diff --git a/src/Ast/Statements/ImplementSt.hpp b/src/Ast/Statements/ImplementSt.hpp
index e69de29..4d021b1 100644
--- a/src/Ast/Statements/ImplementSt.hpp
+++ b/src/Ast/Statements/ImplementSt.hpp
@@ -0,0 +1,46 @@
+#pragma once
+#include <Ast/functionParameters.hpp>
+#include <Ast/Statements/FunctionDefSt.hpp>
+#include <Ast/astBase.hpp>
+
+namespace Fig::Ast
+{
+    /*
+    
+    impl Readable for File
+    {
+        read() -> String
+        {
+            ...
+    }
+    
+    */
+    struct ImplementMethod
+    {
+        FString name;
+        FunctionParameters paras;
+        BlockStatement body;
+    };
+
+    class ImplementAst final : public StatementAst
+    {
+    public:
+        FString interfaceName;
+        FString structName;
+
+        std::vector<ImplementMethod> methods;
+
+        ImplementAst()
+        {
+            type = AstType::ImplementSt;
+        }
+
+        ImplementAst(FString _interfaceName, FString _structName, std::vector<ImplementMethod> _methods) :
+            interfaceName(std::move(_interfaceName)), structName(std::move(_structName)), methods(std::move(_methods))
+        {
+            type = AstType::ImplementSt;
+        }
+    };
+
+    using Implement = std::shared_ptr<ImplementAst>;
+};
\ No newline at end of file
diff --git a/src/Ast/Statements/InterfaceDefSt.hpp b/src/Ast/Statements/InterfaceDefSt.hpp
new file mode 100644
index 0000000..d062fa4
--- /dev/null
+++ b/src/Ast/Statements/InterfaceDefSt.hpp
@@ -0,0 +1,59 @@
+#pragma once
+
+#include <Ast/functionParameters.hpp>
+#include <Ast/astBase.hpp>
+
+
+namespace Fig::Ast
+{
+    /*
+    
+    interface Readable
+    {
+        read() -> String
+        {
+            // default
+        }
+        
+        flush() -> Null; // non-default
+    }
+    
+    */
+
+
+    struct InterfaceMethod
+    {
+        FString name;
+        FunctionParameters paras;
+        FString returnType;
+
+        BlockStatement defaultBody = nullptr; // nullptr is non-default func
+
+        bool hasDefaultBody() const
+        {
+            return defaultBody != nullptr;
+        }
+    };
+
+    class InterfaceDefAst final : public StatementAst
+    {
+    public:
+        FString name;
+        std::vector<InterfaceMethod> methods;
+        std::vector<FString> parents; // Feature, NOT NOW
+        bool isPublic;
+
+        InterfaceDefAst()
+        {
+            type = AstType::InterfaceDefSt;
+        }
+
+        InterfaceDefAst(FString _name, std::vector<InterfaceMethod> _methods, bool _isPublic) :
+            name(std::move(_name)), methods(std::move(_methods)), isPublic(_isPublic)
+        {
+            type = AstType::InterfaceDefSt;
+        }
+    };
+
+    using InterfaceDef = std::shared_ptr<InterfaceDefAst>;
+};
\ No newline at end of file
diff --git a/src/Ast/ast.hpp b/src/Ast/ast.hpp
index 0a038b4..c2b7741 100644
--- a/src/Ast/ast.hpp
+++ b/src/Ast/ast.hpp
@@ -21,6 +21,7 @@
 #include <Ast/Statements/IfSt.hpp>
 #include <Ast/Statements/ImplementSt.hpp>
 #include <Ast/Statements/ImportSt.hpp>
+#include <Ast/Statements/InterfaceDefSt.hpp>
 #include <Ast/Statements/FunctionDefSt.hpp>
 #include <Ast/Statements/ControlSt.hpp>
 #include <Ast/Statements/ExpressionStmt.hpp>
diff --git a/src/Ast/astBase.hpp b/src/Ast/astBase.hpp
index d64a165..05e8f51 100644
--- a/src/Ast/astBase.hpp
+++ b/src/Ast/astBase.hpp
@@ -44,6 +44,7 @@ namespace Fig::Ast
         VarDefSt,
         FunctionDefSt,
         StructSt,
+        InterfaceDefSt,
         ImplementSt,
 
         IfSt,
diff --git a/src/Ast/functionParameters.hpp b/src/Ast/functionParameters.hpp
index 3c8ae5c..4f1e6c6 100644
--- a/src/Ast/functionParameters.hpp
+++ b/src/Ast/functionParameters.hpp
@@ -4,6 +4,8 @@
 #include <Value/Type.hpp>
 #include <Core/fig_string.hpp>
 
+#include <format>
+
 namespace Fig::Ast
 {
     struct FunctionParameters // for define
@@ -26,7 +28,6 @@ namespace Fig::Ast
 
         FunctionParameters()
         {
-            
         }
         FunctionParameters(PosParasType _posParas, DefParasType _defParas)
         {
@@ -43,5 +44,52 @@ namespace Fig::Ast
         {
             return posParas.size() + defParas.size();
         }
+
+        bool operator==(const FunctionParameters &other) const
+        {
+            return posParas == other.posParas && defParas == other.defParas && variadicPara == other.variadicPara && variadic == other.variadic;
+        }
+
+        FString toString() const
+        {
+            if (variadic)
+            {
+                return FString(variadicPara + u8"...");
+            }
+            static const auto posParasToString = [this]() {
+                FString out;
+                for (auto &p : posParas)
+                {
+                    out += p.first;
+                    if (!p.second.empty())
+                    {
+                        out += FString(u8":" + p.second);
+                    }
+                    out += u8",";
+                }
+                out.pop_back();
+                return out;
+            };
+            static const auto defParasToString = [this]() {
+                FString out;
+                for (auto &p : defParas)
+                {
+                    out += p.first;
+                    if (!p.second.first.empty())
+                    {
+                        out += FString(u8":" + p.second.first);
+                    }
+                    if (p.second.second != nullptr)
+                    {
+                        out += u8"=";
+                        out += p.second.second->toString();
+                    }
+                    out += u8",";
+                }
+                out.pop_back();
+                return out;
+            };
+            return FString(std::format("{},{}", posParasToString().toBasicString(), defParasToString().toBasicString()));
+        }
     };
-}
\ No newline at end of file
+} // namespace Fig::Ast
\ No newline at end of file
diff --git a/src/Context/context.hpp b/src/Context/context.hpp
index 1c87db9..7b60d9d 100644
--- a/src/Context/context.hpp
+++ b/src/Context/context.hpp
@@ -1,5 +1,8 @@
 #pragma once
 
