added some sort of error generation when parsing

errors occur

.clangd

@@ -0,0 +1,2 @@
+SemanticTokens:
+  DisabledKinds: [Operator]

Makefile

@@ -1,70 +1,78 @@
-# ────────────────────────────────────
-# Compiler and flags
-CXX      := g++
-CXXFLAGS := -Wall -WError -I./include
-
-# ────────────────────────────────────
-# Paths
+# -------------------------------------------------------------------
+# Project‐wide settings
+# -------------------------------------------------------------------
+CXX       := g++
+CXXFLAGS  := -std=c++17 -Wall -Wextra -Iinclude -Isrc -MMD -MP
+LDFLAGS   :=
 SRC_DIR   := src
-TEST_SRC  := test/test.cpp
-REPL_SRC  := $(SRC_DIR)/main.cpp
+TEST_DIR  := test
+BUILD_DIR := build
+OBJ_DIR   := $(BUILD_DIR)/objs
+BIN_DIR   := $(BUILD_DIR)/bin
 
-OBJ_DIR   := build/obj
-BIN_DIR   := build/bin
+# -------------------------------------------------------------------
+# Source & object lists
+# -------------------------------------------------------------------
+SRC_CPP    := $(shell find $(SRC_DIR)  -name '*.cpp')
+TEST_CPP   := $(shell find $(TEST_DIR) -name '*.cpp')
+OBJ        := $(patsubst $(SRC_DIR)/%.cpp,$(OBJ_DIR)/%.o,$(SRC_CPP))
+TEST_OBJ   := $(patsubst $(TEST_DIR)/%.cpp,$(OBJ_DIR)/test/%.o,$(TEST_CPP))
+DEPFILES   := $(OBJ:.o=.d) $(TEST_OBJ:.o=.d)
 
-# ────────────────────────────────────
-# Source listings
-# All .cpp under src/, but exclude your REPL main
-LIB_SRCS := $(filter-out $(REPL_SRC),$(shell find $(SRC_DIR) -name '*.cpp'))
-
-# Mirror src/.../*.cpp → build/obj/src/.../*.o
-LIB_OBJS := $(patsubst $(SRC_DIR)/%.cpp,$(OBJ_DIR)/$(SRC_DIR)/%.o,$(LIB_SRCS))
+# Identify your “real” main.cpp so we can exclude it from tests
+MAIN_SRC        := $(SRC_DIR)/main.cpp
+MAIN_OBJ        := $(MAIN_SRC:$(SRC_DIR)/%.cpp=$(OBJ_DIR)/%.o)
+SRC_OBJS_NO_MAIN := $(filter-out $(MAIN_OBJ),$(OBJ))
 
 # Binaries
-TEST_BIN := $(BIN_DIR)/tests
-REPL_BIN := $(BIN_DIR)/repl
+TARGET        := $(BIN_DIR)/monkey
+TEST_TARGET   := $(BIN_DIR)/monkey_tests
 
-# ────────────────────────────────────
-# Default target: build & run tests
-all: test
+# -------------------------------------------------------------------
+# Top‐level rules
+# -------------------------------------------------------------------
+.PHONY: all clean run tests valgrind
+all: $(TARGET) $(TEST_TARGET)
 
-# ─ Link test runner (test.cpp defines main via DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN)
-$(TEST_BIN): $(LIB_OBJS) | $(BIN_DIR)
-    @echo "⏳ Linking tests..."
-    $(CXX) $(CXXFLAGS) $(TEST_SRC) $(LIB_OBJS) -o $@
+valgrind: CXXFLAGS += -O0 -g
+valgrind: $(TEST_TARGET)
+	valgrind -s --leak-check=full --show-leak-kinds=all --track-origins=yes $(TEST_TARGET)
 
