::Mut(n) => n.borrow().is_nat(),
+            _ => false,
+        }
+    }
+
     pub fn is_nat(&self) -> bool {
         match self {
             Self::Nat(_) | Self::Bool(_) => true,
diff --git a/examples/mut.er b/examples/mut.er
new file mode 100644
index 00000000..55468984
--- /dev/null
+++ b/examples/mut.er
@@ -0,0 +1,8 @@
+i = !1
+i.update! i -> i * 3
+assert i == 3
+
+i.inc!()
+assert i == 4
+i.dec!()
+assert i == 3
diff --git a/tests/test.rs b/tests/test.rs
index c441f8c4..e47897fb 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -84,6 +84,11 @@ fn exec_infer_trait() -> Result<(), ()> {
     expect_success("tests/should_ok/infer_trait.er")
 }
 
+#[test]
+fn exec_mut() -> Result<(), ()> {
+    expect_success("examples/mut.er")
+}
+
 #[test]
 fn exec_pattern() -> Result<(), ()> {
     expect_success("tests/should_ok/pattern.er")