How to Write Your Own Language Interpreter?
Introduction
Interpreters play a crucial role in programming by executing code written in high-level languages. Creating your own interpreter can serve various purposes, such as educational goals or developing a Domain-Specific Language (DSL). This article will guide you through the process of building a simple interpreter from scratch.
1. Theoretical Part
1.1. What is an Interpreter?
An interpreter is a program that reads and executes code written in a programming language. Its primary functions include parsing the code, converting it into an intermediate representation, and executing it. The key difference between an interpreter and a compiler is that an interpreter executes code line-by-line, while a compiler translates the entire code into machine language before execution.
1.2. Main Components of an Interpreter
- Lexical Analysis (Lexer): Breaks the input code into tokens.
- Syntactic Analysis (Parser): Analyzes the token sequence to create a parse tree.
- Semantic Analysis: Checks for semantic errors in the parse tree.
- Interpretation (Code Execution): Executes the code based on the parse tree.
1.3. Choosing a Language for Creating an Interpreter
Popular languages for building interpreters include:
- Python: Easy to use, great for prototyping.
- JavaScript: Good for web-based interpreters.
- C++: Offers performance benefits but is more complex.
Each language has its pros and cons, depending on your goals and requirements.
2. Practical Part
2.1. Defining the Language Grammar
Start by defining a simple grammar for your language. For example, you can create a language for arithmetic expressions. Use Backus-Naur Form (BNF) to describe the grammar:
```
<expression> ::= <term> | <expression> '+' <term> | <expression> '-' <term>
<term> ::= <factor> | <term> '*' <factor> | <term> '/' <factor>
<factor> ::= <number> | '(' <expression> ')'
<number> ::= [0-9]+
```
2.2. Implementing the Lexer
The lexer converts the input code into tokens. Here’s a simple example in Python:
```python
import re
class Lexer:
def __init__(self, text):
self.text = text
self.pos = 0
self.current_char = self.text[self.pos]
def error(self):
raise Exception('Invalid character')
def advance(self):
self.pos += 1
if self.pos > len(self.text) - 1:
self.current_char = None
else:
self.current_char = self.text[self.pos]
def skip_whitespace(self):
while self.current_char is not None and self.current_char.isspace():
self.advance()
def variable(self):
result = ''
while self.current_char is not None and self.current_char.isalnum():
result += self.current_char
self.advance()
return result
def number(self):
result = ''
while self.current_char is not None and self.current_char.isdigit():
result += self.current_char
self.advance()
return int(result)
def get_next_token(self):
while self.current_char is not None:
if self.current_char.isspace():
self.skip_whitespace()
continue
if self.current_char.isalpha():
return Token('VARIABLE', self.variable())
if self.current_char.isdigit():
return Token('NUMBER', self.number())
if self.current_char == '+':
self.advance()
return Token('PLUS', '+')
self.error()
return Token('EOF', None)
```
2.3. Implementing the Parser
The parser takes tokens from the lexer and builds a parse tree. Here’s a simple parser example:
```python
class Parser:
def __init__(self, lexer):
self.lexer = lexer
self.current_token = self.lexer.get_next_token()
def error(self):
raise Exception('Invalid syntax')
def eat(self, token_type):
if self.current_token.type == token_type:
self.current_token = self.lexer.get_next_token()
else:
self.error()
def factor(self):
token = self.current_token
self.eat('NUMBER')
return token.value
def term(self):
result = self.factor()
while self.current_token.type in ('MUL', 'DIV'):
token = self.current_token
if token.type == 'MUL':
self.eat('MUL')
result *= self.factor()
elif token.type == 'DIV':
self.eat('DIV')
result /= self.factor()
return result
def expression(self):
result = self.term()
while self.current_token.type in ('PLUS', 'MINUS'):
token = self.current_token
if token.type == 'PLUS':
self.eat('PLUS')
result += self.term()
elif token.type == 'MINUS':
self.eat('MINUS')
result -= self.term()
return result
```
2.4. Semantic Analysis
Semantic analysis checks for logical errors in the code. For example, if a variable is used before it is defined, the interpreter should raise an error. Implement checks in the parser to validate variable usage.
2.5. Implementing the Interpreter