-# ─ Link REPL
-$(REPL_BIN): $(LIB_OBJS) | $(BIN_DIR)
-    @echo "🚀 Linking REPL..."
-    $(CXX) $(CXXFLAGS) $(REPL_SRC) $(LIB_OBJS) -o $@
-
-# ─ Compile each library .cpp → mirrored .o
-$(OBJ_DIR)/$(SRC_DIR)/%.o: $(SRC_DIR)/%.cpp
-    @echo "🛠  Compiling $<"
-    @mkdir -p $(dir $@)
-    $(CXX) $(CXXFLAGS) -c $< -o $@
-
-# ────────────────────────────────────
-# Run or launch targets
-.PHONY: test repl clean all
-
-test: $(TEST_BIN)
-    @echo "\n✅ Running tests..."
-    @$(TEST_BIN)
-    # @$(TEST_BIN) $(if $(TESTCASE),--test-case=$(TESTCASE))
-
-repl: $(REPL_BIN)
-    @echo "\n🔧 Starting REPL..."
-    @$(REPL_BIN)
-
-# ────────────────────────────────────
-# Ensure bin/ exists before linking
-$(BIN_DIR):
-    @mkdir -p $@
-
-# ────────────────────────────────────
-# Clean up everything
 clean:
-    @echo "🧹 Cleaning build artifacts"
-    @rm -rf $(OBJ_DIR) $(BIN_DIR)
+	@rm -rf $(BUILD_DIR)
 
+# -------------------------------------------------------------------
+# Build & run
+# -------------------------------------------------------------------
+run: $(TARGET)
+	@$(TARGET)
+
+test: $(TEST_TARGET)
+	@$(TEST_TARGET) $(if $(TEST),--test-case=$(TEST))
+
+# -------------------------------------------------------------------
+# Link binaries
+# -------------------------------------------------------------------
+$(TARGET): $(OBJ)
+	@mkdir -p $(BIN_DIR)
+	$(CXX) $(LDFLAGS) $^ -o $@
+
+$(TEST_TARGET): $(SRC_OBJS_NO_MAIN) $(TEST_OBJ)
+	@mkdir -p $(BIN_DIR)
+	$(CXX) $(LDFLAGS) $^ -o $@
+
+# -------------------------------------------------------------------
+# Compile rules
+# -------------------------------------------------------------------
+$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp
+	@mkdir -p $(@D)
+	$(CXX) $(CXXFLAGS) -c $< -o $@
+
+$(OBJ_DIR)/test/%.o: $(TEST_DIR)/%.cpp
+	@mkdir -p $(@D)
+	$(CXX) $(CXXFLAGS) -c $< -o $@
+
+# -------------------------------------------------------------------
+# Auto‐include dependencies
+# -------------------------------------------------------------------
+-include $(DEPFILES)

compile_flags.txt

@@ -0,0 +1,2 @@
+-I./include
+-I./src

src/ast/ast.cpp

@@ -0,0 +1,9 @@
+#include "ast.hpp"
+
+namespace ast {
+    std::string program ::token_literal() const {
+        if (statements.size() > 0)
+            return statements[0]->token_literal();
+        return "";
+    }
+} // namespace ast

src/ast/ast.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace ast {
+    struct node {
+        virtual std::string token_literal() const = 0;
+        virtual ~node() = default;
+    };
+
+    struct statement : node {
+        virtual std::string token_literal() const override = 0;
+    };
+
+    struct expression : node {
+        virtual std::string token_literal() const override = 0;
+    };
+
+    struct program : public node {
+        std::vector<statement*> statements;
+        std::string token_literal() const override;
+
+        ~program() {
+            for (const auto& ref : statements)
+                delete ref;
+        };
+    };
+} // namespace ast

src/ast/errors/error.hpp

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "token/type.hpp"
+
+namespace ast::error {
+    struct error : public std::runtime_error {
+        explicit error(const std::string& message)
+            : std::runtime_error(message) {}
+    };
+
+    struct parser_error : error {
+        explicit parser_error(const std::string& message): error(message) {}
+    };
+
+    struct expected_next : parser_error {
+        token::type expected_type;
+
+        explicit expected_next(
+            token::type expected_type, const std::string& message
+        )
+            : parser_error(message),
+              expected_type(expected_type) {}
+    };
+
+} // namespace ast::error

src/ast/expressions/identifier.cpp

@@ -0,0 +1,11 @@
+#include "identifier.hpp"
+
+namespace ast {
+    identifier::identifier(token::token token, std::string value)
+        : token(std::move(token)),
+          value(std::move(value)) {}
+
+    std::string identifier::token_literal() const {
+        return token.literal;
+    }
+} // namespace ast