+#include "Value/interface.hpp"
+#include <Value/Type.hpp>
+#include <algorithm>
 #include <unordered_map>
 #include <iostream>
 #include <memory>
@@ -12,6 +15,14 @@
 namespace Fig
 {
 
+    struct ImplRecord
+    {
+        TypeInfo interfaceType;
+        TypeInfo structType;
+
+        std::unordered_map<FString, Function> implMethods;
+    };
+
     class Context : public std::enable_shared_from_this<Context>
     {
     private:
@@ -20,7 +31,9 @@ namespace Fig
 
         std::unordered_map<std::size_t, Function> functions;
         std::unordered_map<std::size_t, FString> functionNames;
-        // std::unordered_map<std::size_t, FString> structTypeNames;
+
+        // implRegistry <Struct, ordered list of ImplRecord>
+        std::unordered_map<TypeInfo, std::vector<ImplRecord>, TypeInfoHash> implRegistry;
 
     public:
         ContextPtr parent;
@@ -28,7 +41,7 @@ namespace Fig
         Context(const Context &) = default;
         Context(const FString &name, ContextPtr p = nullptr) :
             scopeName(name), parent(p) {}
-            
+
         void setParent(ContextPtr _parent)
         {
             parent = _parent;
@@ -49,6 +62,7 @@ namespace Fig
             variables.insert(c.variables.begin(), c.variables.end());
             functions.insert(c.functions.begin(), c.functions.end());
             functionNames.insert(c.functionNames.begin(), c.functionNames.end());
+            implRegistry.insert(c.implRegistry.begin(), c.implRegistry.end());
             // structTypeNames.insert(c.structTypeNames.begin(), c.structTypeNames.end());
         }
 
@@ -243,6 +257,169 @@ namespace Fig
             }
             return false;
         }
+
+        bool hasImplRegisted(const TypeInfo &structType, const TypeInfo &interfaceType) const
+        {
+            auto it = implRegistry.find(structType);
+            if (it != implRegistry.end())
+            {
+                for (auto &r : it->second)
+                {
+                    if (r.interfaceType == interfaceType)
+                        return true;
+                }
+            }
+            return parent && parent->hasImplRegisted(structType, interfaceType);
+        }
+
+        std::optional<ImplRecord> getImplRecord(const TypeInfo &structType, const TypeInfo &interfaceType) const
+        {
+            auto it = implRegistry.find(structType);
+            if (it != implRegistry.end())
+            {
+                for (auto &r : it->second)
+                {
+                    if (r.interfaceType == interfaceType)
+                        return r;
+                }
+            }
+
+            if (parent)
+                return parent->getImplRecord(structType, interfaceType);
+
+            return std::nullopt;
+        }
+
+        void setImplRecord(const TypeInfo &structType, const TypeInfo &interfaceType, const ImplRecord &record)
+        {
+            auto &list = implRegistry[structType];
+
+            for (auto &r : list)
+            {
+                if (r.interfaceType == interfaceType)
+                    return;
+            }
+
+            list.push_back(record); // order is the level
+        }
+
+        bool hasMethodImplemented(const TypeInfo &structType, const FString &functionName) const
+        {
+            auto it = implRegistry.find(structType);
+            if (it == implRegistry.end())
+                return false;
+
+            for (auto &record : it->second)
+            {
+                if (record.implMethods.contains(functionName))
+                    return true;
+            }
+
+            return false;
+        }
+
+        bool hasDefaultImplementedMethod(const TypeInfo &structType, const FString &functionName) const
+        {
+            auto it = implRegistry.find(structType);
+            if (it == implRegistry.end())
+                return false;
+
+            std::vector<TypeInfo> implementedInterfaces;
+            for (auto &record : it->second)
+                implementedInterfaces.push_back(record.interfaceType);
+
+            for (auto &[_, slot] : variables)
+            {
+                if (!slot->value->is<InterfaceType>())
+                    continue;
+
+                InterfaceType &interface = slot->value->as<InterfaceType>();
+
+                bool implemented =
+                    std::any_of(
+                        implementedInterfaces.begin(),
+                        implementedInterfaces.end(),
+                        [&](const TypeInfo &ti) { return ti == interface.type; });
+
+                if (!implemented)
+                    continue;
+
+                for (auto &method : interface.methods)
+                {
+                    if (method.name == functionName && method.hasDefaultBody())
+                        return true;
+                }
+            }
+
+            return false;
+        }
+
+        Function getDefaultImplementedMethod(const TypeInfo &structType, const FString &functionName)
+        {
+            // O(N²)
+            // SLOW
+            // wwww (sad)
+
+            // huh
+            // just let it be SLOW
+            // i dont give a fuck
+
+            auto it = implRegistry.find(structType);
+            if (it == implRegistry.end())
+                assert(false);
+
+            std::vector<TypeInfo> implementedInterfaces;
+            for (auto &record : it->second)
+                implementedInterfaces.push_back(record.interfaceType);
+
+            for (auto &[_, slot] : variables)
+            {
+                if (!slot->value->is<InterfaceType>())
+                    continue;
+
+                InterfaceType &interface = slot->value->as<InterfaceType>();
+
+                bool implemented =
+                    std::any_of(
+                        implementedInterfaces.begin(),
+                        implementedInterfaces.end(),
+                        [&](const TypeInfo &ti) { return ti == interface.type; });
+
+                if (!implemented)
+                    continue;
+
+                for (auto &method : interface.methods)
+                {
+                    if (method.name == functionName)
+                    {
+                        if (!method.hasDefaultBody())
+                            assert(false);
+
+                        return Function(
+                            method.paras,
+                            TypeInfo(method.returnType),
+                            method.defaultBody,
+                            shared_from_this());
+                    }
+                }
+            }
+
+            assert(false);
+        }
+
+        const Function &getImplementedMethod(const TypeInfo &structType, const FString &functionName) const
+        {
+            const auto &list = implRegistry.at(structType);
+            for (auto &record : list)
+            {
+                auto it = record.implMethods.find(functionName);
+                if (it != record.implMethods.end())
+                    return it->second;
+            }
+
+            assert(false);
+        }
+
         void printStackTrace(std::ostream &os = std::cerr, int indent = 0) const
         {
             const Context *ctx = this;
diff --git a/src/Evaluator/evaluator.cpp b/src/Evaluator/evaluator.cpp
index f25b7bc..2ad7b10 100644
--- a/src/Evaluator/evaluator.cpp
+++ b/src/Evaluator/evaluator.cpp
@@ -1,3 +1,14 @@
+#include "Value/VariableSlot.hpp"
+#include "Value/value.hpp"
+#include <Ast/AccessModifier.hpp>
+#include <Ast/Statements/ImplementSt.hpp>
+#include <Ast/Statements/InterfaceDefSt.hpp>
+#include <Ast/astBase.hpp>
+#include <Context/context_forward.hpp>
+#include <Error/error.hpp>
+#include <Value/Type.hpp>
+#include <Value/interface.hpp>
+#include <Value/structInstance.hpp>
 #include <Error/errorLog.hpp>
 #include <Evaluator/evaluator.hpp>
 #include <Evaluator/evaluator_error.hpp>
@@ -8,21 +19,14 @@
 
