Breakdown
Creating a simple interpreter that can parse a custom command and execute it in C++ is an exciting project for anyone interested in programming languages and compilers. In this post, we’ll walk through the process of building a minimal interpreter that recognizes a single command: tell("Your Message"), which outputs the given message using std::cout.
1. Setting Up the Lexer
The first step in building our interpreter is to create a lexer. The lexer takes the input string and breaks it down into meaningful tokens. Here’s a simple implementation:
1.1 Defining the Token Enumeration
First, we create the Token enum class. This enumeration represents different types of tokens that our lexer will recognize. We include tokens for our command, strings, the end of input, invalid tokens, and parentheses.
enum class Token {
Tell, // The token for the command "tell"
String, // The token for a string literal
End, // Token indicating the end of the input
Invalid, // Token for invalid input
OpenParen, // Token for an opening parenthesis
CloseParen // Token for a closing parenthesis
};1.2 Creating the Lexeme Structure
Then we create a Lexeme struct which takes our Token type and the
struct Lexeme {
Token type; // The type of the token
std::string value; // The value associated with the token
};1.3 Implementing the Lexer Class
Now, we implement the Lexer class, which will use the input string to generate tokens. The constructor initializes the lexer with the source string and sets the starting index.
class Lexer {
public:
Lexer(const std::string& src) : src(src), index(0) {}1.4 Token Generation Logic
Within the Lexer class, we define the nextToken method. This method scans the input string and generates the next token based on the current index.
Lexeme nextToken() {
while (index < src.length() && std::isspace(src[index])) {
index++; // Skip whitespace
}
if (index >= src.length()) {
return {Token::End, ""}; // Return End token if we reach the end of the input
}
// Check for the "tell" command
if (src.substr(index, 3) == "tell") {
index += 3;
return {Token::Tell, "tell"};
}
// Check for string literals
if (src[index] == '"') {
size_t start = index++;
while (index < src.length() && src[index] != '"') {
index++; // Continue until the closing quote
}
if (index < src.length()) {
index++; // Skip closing quote
return {Token::String, src.substr(start, index - start)};
}
}
// Check for parentheses
if (src[index] == '(') {
index++;
return {Token::OpenParen, "("};
}
if (src[index] == ')') {
index++;
return {Token::CloseParen, ")"};
}
return {Token::Invalid, ""}; // Return Invalid token for unrecognized input
}1.5 Private Members of the Lexer
Finally, we define the private members of the Lexer class, which include the source string and the current index position.
private:
std::string src; // The source input string
size_t index; // Current index in the input string
};2. Implementing the Parser
The next step in our interpreter is to create a parser. The parser takes the tokens generated by the lexer and interprets them to perform actions based on the input. Here’s how we can implement it:
2.1 Defining the Parser Class
We start by defining the Parser class. This class will take a Lexer instance and manage the parsing process.
class Parser {
public:
Parser(Lexer& lexer) : lexer(lexer), currentToken(lexer.nextToken()) {}2.2 Parsing Logic
In the parse method, we define the logic for interpreting the tokens. We check if the current token is the Tell command and process it accordingly.
void parse() {
if (currentToken.type == Token::Tell) {
currentToken = lexer.nextToken(); // Get the next token
if (currentToken.type == Token::OpenParen) {
currentToken = lexer.nextToken(); // Get the next token
if (currentToken.type == Token::String) {
std::string code = generateCode(currentToken.value);
compileAndRun(code); // Compile and run the generated code
return;
}
}
std::cerr << "Syntax error!" << std::endl; // Handle syntax errors
}
}2.3 Private Members of the Parser
The Parser class also contains private members. These include a reference to the Lexer and the current token being processed.
private:
Lexer& lexer; // Reference to the lexer
Lexeme currentToken; // Current token being processed2.4 Code Generation Method
We define the generateCode method, which takes a string message and creates C++ code that outputs that message. It also escapes any double quotes in the string to ensure valid syntax.
std::string generateCode(const std::string& message) {
// Escape double quotes in the message
std::string escapedMessage = message;
size_t pos = 0;
while ((pos = escapedMessage.find("\"", pos)) != std::string::npos) {
escapedMessage.insert(pos, "\""); // Insert escape character before double quotes
pos += 2; // Move past the newly inserted character
}
return "#include <iostream>\n"
"int main() {\n"
" std::cout << \"" + escapedMessage + "\" << std::endl;\n"
" return 0;\n"
"}\n"; // Return the generated C++ code
}2.5 Compilation and Execution Method
Finally, we implement the compileAndRun method, which compiles the generated C++ code and executes it. It also cleans up the temporary files created during the process.
void compileAndRun(const std::string& code) {
std::ofstream outFile("temp.cpp");
outFile << code; // Write the generated code to a temporary file
outFile.close();
system("g++ temp.cpp -o temp && ./temp"); // Compile and run the code
system("rm temp.cpp temp"); // Clean up temporary files
}3. Putting It All Together
Finally, in our main function, we set up the lexer and parser, and pass a command to be interpreted.
int main() {
std::string input = "yap(\"Hello World\")";
Lexer lexer(input);
Parser parser(lexer);
parser.parse(); // Parse and execute the input
return 0;
}Conclusion
With this setup, we can parse and execute a simple command that prints a message to the console. This is a foundational step towards creating more complex interpreters and programming languages. By exploring the concepts of lexers and parsers, we gain valuable insights into how programming languages are designed and implemented. Happy coding!