src/ast/expressions/identifier.hpp

@@ -0,0 +1,16 @@
+#pragma once
+
+#include "ast/ast.hpp"
+#include "token/token.hpp"
+
+#include <string>
+
+namespace ast {
+    struct identifier : expression {
+        identifier(token::token token, std::string value);
+        token::token token;
+        std::string value;
+
+        std::string token_literal() const override;
+    };
+} // namespace ast

src/ast/statements/let.cpp

@@ -0,0 +1,17 @@
+#include "let.hpp"
+
+namespace ast {
+    let::let(token::token token)
+        : token(std::move(token)),
+          name(nullptr),
+          value(nullptr) {}
+
+    std::string let::token_literal() const {
+        return token.literal;
+    }
+
+    let::~let() {
+        delete name;
+        delete value;
+    };
+} // namespace ast

src/ast/statements/let.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "ast/ast.hpp"
+#include "ast/expressions/identifier.hpp"
+#include "token/token.hpp"
+
+namespace ast {
+    struct let : statement {
+        let(token::token token);
+
+        token::token token;
+        identifier* name;
+        expression* value;
+
+        std::string token_literal() const override;
+
+        ~let();
+    };
+} // namespace ast

src/lexer/lexer.cpp

@@ -0,0 +1,83 @@
+#include "lexer.hpp"
+
+#include "token/token.hpp"
+#include "token/type.hpp"
+
+#include <cctype>
+#include <iostream>
+
+namespace lexer {
+    token::token lexer::next_token() {
+        if (!(input >> c))
+            return {token::type::END_OF_FILE, ""};
+
+        switch (c) {
+        case '=':
+            if (input.peek() == '=')
+                return {token::type::EQ, std::string{c, (char) input.get()}};
+            return {token::type::ASSIGN, c};
+        case '+':
+            return {token::type::PLUS, c};
+        case '-':
+            return {token::type::MINUS, c};
+        case '!':
+            if (input.peek() == '=')
+                return {token::type::NEQ, std::string{c, (char) input.get()}};
+            return {token::type::BANG, c};
+        case '*':
+            return {token::type::ASTERISK, c};
+        case '/':
+            return {token::type::SLASH, c};
+        case '<':
+            return {token::type::LT, c};
+        case '>':
+            return {token::type::GT, c};
+        case ',':
+            return {token::type::COMMA, c};
+        case ';':
+            return {token::type::SEMICOLON, c};
+        case '(':
+            return {token::type::LPAREN, c};
+        case ')':
+            return {token::type::RPAREN, c};
+        case '{':
+            return {token::type::LBRACE, c};
+        case '}':
+            return {token::type::RBRACE, c};
+        default:
+            if (is_letter(c)) {
+                std::string identifier_or_keyword = read_string();
+                return {
+                    token::lookup_identifier(identifier_or_keyword),
+                    identifier_or_keyword
+                };
+            }
+            if (std::isdigit(c))
+                return {token::type::INT, read_int()};
+
+            return {token::type::ILLEGAL, c};
+        }
+    }
+
+    bool lexer::is_letter(char c) {
+        return c == '_' || std::isalpha(static_cast<unsigned char>(c));
+    }
+
+    std::string lexer::read_string() {
+        std::string result;
+        result.push_back(c);
+        for (char c = input.peek(); is_letter(c); c = input.peek())
+            result.push_back(input.get());
+        return result;
+    }
+
+    std::string lexer::read_int() {
+        std::string result;
+        result.push_back(c);
+        for (char c = input.peek(); std::isdigit(c); c = input.peek())
+            result.push_back(input.get());
+        return result;
+    }
+
+
+} // namespace lexer

src/lexer/lexer.hpp

@@ -0,0 +1,18 @@
+#pragma once
+#include "token/token.hpp"
+
+#include <istream>
+
+namespace lexer {
+    struct lexer {
+        std::istream& input;
+        char c = 0;
+        token::token next_token();
+
+      private:
+        bool is_letter(char);
+
+        std::string read_string();
+        std::string read_int();
+    };
+} // namespace lexer

src/main.cpp

@@ -0,0 +1,8 @@
+#include "repl/repl.hpp"
+
+#include <iostream>
+
+int main() {
+    repl::start(std::cin, std::cout);
+    return 0;
+}