 #include <filesystem>
 #include <fstream>
+#include <memory>
 
 namespace Fig
 {
-    bool Evaluator::isTypeMatch(const TypeInfo &expected, ObjectPtr obj)
+
+    bool Evaluator::isInterfaceSignatureMatch(const Ast::ImplementMethod &implMethod, const Ast::InterfaceMethod &ifMethod)
     {
-        if (expected == ValueType::Any)
-            return true;
-
-        TypeInfo actual = obj->getTypeInfo();
-
-        if (actual != ValueType::StructInstance)
-            return expected == actual;
-
-        const StructInstance &si = obj->as<StructInstance>();
-        return si.parentType == expected;
+        return implMethod.name == ifMethod.name && implMethod.paras == ifMethod.paras;
     }
 
     LvObject Evaluator::evalVarExpr(Ast::VarExpr var, ContextPtr ctx)
@@ -32,7 +36,7 @@ namespace Fig
         {
             throw EvaluatorError(u8"UndeclaredIdentifierError", name, var);
         }
-        return LvObject(ctx->get(name));
+        return LvObject(ctx->get(name), ctx);
     }
     LvObject Evaluator::evalMemberExpr(Ast::MemberExpr me, ContextPtr ctx)
     {
@@ -45,7 +49,8 @@ namespace Fig
             if (mod.ctx->contains(member) && mod.ctx->isVariablePublic(member))
             {
                 return LvObject(
-                    mod.ctx->get(member));
+                    mod.ctx->get(member),
+                    ctx);
             }
             else
             {
@@ -61,13 +66,14 @@ namespace Fig
         if (baseVal->hasMemberFunction(member))
         {
             return LvObject(std::make_shared<VariableSlot>(
-                member,
-                std::make_shared<Object>(
-                    Function(
-                        baseVal->getMemberFunction(member),
-                        baseVal->getMemberFunctionParaCount(member))),
-                ValueType::Function,
-                AccessModifier::PublicConst)); // fake l-value
+                                member,
+                                std::make_shared<Object>(
+                                    Function(
+                                        baseVal->getMemberFunction(member),
+                                        baseVal->getMemberFunctionParaCount(member))),
+                                ValueType::Function,
+                                AccessModifier::PublicConst),
+                            ctx); // fake l-value
         }
         if (baseVal->getTypeInfo() != ValueType::StructInstance) // and not member function found
         {
@@ -80,17 +86,40 @@ namespace Fig
                 me->base);
         }
         const StructInstance &si = baseVal->as<StructInstance>();
-        if (!si.localContext->containsInThisScope(member) || !si.localContext->isVariablePublic(member))
+        if (ctx->hasMethodImplemented(si.parentType, member))
+        {
+            return LvObject(std::make_shared<VariableSlot>(
+                                member,
+                                std::make_shared<Object>(
+                                    ctx->getImplementedMethod(si.parentType, member)),
+                                ValueType::Function,
+                                AccessModifier::PublicConst),
+                            ctx);
+        }
+        else if (si.localContext->containsInThisScope(member) && si.localContext->isVariablePublic(member))
+        {
+            return LvObject(si.localContext->get(member), ctx);
+        }
+        else if (ctx->hasDefaultImplementedMethod(si.parentType, member))
+        {
+            return LvObject(std::make_shared<VariableSlot>(
+                                member,
+                                std::make_shared<Object>(
+                                    ctx->getDefaultImplementedMethod(si.parentType, member)),
+                                ValueType::Function,
+                                AccessModifier::PublicConst),
+                            ctx);
+        }
+        else
         {
             throw EvaluatorError(
                 u8"NoAttributeError",
                 std::format(
-                    "`{}` has not attribute '{}'",
+                    "`{}` has not attribute '{}' and no interfaces have been implemented it",
                     baseVal->toString().toBasicString(),
                     member.toBasicString()),
                 me->base);
         }
-        return LvObject(si.localContext->get(member));
     }
     LvObject Evaluator::evalIndexExpr(Ast::IndexExpr ie, ContextPtr ctx)
     {
@@ -125,14 +154,16 @@ namespace Fig
             return LvObject(
                 base.get(),
                 indexVal,
-                LvObject::Kind::ListElement);
+                LvObject::Kind::ListElement,
+                ctx);
         }
         else if (type == ValueType::Map)
         {
             return LvObject(
                 base.get(),
                 index,
-                LvObject::Kind::MapElement);
+                LvObject::Kind::MapElement,
+                ctx);
         }
         else if (type == ValueType::String)
         {
@@ -160,7 +191,8 @@ namespace Fig
             return LvObject(
                 base.get(),
                 indexVal,
-                LvObject::Kind::StringElement);
+                LvObject::Kind::StringElement,
+                ctx);
         }
         else
         {
@@ -450,7 +482,7 @@ namespace Fig
             TypeInfo expectedType(fnParas.posParas[i].second); // look up type info, if exists a type with the name, use it, else throw
             ObjectPtr argVal = eval(fnArgs.argv[i], ctx);
             TypeInfo actualType = argVal->getTypeInfo();
-            if (!isTypeMatch(expectedType, argVal))
+            if (!isTypeMatch(expectedType, argVal, ctx))
             {
                 throw EvaluatorError(
                     u8"ArgumentTypeMismatchError",
@@ -467,10 +499,10 @@ namespace Fig
         for (; i < fnArgs.getLength(); i++)
         {
             size_t defParamIndex = i - fnParas.posParas.size();
-            TypeInfo expectedType = fnParas.defParas[defParamIndex].second.first;
+            TypeInfo expectedType(fnParas.defParas[defParamIndex].second.first);
 
