diff --git a/bootstrap/emit.cc b/bootstrap/emit.cc
index 1e6e3c1..e121da8 100644
--- a/bootstrap/emit.cc
+++ b/bootstrap/emit.cc
@@ -136,7 +136,7 @@ std::any EmitVisitor::visitFactor(xlangParser::FactorContext *ctx) {
       if (auto expr_list = ctx->exprList()) {
         for (auto expr : expr_list->expr()) {
           visitExpr(expr);
-          args.push_back(last);
+          args.emplace_back(last);
         }
       }
       output << "  " << tmp() << " = w call $"
diff --git a/bootstrap/error.cc b/bootstrap/error.cc
index 0ccc5d2..a594bbc 100644
--- a/bootstrap/error.cc
+++ b/bootstrap/error.cc
@@ -15,8 +15,15 @@ void ErrorListener::syntaxError([[maybe_unused]] antlr4::Recognizer *recognizer,
                                 size_t line, size_t charPositionInLine,
                                 const std::string &msg,
                                 [[maybe_unused]] std::exception_ptr e) {
-  std::cerr << file << ":" << line << ":" << charPositionInLine << ": " << msg
-            << std::endl;
+  std::cerr << file << ":" << line << ":" << charPositionInLine + 1 << ": "
+            << msg << std::endl;
+  has_error = true;
+}
+
+void ErrorListener::typeError(size_t line, size_t charPositionInLine,
+                              std::string_view msg) {
+  std::cerr << file << ":" << line << ":" << charPositionInLine + 1 << ": "
+            << msg << std::endl;
   has_error = true;
 }
 
diff --git a/bootstrap/error.hh b/bootstrap/error.hh
index 7772106..6208ef4 100644
--- a/bootstrap/error.hh
+++ b/bootstrap/error.hh
@@ -11,11 +11,13 @@ class ErrorListener : public antlr4::BaseErrorListener {
 
  public:
   ErrorListener(std::string_view inputfile);
+
   bool hasError();
   void syntaxError(antlr4::Recognizer *recognizer,
                    antlr4::Token *offendingSymbol, size_t line,
                    size_t charPositionInLine, const std::string &msg,
                    std::exception_ptr e) override;
+  void typeError(size_t line, size_t charPositionInLine, std::string_view msg);
 };
 
 }  // namespace xlang
diff --git a/bootstrap/main.cc b/bootstrap/main.cc
index 0ea2670..3103813 100644
--- a/bootstrap/main.cc
+++ b/bootstrap/main.cc
@@ -5,6 +5,7 @@
 #include <error.hh>
 #include <iostream>
 #include <string>
+#include <typecheck.hh>
 #include <xlangLexer.h>
 #include <xlangParser.h>
 