src/parser/parser.cpp

@@ -0,0 +1,84 @@
+#include "parser.hpp"
+
+#include "ast/errors/error.hpp"
+#include "token/type.hpp"
+
+#include <sstream>
+
+namespace parser {
+    parser::parser(lexer::lexer& lexer)
+        : lexer(lexer),
+          current(token::type::ILLEGAL, ""),
+          next(token::type::ILLEGAL, "") {
+        next_token();
+        next_token();
+    }
+
+    void parser::next_token() {
+        current = next;
+        next = lexer.next_token();
+    }
+
+    ast::program* parser::parse_program() {
+        ast::program* p = new ast::program();
+
+        for (; current.type != token::type::END_OF_FILE; next_token()) {
+            ast::statement* stmt = parse_statement();
+            if (stmt != nullptr)
+                p->statements.push_back(stmt);
+        }
+
+        return p;
+    }
+
+    ast::statement* parser::parse_statement() {
+        switch (current.type) {
+        case token::type::LET:
+            return parse_let();
+        default:
+            return nullptr;
+        }
+    }
+
+    bool parser::expect_next(token::type t) {
+        if (next.type == t) {
+            next_token();
+            return true;
+        }
+        next_error(t);
+        return false;
+    }
+
+    ast::let* parser::parse_let() {
+        ast::let* stmt = new ast::let(current);
+
+        if (!expect_next(token::type::IDENTIFIER)) {
+            delete stmt;
+            return nullptr;
+        }
+
+        stmt->name = new ast::identifier{current, current.literal};
+
+        if (!expect_next(token::type::ASSIGN)) {
+            delete stmt;
+            return nullptr;
+        }
+
+        // TODO: we are currently skipping expressions until we encounter a
+        // semicolon
+        for (; current.type != token::type::SEMICOLON; next_token()) {}
+        return stmt;
+    }
+
+    void parser::next_error(token::type t) {
+        std::stringstream ss;
+        ss << "Expected next token to be " << t << " but instead got "
+           << next.type;
+        errors.push_back(new ast::error::expected_next(t, ss.str()));
+    }
+
+    parser::~parser() {
+        for (const auto& e : errors)
+            delete e;
+    }
+} // namespace parser

src/parser/parser.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "ast/ast.hpp"
+#include "ast/errors/error.hpp"
+#include "ast/statements/let.hpp"
+#include "lexer/lexer.hpp"
+#include "token/token.hpp"
+
+namespace parser {
+    struct parser {
+        parser(lexer::lexer& lexer);
+        ~parser();
+        std::vector<ast::error::error*> errors;
+
+        ast::program* parse_program();
+
+      private:
+        lexer::lexer& lexer;
+        token::token current, next;
+
+        void next_token();
+        ast::statement* parse_statement();
+        ast::let* parse_let();
+        bool expect_next(token::type);
+        void next_error(token::type);
+    };
+} // namespace parser

src/repl/repl.cpp

@@ -0,0 +1,30 @@
+#include "repl.hpp"
+
+#include "lexer/lexer.hpp"
+
+#include <sstream>
+#include <string>
+
+static const std::string PROMPT = ">> ";
+
+namespace repl {
+
+    void start(std::istream& in, std::ostream& out) {
+        while (true) {
+            out << PROMPT;
+            std::string line;
+            if (!std::getline(in, line))
+                return;
+
+            std::istringstream ss(line);
+
+            lexer::lexer l{ss};
+            for (token::token tok = l.next_token();
+                 tok.type != token::type::END_OF_FILE;
+                 tok = l.next_token())
+                out << tok << " ";
+            out << std::endl;
+        }
+    }
+
+} // namespace repl

src/repl/repl.hpp

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <istream>
+#include <ostream>
+
+namespace repl {
+    void start(std::istream&, std::ostream&);
+}

src/token/token.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "type.hpp"
+
+#include <string>
+
+namespace token {
+    struct token {
+        ::token::type type;
+        std::string literal;
+
+        token(::token::type t, std::string s): type(t), literal(s) {}
+
+        token(::token::type t, char c): type(t), literal(1, c) {}
+    };
+
+    inline std::ostream& operator<<(std::ostream& os, token tok) {
+        return os << tok.type << '(' << tok.literal << ')';
+    }
+} // namespace token