             ObjectPtr defaultVal = eval(fnParas.defParas[defParamIndex].second.second, ctx);
-            if (!isTypeMatch(expectedType, defaultVal))
+            if (!isTypeMatch(expectedType, defaultVal, ctx))
             {
                 throw EvaluatorError(
                     u8"DefaultParameterTypeError",
@@ -484,7 +516,7 @@ namespace Fig
 
             ObjectPtr argVal = eval(fnArgs.argv[i], ctx);
             TypeInfo actualType = argVal->getTypeInfo();
-            if (!isTypeMatch(expectedType, argVal))
+            if (!isTypeMatch(expectedType, argVal, ctx))
             {
                 throw EvaluatorError(
                     u8"ArgumentTypeMismatchError",
@@ -512,13 +544,13 @@ namespace Fig
             if (j < fnParas.posParas.size())
             {
                 paramName = fnParas.posParas[j].first;
-                paramType = fnParas.posParas[j].second;
+                paramType = TypeInfo(fnParas.posParas[j].second);
             }
             else
             {
                 size_t defParamIndex = j - fnParas.posParas.size();
                 paramName = fnParas.defParas[defParamIndex].first;
-                paramType = fnParas.defParas[defParamIndex].second.first;
+                paramType = TypeInfo(fnParas.defParas[defParamIndex].second.first);
             }
             AccessModifier argAm = AccessModifier::Const;
             newContext->def(paramName, paramType, argAm, evaluatedArgs.argv[j]);
@@ -553,7 +585,7 @@ namespace Fig
                 break;
             }
         }
-        if (!isTypeMatch(fnStruct.retType, retVal))
+        if (!isTypeMatch(fnStruct.retType, retVal, ctx))
         {
             throw EvaluatorError(
                 u8"ReturnTypeMismatchError",
@@ -741,7 +773,7 @@ namespace Fig
                                 ObjectPtr defaultVal = eval(field.defaultValue, ctx); // it can't be null here
 
                                 // type check
-                                if (!isTypeMatch(expectedType, defaultVal))
+                                if (!isTypeMatch(expectedType, defaultVal, ctx))
                                 {
                                     throw EvaluatorError(
                                         u8"StructFieldTypeMismatchError",
@@ -759,7 +791,7 @@ namespace Fig
                             }
 
                             const ObjectPtr &argVal = evaluatedArgs[i].second;
-                            if (!isTypeMatch(expectedType, argVal))
+                            if (!isTypeMatch(expectedType, argVal, ctx))
                             {
                                 throw EvaluatorError(
                                     u8"StructFieldTypeMismatchError",
@@ -797,7 +829,7 @@ namespace Fig
 
                                 // type check
                                 const TypeInfo &expectedType = field.type;
-                                if (!isTypeMatch(expectedType, defaultVal))
+                                if (!isTypeMatch(expectedType, defaultVal, ctx))
                                 {
                                     throw EvaluatorError(
                                         u8"StructFieldTypeMismatchError",
@@ -814,7 +846,7 @@ namespace Fig
                                 continue;
                             }
                             const ObjectPtr &argVal = evaluatedArgs[i].second;
-                            if (!isTypeMatch(field.type, argVal))
+                            if (!isTypeMatch(field.type, argVal, ctx))
                             {
                                 throw EvaluatorError(
                                     u8"StructFieldTypeMismatchError",
@@ -918,7 +950,7 @@ namespace Fig
                 else if (!declaredTypeName.empty())
                 {
                     declaredType = TypeInfo(declaredTypeName);
-                    if (value != nullptr && !isTypeMatch(declaredType, value))
+                    if (value != nullptr && !isTypeMatch(declaredType, value, ctx))
                     {
                         throw EvaluatorError(
                             u8"TypeError",
@@ -1031,6 +1063,194 @@ namespace Fig
                 return StatementResult::normal();
             }
 