@@ -51,6 +52,12 @@ int main(int argc, char **argv) {
     exit(1);
   }
 
+  xlang::TypeCheckVisitor typecheck{errorlistener};
+  typecheck.visitFile(tree);
+  if (errorlistener.hasError()) {
+    exit(1);
+  }
+
   xlang::EmitVisitor emit{outputfile};
   emit.visitFile(tree);
   return 0;
diff --git a/bootstrap/meson.build b/bootstrap/meson.build
index 6fa7c8e..c941f72 100644
--- a/bootstrap/meson.build
+++ b/bootstrap/meson.build
@@ -3,6 +3,7 @@ xc_exe = executable('xc',
       'main.cc',
       'emit.cc',
       'error.cc',
+      'typecheck.cc',
       antlr4.process('xlang.g4'),
     ],
   dependencies : [
diff --git a/bootstrap/typecheck.cc b/bootstrap/typecheck.cc
new file mode 100644
index 0000000..a4f7175
--- /dev/null
+++ b/bootstrap/typecheck.cc
@@ -0,0 +1,109 @@
+#include <typecheck.hh>
+
+namespace xlang {
+
+bool TypeCheckVisitor::inScope(const std::string &name) {
+  for (auto it = scope.end() - 1;; it--) {
+    if (it->find(name) != it->end()) {
+      return true;
+    }
+    if (it == scope.begin()) {
+      break;
+    }
+  }
+  return false;
+}
+
+TypeCheckVisitor::TypeCheckVisitor(ErrorListener &errorlistener)
+    : errorlistener{errorlistener} {}
+
+std::any TypeCheckVisitor::visitFile(xlangParser::FileContext *ctx) {
+  for (auto function : ctx->function()) {
+    auto token = function->Identifier()->getSymbol();
+    auto name = token->getText();
+    if (function_arity.find(name) != function_arity.end()) {
+      errorlistener.typeError(token->getLine(), token->getCharPositionInLine(),
+                              "duplicate function '" + name + "'");
+      continue;
+    }
+    if (auto arg_list = function->argumentList()) {
+      function_arity[name] = arg_list->Identifier().size();
+    } else {
+      function_arity[name] = 0;
+    }
+  }
+  visitChildren(ctx);
+  return {};
+}
+
+std::any TypeCheckVisitor::visitFunction(xlangParser::FunctionContext *ctx) {
+  scope.emplace_back();
+  if (auto arg_list = ctx->argumentList()) {
+    for (const auto arg : arg_list->Identifier()) {
+      scope.back().emplace(arg->getSymbol()->getText());
+    }
+  }
+  visitBlock(ctx->block());
+  scope.pop_back();
+  return {};
+}
+
+std::any TypeCheckVisitor::visitBlock(xlangParser::BlockContext *ctx) {
+  scope.emplace_back();
+  visitChildren(ctx);
+  scope.pop_back();
+  return {};
+}
+
+std::any TypeCheckVisitor::visitStatement(xlangParser::StatementContext *ctx) {
+  if (auto identifier = ctx->Identifier()) {
+    visitExpr(ctx->expr());
+    scope.back().emplace(identifier->getSymbol()->getText());
+    return {};
+  }
+  visitChildren(ctx);
+  return {};
+}
+
+std::any TypeCheckVisitor::visitFactor(xlangParser::FactorContext *ctx) {
+  if (auto identifier = ctx->Identifier()) {
+    auto token = identifier->getSymbol();
+    auto name = token->getText();
+    if (ctx->LeftParen()) {
+      if (function_arity.find(name) == function_arity.end()) {
+        errorlistener.typeError(token->getLine(),
+                                token->getCharPositionInLine(),
+                                "unknown function '" + name + "'");
+      }
+      if (auto expr_list = ctx->exprList()) {
+        auto arity = function_arity[name];
+        auto arg_num = expr_list->expr().size();
+        if (arity != arg_num) {
+          errorlistener.typeError(
+              token->getLine(), token->getCharPositionInLine(),
+              "function '" + name + "' expects " + std::to_string(arity) +
+                  " arguments, but got " + std::to_string(arg_num));
+        }
+      } else {
+        if (auto arity = function_arity[name]) {
+          errorlistener.typeError(
+              token->getLine(), token->getCharPositionInLine(),
+              "function '" + name + "' expects " + std::to_string(arity) +
+                  " arguments, but got none");
+        }
+      }
+      visitChildren(ctx);
+    } else {
+      if (!inScope(name)) {
+        errorlistener.typeError(token->getLine(),
+                                token->getCharPositionInLine(),
+                                "variable '" + name + "' is not in scope");
+      }
+    }
+    return {};
+  }
+  visitChildren(ctx);
+  return {};
+}
+
+}  // namespace xlang
diff --git a/bootstrap/typecheck.hh b/bootstrap/typecheck.hh
new file mode 100644
index 0000000..b60290f
--- /dev/null
+++ b/bootstrap/typecheck.hh
@@ -0,0 +1,29 @@
+#pragma once
+
+#include <error.hh>
+#include <string>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+#include <xlangBaseVisitor.h>
+
+namespace xlang {
+
+class TypeCheckVisitor : public xlangBaseVisitor {
+  ErrorListener &errorlistener;
+  std::unordered_map<std::string, unsigned int> function_arity;
+  std::vector<std::unordered_set<std::string>> scope;
+
+  bool inScope(const std::string &name);
+
+ public:
+  TypeCheckVisitor(ErrorListener &errorlistener);
+
+  std::any visitFile(xlangParser::FileContext *ctx) override;
+  std::any visitFunction(xlangParser::FunctionContext *ctx) override;
+  std::any visitBlock(xlangParser::BlockContext *ctx) override;
+  std::any visitStatement(xlangParser::StatementContext *ctx) override;
+  std::any visitFactor(xlangParser::FactorContext *ctx) override;
+};
+
+}  // namespace xlang