src/token/type.cpp

@@ -0,0 +1,43 @@
+#include "type.hpp"
+
+#include <array>
+#include <unordered_map>
+
+namespace token {
+
+    // Array mapping enum values to their string representations
+    constexpr std::
+        array<std::string_view, static_cast<size_t>(type::RETURN) + 1>
+            tokenTypeStrings = {
+#define X(name, str) str,
+                TOKEN_LIST
+#undef X
+    };
+
+    // Stream insertion operator using the lookup array
+    std::ostream& operator<<(std::ostream& os, type type) {
+        auto idx = static_cast<size_t>(type);
+        if (idx < tokenTypeStrings.size())
+            return os << tokenTypeStrings[idx];
+        return os << "Unknown";
+    }
+
+    static std::unordered_map<std::string, type> keywords{
+        {"fn", type::FUNCTION},
+        {"let", type::LET},
+        {"if", type::IF},
+        {"else", type::ELSE},
+        {"true", type::TRUE},
+        {"false", type::FALSE},
+        {"return", type::RETURN},
+    };
+
+    type lookup_identifier(std::string ident) {
+        try {
+            return keywords.at(ident);
+        } catch (const std::out_of_range&) {
+            return type::IDENTIFIER;
+        }
+    }
+
+} // namespace token

src/token/type.hpp

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <ostream>
+
+namespace token {
+
+// X-macro list of token types and their string representations
+#define TOKEN_LIST                                                             \
+    X(ILLEGAL, "ILLEGAL")                                                      \
+    X(END_OF_FILE, "EOF")                                                      \
+    X(IDENTIFIER, "IDENTIFIER")                                                \
+    X(INT, "INT")                                                              \
+    X(ASSIGN, "=")                                                             \
+    X(PLUS, "+")                                                               \
+    X(MINUS, "-")                                                              \
+    X(BANG, "!")                                                               \
+    X(ASTERISK, "*")                                                           \
+    X(SLASH, "/")                                                              \
+    X(LT, "<")                                                                 \
+    X(GT, ">")                                                                 \
+    X(COMMA, ",")                                                              \
+    X(SEMICOLON, ";")                                                          \
+    X(LPAREN, "(")                                                             \
+    X(RPAREN, ")")                                                             \
+    X(LBRACE, "{")                                                             \
+    X(RBRACE, "}")                                                             \
+    X(LET, "LET")                                                              \
+    X(FUNCTION, "FUNCTION")                                                    \
+    X(IF, "IF")                                                                \
+    X(ELSE, "ELSE")                                                            \
+    X(TRUE, "TRUE")                                                            \
+    X(FALSE, "FALSE")                                                          \
+    X(EQ, "==")                                                                \
+    X(NEQ, "!=")                                                               \
+    X(RETURN, "RETURN")
+
+    // Define the TokenType enum using the X-macro
+    enum class type {
+#define X(name, str) name,
+        TOKEN_LIST
+#undef X
+    };
+
+    std::ostream& operator<<(std::ostream&, type);
+    type lookup_identifier(std::string);
+} // namespace token