+            case InterfaceDefSt: {
+                auto ifd = std::dynamic_pointer_cast<Ast::InterfaceDefAst>(stmt);
+                assert(ifd != nullptr);
+
+                const FString &interfaceName = ifd->name;
+
+                if (ctx->containsInThisScope(interfaceName))
+                {
+                    throw EvaluatorError(
+                        u8"RedeclarationError",
+                        std::format("Interface `{}` already declared in this scope",
+                                    interfaceName.toBasicString()),
+                        ifd);
+                }
+                TypeInfo type(interfaceName, true); // register interface
+                ctx->def(
+                    interfaceName,
+                    type,
+                    (ifd->isPublic ? AccessModifier::PublicConst : AccessModifier::Const),
+                    std::make_shared<Object>(InterfaceType(
+                        type,
+                        ifd->methods)));
+                return StatementResult::normal();
+            }
+
+            case ImplementSt: {
+                auto ip = std::dynamic_pointer_cast<Ast::ImplementAst>(stmt);
+                assert(ip != nullptr);
+
+                TypeInfo structType(ip->structName);
+                TypeInfo interfaceType(ip->interfaceName);
+                if (ctx->hasImplRegisted(structType, interfaceType))
+                {
+                    throw EvaluatorError(
+                        u8"DuplicateImplError",
+                        std::format(
+                            "Duplicate implement `{}` for `{}`",
+                            interfaceType.toString().toBasicString(),
+                            structType.toString().toBasicString()),
+                        ip);
+                }
+                if (!ctx->contains(ip->interfaceName))
+                {
+                    throw EvaluatorError(
+                        u8"InterfaceNotFoundError",
+                        std::format("Interface '{}' not found", ip->interfaceName.toBasicString()),
+                        ip);
+                }
+                if (!ctx->contains(ip->structName))
+                {
+                    throw EvaluatorError(
+                        u8"StructNotFoundError",
+                        std::format("Struct '{}' not found", ip->structName.toBasicString()),
+                        ip);
+                }
+                auto interfaceSlot = ctx->get(ip->interfaceName);
+                auto structSlot = ctx->get(ip->structName);
+
+                LvObject interfaceLv(interfaceSlot, ctx);
+                LvObject structLv(structSlot, ctx);
+
+                ObjectPtr interfaceObj = interfaceLv.get();
+                ObjectPtr structTypeObj = structLv.get();
+
+                if (!interfaceObj->is<InterfaceType>())
+                {
+                    throw EvaluatorError(
+                        u8"NotAInterfaceError",
+                        std::format("Variable `{}` is not a interface", ip->interfaceName.toBasicString()),
+                        ip);
+                }
+                if (!structTypeObj->is<StructType>())
+                {
+                    throw EvaluatorError(
+                        u8"NotAStructType",
+                        std::format("Variable `{}` is not a struct type", ip->structName.toBasicString()),
+                        ip);
+                }
+                auto &implementMethods = ip->methods;
+                InterfaceType &interface = interfaceObj->as<InterfaceType>();
+
+                // ===== interface implementation validation =====
+                ImplRecord record{interfaceType, structType, {}};
+
+                std::unordered_map<FString, Ast::InterfaceMethod> ifaceMethods;
+                for (auto &m : interface.methods)
+                {
+                    if (ifaceMethods.contains(m.name))
+                    {
+                        throw EvaluatorError(
+                            u8"InterfaceDuplicateMethodError",
+                            std::format("Interface '{}' has duplicate method '{}'",
+                                        interfaceType.toString().toBasicString(),
+                                        m.name.toBasicString()),
+                            ip);
+                    }
+                    ifaceMethods[m.name] = m;
+                }
+
+                std::unordered_set<FString> implemented;
+
+                for (auto &implMethod : implementMethods)
+                {
+                    const FString &name = implMethod.name;
+
+                    // ---- redundant impl ----
+                    if (!ifaceMethods.contains(name))
+                    {
+                        throw EvaluatorError(
+                            u8"RedundantImplementationError",
+                            std::format("Struct '{}' implements extra method '{}' "
+                                        "which is not required by interface '{}'",
+                                        structType.toString().toBasicString(),
+                                        name.toBasicString(),
+                                        interfaceType.toString().toBasicString()),
+                            ip);
+                    }
+
+                    if (implemented.contains(name))
+                    {
+                        throw EvaluatorError(
+                            u8"DuplicateImplementMethodError",
+                            std::format("Duplicate implement method '{}'",
+                                        name.toBasicString()),
+                            ip);
+                    }
+
+                    auto &ifMethod = ifaceMethods[name];
+
+                    // ---- signature check ----
+                    if (!isInterfaceSignatureMatch(implMethod, ifMethod))
+                    {
+                        throw EvaluatorError(
+                            u8"InterfaceSignatureMismatch",
+                            std::format(
+                                "Interface method '{}({})' signature mismatch with "
+                                "implementation '{}({})'",
+                                ifMethod.name.toBasicString(),
+                                ifMethod.paras.toString().toBasicString(),
+                                implMethod.name.toBasicString(),
+                                implMethod.paras.toString().toBasicString()),
+                            ip);
+                    }
+
+                    if (ctx->hasMethodImplemented(structType, name))
+                    {
+                        throw EvaluatorError(
+                            u8"DuplicateImplementMethodError",
+                            std::format(
+                                "Method '{}' already implemented by another interface "
+                                "for struct '{}'",
+                                name.toBasicString(),
+                                structType.toString().toBasicString()),
+                            ip);
+                    }
+
+                    implemented.insert(name);
+
+                    record.implMethods[name] = Function(
+                        implMethod.paras,
+                        TypeInfo(ifMethod.returnType),
+                        implMethod.body,
+                        ctx);
+                }
+
+                for (auto &m : interface.methods)
+                {
+                    if (implemented.contains(m.name))
+                        continue;
+
+                    if (m.hasDefaultBody())
+                        continue;
+
+                    throw EvaluatorError(
+                        u8"MissingImplementationError",
+                        std::format(
+                            "Struct '{}' does not implement required interface method '{}' "
+                            "and interface '{}' provides no default implementation",
+                            structType.toString().toBasicString(),
+                            m.name.toBasicString(),
+                            interfaceType.toString().toBasicString()),
+                        ip);
+                }
+
+                ctx->setImplRecord(structType, interfaceType, record);
+                return StatementResult::normal();
+            }
+
             case IfSt: {
                 auto ifSt = std::dynamic_pointer_cast<Ast::IfSt>(stmt);
                 ObjectPtr condVal = eval(ifSt->condition, ctx);
diff --git a/src/Evaluator/evaluator.hpp b/src/Evaluator/evaluator.hpp
index 6d18b05..5b0cc9b 100644
--- a/src/Evaluator/evaluator.hpp
+++ b/src/Evaluator/evaluator.hpp
@@ -1,3 +1,6 @@
+#include "Ast/Statements/ImplementSt.hpp"
+#include "Ast/Statements/InterfaceDefSt.hpp"
+#include "Value/Type.hpp"
 #include <Ast/ast.hpp>
 
 #include <Context/context.hpp>
@@ -102,7 +105,7 @@ namespace Fig
             }
         }
 
-        bool isTypeMatch(const TypeInfo &, ObjectPtr);
+        bool isInterfaceSignatureMatch(const Ast::ImplementMethod &, const Ast::InterfaceMethod &);
 