test/lexer.cpp

@@ -0,0 +1,161 @@
+#include "lexer/lexer.hpp"
+
+#include "token/type.hpp"
+
+#include <doctest.h>
+#include <sstream>
+#include <string>
+
+TEST_CASE("Single character token") {
+    struct test {
+        token::type expectedType;
+        std::string expectedLiteral;
+    };
+
+    std::string input = "=+(){},;";
+    std::istringstream ss(input);
+
+    lexer::lexer l{ss};
+
+    test tests[] = {
+        {token::type::ASSIGN, "="},
+        {token::type::PLUS, "+"},
+        {token::type::LPAREN, "("},
+        {token::type::RPAREN, ")"},
+        {token::type::LBRACE, "{"},
+        {token::type::RBRACE, "}"},
+        {token::type::COMMA, ","},
+        {token::type::SEMICOLON, ";"},
+        {token::type::END_OF_FILE, ""},
+    };
+
+    for (const auto& t : tests) {
+        token::token tok = l.next_token();
+        REQUIRE(tok.type == t.expectedType);
+        REQUIRE(tok.literal == t.expectedLiteral);
+    }
+};
+
+TEST_CASE("More tokens") {
+    struct test {
+        token::type expectedType;
+        std::string expectedLiteral;
+    };
+
+    std::istringstream ss("let five = 5;\
+let ten = 10;\
+let add = fn(x, y) {\
+    x + y;\
+};\
+let result = add(five, ten);\
+!-/*5;\
+5 < 10 > 5;\
+\
+if (5 < 10) {\
+    return true;\
+} else {\
+    return false;\
+}\
+\
+10 == 10;\
+10 != 9;\
+");
+
+    lexer::lexer l{ss};
+
+    test tests[] = {
+        // clang-format off
+        {token::type::LET, "let"},
+        {token::type::IDENTIFIER, "five"},
+        {token::type::ASSIGN, "="},
+        {token::type::INT, "5"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::LET, "let"},
+        {token::type::IDENTIFIER, "ten"},
+        {token::type::ASSIGN, "="},
+        {token::type::INT, "10"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::LET, "let"},
+        {token::type::IDENTIFIER, "add"},
+        {token::type::ASSIGN, "="},
+        {token::type::FUNCTION, "fn"},
+        {token::type::LPAREN, "("},
+        {token::type::IDENTIFIER, "x"},
+        {token::type::COMMA, ","},
+        {token::type::IDENTIFIER, "y"},
+        {token::type::RPAREN, ")"},
+        {token::type::LBRACE, "{"},
+        {token::type::IDENTIFIER, "x"},
+        {token::type::PLUS, "+"},
+        {token::type::IDENTIFIER, "y"},
+        {token::type::SEMICOLON, ";"},
+        {token::type::RBRACE, "}"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::LET, "let"},
+        {token::type::IDENTIFIER, "result"},
+        {token::type::ASSIGN, "="},
+        {token::type::IDENTIFIER, "add"},
+        {token::type::LPAREN, "("},
+        {token::type::IDENTIFIER, "five"},
+        {token::type::COMMA, ","},
+        {token::type::IDENTIFIER, "ten"},
+        {token::type::RPAREN, ")"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::BANG, "!"},
+        {token::type::MINUS, "-"},
+        {token::type::SLASH, "/"},
+        {token::type::ASTERISK, "*"},
+        {token::type::INT, "5"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::INT, "5"},
+        {token::type::LT, "<"},
+        {token::type::INT, "10"},
+        {token::type::GT, ">"},
+        {token::type::INT, "5"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::IF, "if"},
+        {token::type::LPAREN, "("},
+        {token::type::INT, "5"},
+        {token::type::LT, "<"},
+        {token::type::INT, "10"},
+        {token::type::RPAREN, ")"},
+        {token::type::LBRACE, "{"},
+
+        {token::type::RETURN, "return"},
+        {token::type::TRUE, "true"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::RBRACE, "}"},
+        {token::type::ELSE, "else"},
+        {token::type::LBRACE, "{"},
+
+        {token::type::RETURN, "return"},
+        {token::type::FALSE, "false"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::RBRACE, "}"},
+
+        {token::type::INT, "10"},
+        {token::type::EQ, "=="},
+        {token::type::INT, "10"},
+        {token::type::SEMICOLON, ";"},
+
+        {token::type::INT, "10"},
+        {token::type::NEQ, "!="},
+        {token::type::INT, "9"},
+        {token::type::SEMICOLON, ";"},
+        // clang-format on
+    };
+
+    for (const auto& t : tests) {
+        token::token tok = l.next_token();
+        REQUIRE(tok.type == t.expectedType);
+        REQUIRE(tok.literal == t.expectedLiteral);
+    }
+};