         /* Left-value eval*/
         LvObject evalVarExpr(Ast::VarExpr, ContextPtr);
diff --git a/src/Parser/parser.cpp b/src/Parser/parser.cpp
index f0f81d6..695d051 100644
--- a/src/Parser/parser.cpp
+++ b/src/Parser/parser.cpp
@@ -1,3 +1,8 @@
+#include "Ast/Statements/ImplementSt.hpp"
+#include "Ast/astBase.hpp"
+#include "Ast/functionParameters.hpp"
+#include "Error/error.hpp"
+#include "Token/token.hpp"
 #include <Parser/parser.hpp>
 
 namespace Fig
@@ -365,6 +370,114 @@ namespace Fig
         }
         return makeAst<Ast::StructDefSt>(isPublic, structName, fields, makeAst<Ast::BlockStatementAst>(stmts));
     }
+
+    Ast::InterfaceDef Parser::__parseInterfaceDef(bool isPublic)
+    {
+        // entry: current is interface name
+        FString interfaceName = currentToken().getValue();
+        next();                       // consume name
+        expect(TokenType::LeftBrace); // `{
+        next();                       // consume `{`
+
+        std::vector<Ast::InterfaceMethod> methods;
+
+        while (true)
+        {
+            if (isThis(TokenType::RightBrace))
+            {
+                next(); // consume `}`
+                break;
+            }
+            if (isThis(TokenType::Identifier))
+            {
+                FString funcName = currentToken().getValue();
+                next(); // consume func name
+
+                expect(TokenType::LeftParen);
+                Ast::FunctionParameters paras = __parseFunctionParameters();
+
+                expect(TokenType::RightArrow); // ->
+                next();                        // consume `->`
+
+                expect(TokenType::Identifier, u8"return type");
+                FString returnType = currentToken().getValue();
+                next(); // consume return type
+
+                if (isThis(TokenType::LeftBrace))
+                {
+                    Ast::BlockStatement block = __parseBlockStatement();
+
+                    methods.push_back(Ast::InterfaceMethod(
+                        funcName,
+                        paras,
+                        returnType,
+                        block));
+                    continue;
+                }
+                expect(TokenType::Semicolon);
+                next(); // consume `;`
+
+                methods.push_back(Ast::InterfaceMethod(
+                    funcName,
+                    paras,
+                    returnType));
+            }
+            else
+            {
+                throw SyntaxError(FString(u8"Invalid syntax"), currentAAI.line, currentAAI.column);
+            }
+        }
+        return makeAst<Ast::InterfaceDefAst>(interfaceName, methods, isPublic);
+    }
+
+    Ast::Implement Parser::__parseImplement()
+    {
+        // entry: current is `impl`
+        next(); // consume `impl`
+        expect(TokenType::Identifier, u8"interface name");
+        FString interfaceName = currentToken().getValue();
+        next(); // consume interface name
+
+        expect(TokenType::For);
+        next(); // consume `for`
+
+        expect(TokenType::Identifier, u8"struct name");
+        FString structName = currentToken().getValue();
+        next();                       // consume name
+        expect(TokenType::LeftBrace); // {
+        next();  // consume `{`
+
+        std::vector<Ast::ImplementMethod> methods;
+
+        while (true)
+        {
+            if (isThis(TokenType::RightBrace))
+            {
+                next(); // consume `}`
+                break;
+            }
+            if (isThis(TokenType::Identifier))
+            {
+                FString funcName = currentToken().getValue();
+                next(); // consume func name
+                expect(TokenType::LeftParen);
+                Ast::FunctionParameters paras = __parseFunctionParameters();
+                expect(TokenType::LeftBrace);
+                Ast::BlockStatement body = __parseBlockStatement();
+                methods.push_back(Ast::ImplementMethod(
+                    funcName,
+                    paras,
+                    body));
+            }
+            else
+            {
+                throw SyntaxError(FString(u8"Invalid syntax"), currentAAI.line, currentAAI.column);
+            }
+        }
+
+        return makeAst<Ast::ImplementAst>(interfaceName, structName, methods);
+    }
+
     Ast::Statement Parser::__parseStatement()
     {
         Ast::Statement stmt;
@@ -389,9 +502,13 @@ namespace Fig
             {
                 stmt = __parseStructDef(true);
             }
+            else if (isThis(TokenType::Interface))
+            {
+                stmt = __parseInterfaceDef(true);
+            }
             else
             {
-                throwAddressableError<SyntaxError>(FString(u8"Expected `var`, `const`, `function` or `struct` after `public`"));
+                throwAddressableError<SyntaxError>(FString(u8"Expected `var`, `const`, `function`, `struct` or `interface` after `public`"));
             }
         }
         else if (isThis(TokenType::Variable) || isThis(TokenType::Const))
@@ -409,6 +526,16 @@ namespace Fig
             next();
             stmt = __parseStructDef(false);
         }
+        else if (isThis(TokenType::Interface))
+        {
+            expectPeek(TokenType::Identifier, u8"interface name");
+            next();
+            stmt = __parseInterfaceDef(false);
+        }
+        else if (isThis(TokenType::Implement))
+        {
+            stmt = __parseImplement();
+        }
         else if (isThis(TokenType::If))
         {
             stmt = __parseIf();
@@ -501,7 +628,7 @@ namespace Fig
                     expect(TokenType::RightParen);
                     next(); // consume `)`
                 }
-                else 
+                else
                 {
                     elifCondition = parseExpression(0);
                 }
diff --git a/src/Parser/parser.hpp b/src/Parser/parser.hpp
index 4dca735..eeddc49 100644
--- a/src/Parser/parser.hpp
+++ b/src/Parser/parser.hpp
@@ -313,6 +313,8 @@ namespace Fig
         Ast::VarExpr __parseVarExpr(FString);
         Ast::FunctionDef __parseFunctionDef(bool); // entry: current is Token::Identifier (isPublic: Bool)
         Ast::StructDef __parseStructDef(bool);     // entry: current is Token::Identifier (struct name) arg(isPublic: bool)
+        Ast::InterfaceDef __parseInterfaceDef(bool); // entry: current is Token::Identifier (interface name) arg(isPublic: bool)
+        Ast::Implement __parseImplement();         // entry: current is `impl`
 
         Ast::BinaryExpr __parseInfix(Ast::Expression, Ast::Operator, Precedence);
         Ast::UnaryExpr __parsePrefix(Ast::Operator, Precedence);
diff --git a/src/Value/LvObject.hpp b/src/Value/LvObject.hpp
index f971ea4..a1b1686 100644
--- a/src/Value/LvObject.hpp
+++ b/src/Value/LvObject.hpp
@@ -22,18 +22,20 @@ namespace Fig
 
         ObjectPtr mapIndex;
 
-        LvObject(std::shared_ptr<VariableSlot> _slot) :
-            slot(std::move(_slot))
+        ContextPtr ctx;
+
+        LvObject(std::shared_ptr<VariableSlot> _slot, ContextPtr _ctx) :
+            slot(std::move(_slot)), ctx(_ctx)
         {
             kind = Kind::Variable;
         }
-        LvObject(ObjectPtr _v, size_t _index, Kind _kind) :
-            value(_v), numIndex(_index)
+        LvObject(ObjectPtr _v, size_t _index, Kind _kind, ContextPtr _ctx) :
+            value(_v), numIndex(_index), ctx(_ctx)
         {
             kind = _kind;
         }
-        LvObject(ObjectPtr _v, ObjectPtr _index, Kind _kind) :
-            value(_v), mapIndex(_index)
+        LvObject(ObjectPtr _v, ObjectPtr _index, Kind _kind, ContextPtr _ctx) :
+            value(_v), mapIndex(_index), ctx(_ctx)
         {
             kind = _kind;
         }
@@ -78,7 +80,7 @@ namespace Fig
             if (kind == Kind::Variable)
             {
                 auto s = resolve(slot);
-                if (s->declaredType != ValueType::Any && s->declaredType != v->getTypeInfo())
+                if (!isTypeMatch(s->declaredType, v, ctx))
                 {
                     throw RuntimeError(
                         FString(
diff --git a/src/Value/Type.hpp b/src/Value/Type.hpp
index 20f83d8..834926c 100644
--- a/src/Value/Type.hpp
+++ b/src/Value/Type.hpp
@@ -35,7 +35,7 @@ namespace Fig
         }
 
         TypeInfo();
-        TypeInfo(FString _name, bool reg = false);
+        explicit TypeInfo(const FString &_name, bool reg = false);
         TypeInfo(const TypeInfo &other) = default;
 
         bool operator==(const TypeInfo &other) const
@@ -68,7 +68,7 @@ namespace Fig
         extern const TypeInfo List;
         extern const TypeInfo Map;
         extern const TypeInfo Module;
-        // extern const TypeInfo Tuple;
+        extern const TypeInfo InterfaceType;
 
         using IntClass = int64_t;
         using DoubleClass = double;
diff --git a/src/Value/interface.hpp b/src/Value/interface.hpp
new file mode 100644
index 0000000..59697c5
--- /dev/null
+++ b/src/Value/interface.hpp
@@ -0,0 +1,20 @@
+#pragma once
+
+#include <Ast/Statements/InterfaceDefSt.hpp>
+#include <Value/Type.hpp>
+
+#include <vector>
+
+namespace Fig
+{
+    struct InterfaceType
+    {
+        TypeInfo type;
+        std::vector<Ast::InterfaceMethod> methods;
+
+        bool operator==(const InterfaceType &other) const
+        {
+            return type == other.type; // only compare type info (chain -> typeinfo.id)
+        }
+    };
+}
\ No newline at end of file
diff --git a/src/Value/value.cpp b/src/Value/value.cpp
index 0a27964..c7b3db9 100644
--- a/src/Value/value.cpp
+++ b/src/Value/value.cpp
@@ -1,3 +1,4 @@
+#include "Value/Type.hpp"
 #include <Value/value.hpp>
 #include <Context/context.hpp>
 
@@ -7,12 +8,14 @@ namespace Fig
 {
     std::map<FString, size_t> TypeInfo::typeMap = {};
 
-    TypeInfo::TypeInfo() :                              // only allow use in evaluate time !! <---- dynamic type system requirement
-        id(1), name(FString(u8"Any")) {}
-    TypeInfo::TypeInfo(FString _name, bool reg)
+    TypeInfo::TypeInfo() : // only allow use in evaluate time !! <---- dynamic type system requirement
+        id(1), name(FString(u8"Any"))
+    {
+    }
+    TypeInfo::TypeInfo(const FString &_name, bool reg)
     {
         static size_t id_count = 0;
-        name = std::move(_name);
+        name = _name;
         // std::cerr << "TypeInfo constructor called for type name: " << name.toBasicString() << "\n";
         if (reg)
         {
@@ -21,10 +24,17 @@ namespace Fig
         }
         else
         {
+            if (!typeMap.contains(_name))
+            {
+                throw RuntimeError(FString(std::format(
+                    "No type named '{}'",
+                    _name.toBasicString())));
+                // *this = ValueType::String;
+            }
             id = typeMap.at(name); // may throw
         }
     }
-    
+
     size_t ValueKeyHash::operator()(const ValueKey &key) const
     {
         {
@@ -69,7 +79,7 @@ namespace Fig
         }
     }
 
-    FString prettyType(ObjectPtr obj)
+    FString prettyType(std::shared_ptr<const Object> obj)
     {
         auto t = obj->getTypeInfo();
         if (t == ValueType::StructInstance)
@@ -77,16 +87,51 @@ namespace Fig
         return t.toString();
     }
 