test/parser.cpp

@@ -0,0 +1,134 @@
+#include "parser/parser.hpp"
+
+#include "ast/ast.hpp"
+#include "ast/statements/let.hpp"
+#include "lexer/lexer.hpp"
+
+#include <doctest.h>
+#include <iostream>
+#include <sstream>
+
+void test_let_statement(ast::statement* stmt, const std::string name) {
+    REQUIRE(stmt->token_literal() == "let");
+    ast::let* let_stmt;
+    REQUIRE_NOTHROW(let_stmt = dynamic_cast<ast::let*>(stmt));
+    REQUIRE_MESSAGE(
+        let_stmt != nullptr,
+        "Couldn't cast statement to a let statement"
+    );
+
+    REQUIRE(let_stmt->name->value == name);
+    REQUIRE(let_stmt->name->token_literal() == name);
+}
+
+void test_failing_let_parsing(
+    std::string input_s,
+    std::vector<token::type> expected_types,
+    int n_good_statements = 0
+) {
+    std::stringstream input(input_s);
+
+    lexer::lexer l{input};
+    parser::parser p{l};
+
+    ast::program* program = p.parse_program();
+
+    // Check for errors
+    REQUIRE(p.errors.size() == expected_types.size());
+
+    int i = 0;
+    for (auto& e : p.errors) {
+        ast::error::expected_next* en;
+
+        REQUIRE_NOTHROW(en = dynamic_cast<ast::error::expected_next*>(e));
+        REQUIRE_MESSAGE(
+            en != nullptr,
+            "Couldn't cast the error to an 'expected_next'"
+        );
+        REQUIRE(en->expected_type == expected_types[i++]);
+    }
+
+    // normal program check
+    REQUIRE_MESSAGE(
+        program != nullptr,
+        "parse_program() returned a null pointer"
+    );
+    REQUIRE(program->statements.size() == n_good_statements);
+    delete program;
+}
+
+void check_parser_errors(const std::vector<ast::error::error*>& errors) {
+    if (errors.empty())
+        return;
+
+    std::cerr << "parser has " << errors.size() << " errors:\n";
+    for (const auto& error : errors)
+        std::cerr << '\t' << error->what() << "\n";
+
+    // Use doctest's FAIL macro to immediately stop
+    FAIL_CHECK("Parser had errors. See stderr for details.");
+}
+
+TEST_CASE("Malformed let statement (checking for memory leaks)") {
+    SUBCASE("Second token not identifier") {
+        test_failing_let_parsing("let 5 = 5;", {token::type::IDENTIFIER});
+    }
+
+    SUBCASE("Third token not '='") {
+        test_failing_let_parsing("let five ! 5;", {token::type::ASSIGN});
+    }
+
+    SUBCASE("Missing both identifier and '='") {
+        test_failing_let_parsing("let 5;", {token::type::IDENTIFIER});
+    }
+
+    SUBCASE("Multiple parsing errors") {
+        test_failing_let_parsing(
+            "let 5; let ! = 5; let five = 5; let five 5; let;",
+            {token::type::IDENTIFIER,
+             token::type::IDENTIFIER,
+             token::type::ASSIGN,
+             token::type::IDENTIFIER},
+            1
+        );
+    }
+}
+
+TEST_CASE("Parse let statement") {
+    std::stringstream input("\
+let x = 5;\
+let y = 10;\
+let foobar = 103213;\
+");
+
+    lexer::lexer l{input};
+    parser::parser p{l};
+
+    ast::program* program = p.parse_program();
+    check_parser_errors(p.errors);
+
+    REQUIRE_MESSAGE(
+        program != nullptr,
+        "parse_program() returned a null pointer"
+    );
+    REQUIRE(program->statements.size() == 3);
+
+    struct test {
+        std::string expected_identifier;
+    };
+
+    test tests[]{
+        "x",
+        "y",
+        "foobar",
+    };
+
+    int i = 0;
+    for (const auto& t : tests) {
+        ast::statement* stmt = program->statements[i++];
+
+        test_let_statement(stmt, t.expected_identifier);
+    }
+
+    delete program;
+}

test/test.cpp

@@ -0,0 +1,20 @@
+#define DOCTEST_CONFIG_IMPLEMENT_WITH_MAIN
+#include <doctest.h>
+
+int factorial(int number) {
+    return number <= 1 ? number : factorial(number - 1) * number;
+}
+
+TEST_CASE("fact") {
+    CHECK(factorial(1) == 1);
+    CHECK(factorial(2) == 2);
+    CHECK(factorial(3) == 6);
+    CHECK(factorial(10) == 3628800);
+}
+
+TEST_CASE("fact2") {
+    CHECK(factorial(1) == 1);
+    CHECK(factorial(2) == 2);
+    CHECK(factorial(3) == 6);
+    CHECK(factorial(10) == 3628800);
+}