-    const TypeInfo ValueType::Any(FString(u8"Any"), true);           // id: 1
-    const TypeInfo ValueType::Null(FString(u8"Null"), true);         // id: 2
-    const TypeInfo ValueType::Int(FString(u8"Int"), true);           // id: 3
-    const TypeInfo ValueType::String(FString(u8"String"), true);     // id: 4
-    const TypeInfo ValueType::Bool(FString(u8"Bool"), true);         // id: 5
-    const TypeInfo ValueType::Double(FString(u8"Double"), true);     // id: 6
-    const TypeInfo ValueType::Function(FString(u8"Function"), true); // id: 7
-    const TypeInfo ValueType::StructType(FString(u8"StructType"), true);     // id: 8
+    const TypeInfo ValueType::Any(FString(u8"Any"), true);                       // id: 1
+    const TypeInfo ValueType::Null(FString(u8"Null"), true);                     // id: 2
+    const TypeInfo ValueType::Int(FString(u8"Int"), true);                       // id: 3
+    const TypeInfo ValueType::String(FString(u8"String"), true);                 // id: 4
+    const TypeInfo ValueType::Bool(FString(u8"Bool"), true);                     // id: 5
+    const TypeInfo ValueType::Double(FString(u8"Double"), true);                 // id: 6
+    const TypeInfo ValueType::Function(FString(u8"Function"), true);             // id: 7
+    const TypeInfo ValueType::StructType(FString(u8"StructType"), true);         // id: 8
     const TypeInfo ValueType::StructInstance(FString(u8"StructInstance"), true); // id: 9
-    const TypeInfo ValueType::List(FString(u8"List"), true); // id: 10
-    const TypeInfo ValueType::Map(FString(u8"Map"), true); // id: 11
-    const TypeInfo ValueType::Module(FString(u8"Module"), true); // id: 12
+    const TypeInfo ValueType::List(FString(u8"List"), true);                     // id: 10
+    const TypeInfo ValueType::Map(FString(u8"Map"), true);                       // id: 11
+    const TypeInfo ValueType::Module(FString(u8"Module"), true);                 // id: 12
+    const TypeInfo ValueType::InterfaceType(FString(u8"InterfaceType"), true);   // id: 13
+
+
+
+    bool implements(const TypeInfo &structType, const TypeInfo &interfaceType, ContextPtr ctx)
+    {
+        return ctx->hasImplRegisted(structType, interfaceType);
+    }
+
+    bool isTypeMatch(const TypeInfo &expected, ObjectPtr obj, ContextPtr ctx)
+    {
+        if (expected == ValueType::Any)
+            return true;
+
+        TypeInfo actual = obj->getTypeInfo();
+
+        if (obj->is<StructInstance>())
+        {
+            const StructInstance &si = obj->as<StructInstance>();
+            if (si.parentType == expected)
+            {
+                return true;
+            }
+            if (implements(si.parentType, expected, ctx))
+            {
+                return true;
+            }
+            return false;
+        }
+        else
+        {
+            return expected == actual;
+        }
+    }
+
 } // namespace Fig
\ No newline at end of file
diff --git a/src/Value/value.hpp b/src/Value/value.hpp
index 79d0e79..be55814 100644
--- a/src/Value/value.hpp
+++ b/src/Value/value.hpp
@@ -1,11 +1,13 @@
 #pragma once
 #include <Value/function.hpp>
+#include <Value/interface.hpp>
 #include <Value/structType.hpp>
 #include <Value/structInstance.hpp>
 #include <Value/Type.hpp>
 #include <Value/valueError.hpp>
 #include <Value/module.hpp>
 
+#include <memory>
 #include <variant>
 #include <cmath>
 #include <string>
@@ -33,7 +35,7 @@ namespace Fig
     using ObjectPtr = std::shared_ptr<Object>;
     using List = std::vector<ObjectPtr>;
 
-    FString prettyType(ObjectPtr obj);
+    FString prettyType(std::shared_ptr<const Object> obj);
 
     struct ValueKey
     {
@@ -48,7 +50,10 @@ namespace Fig
     };
     using Map = std::unordered_map<ValueKey, ObjectPtr, ValueKeyHash>;
 
-    class Object
+    bool isTypeMatch(const TypeInfo &, ObjectPtr, ContextPtr);
+    bool implements(const TypeInfo &, const TypeInfo &, ContextPtr);
+
+    class Object : public std::enable_shared_from_this<Object>
     {
     public:
         using VariantType = std::variant<
@@ -62,7 +67,8 @@ namespace Fig
             StructInstance,
             List,
             Map,
-            Module>;
+            Module,
+            InterfaceType>;
 
         std::unordered_map<TypeInfo,
                            std::unordered_map<FString,
@@ -207,7 +213,9 @@ namespace Fig
                                               map.contains(index));
                                       }},
                                  }},
-                {ValueType::Module, {}}};
+                {ValueType::Module, {}},
+                {ValueType::InterfaceType, {}},
+            };
         std::unordered_map<TypeInfo, std::unordered_map<FString, int>, TypeInfoHash> memberTypeFunctionsParas{
             {ValueType::Null, {}},
             {ValueType::Int, {}},
@@ -226,8 +234,8 @@ namespace Fig
                                  {u8"get", 1},
                                  {u8"contains", 1},
                              }},
-            {ValueType::Module, {}}
-
+            {ValueType::Module, {}},
+            {ValueType::InterfaceType, {}},
         };
         bool hasMemberFunction(const FString &name) const
         {
@@ -268,6 +276,8 @@ namespace Fig
             data(m) {}
         Object(const Module &m) :
             data(m) {}
+        Object(const InterfaceType &i) :
+            data(i) {}
 
         Object(const Object &) = default;
         Object(Object &&) noexcept = default;
@@ -364,6 +374,9 @@ namespace Fig
                 else if constexpr (std::is_same_v<T, Module>)
                     return ValueType::Module;
 
+                else if constexpr (std::is_same_v<T, InterfaceType>)
+                    return ValueType::InterfaceType;
+
                 else
                     return ValueType::Any;
             },
@@ -397,11 +410,11 @@ namespace Fig
             if (is<ValueType::StringClass>()) return FString(u8"<String \"") + as<ValueType::StringClass>() + FString(u8"\" >");
             if (is<ValueType::BoolClass>()) return as<ValueType::BoolClass>() ? FString(u8"true") : FString(u8"false");
             if (is<Function>())
-                return FString(std::format("<Function {} at {:p}>",
+                return FString(std::format("<Function '{}' at {:p}>",
                                            as<Function>().id,
                                            static_cast<const void *>(&as<Function>())));
             if (is<StructType>())
-                return FString(std::format("<StructType {} at {:p}>",
+                return FString(std::format("<StructType '{}' at {:p}>",
                                            as<StructType>().type.toString().toBasicString(),
                                            static_cast<const void *>(&as<StructType>())));
             if (is<StructInstance>())
@@ -441,9 +454,17 @@ namespace Fig
             if (is<Module>())
             {
                 return FString(std::format(
-                    "<Module at {:p}>",
+                    "<Module '{}' at {:p}>",
+                    as<Module>().name.toBasicString(),
                     static_cast<const void *>(&as<Module>())));
             }
+            if (is<InterfaceType>())
+            {
+                return FString(std::format(
+                    "<InterfaceType '{}' at {:p}",
+                    as<InterfaceType>().type.toString().toBasicString(),
+                    static_cast<const void *>(&as<InterfaceType>())));
+            }
             return FString(u8"<error>");
         }