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<ol class="chapter"><li class="chapter-item expanded "><a href="overview.html"><strong aria-hidden="true">1.</strong> Overview</a></li><li class="chapter-item expanded "><a href="introduction.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="lexer.html" class="active"><strong aria-hidden="true">3.</strong> Lexer</a></li><li class="chapter-item expanded "><a href="parser.html"><strong aria-hidden="true">4.</strong> Parser</a></li><li class="chapter-item expanded "><a href="evaluator.html"><strong aria-hidden="true">5.</strong> Evaluator</a></li><li class="chapter-item expanded "><a href="repl.html"><strong aria-hidden="true">6.</strong> REPL</a></li><li class="chapter-item expanded "><a href="next.html"><strong aria-hidden="true">7.</strong> What's Next</a></li></ol>
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<h1 class="menu-title">Lisp interpreter in Rust</h1>
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<h1 id="lexer"><a class="header" href="#lexer">Lexer</a></h1>
<h2 id="source"><a class="header" href="#source">Source</a></h2>
<p><a href="https://github.com/vishpat/lisp-rs/blob/0.0.1/src/lexer.rs">lexer.rs</a></p>
<h2 id="code-walk-through"><a class="header" href="#code-walk-through">Code Walk Through</a></h2>
<p><strong>Lexer</strong> is a component of the interpreter that takes the program text and converts it to a stream of atomic units known as <strong>tokens</strong>. Every token has a type and may even have a value associated with it. In the case of our interpreter, there are four types of tokens. The four tokens can be represented using a Rust enum as follows.</p>
<pre><code class="language-Rust">pub enum Token {
Integer(i64),
Symbol(String),
LParen,
RParen,
}
</code></pre>
<ul>
<li>Integer: A signed 64-bit integer</li>
<li>Symbol: Any group of characters other than an integer or parenthesis</li>
<li>LParen: Left parenthesis</li>
<li>RParen: Right parenthesis</li>
</ul>
<p>The first step in implementing a lexer is to replace the parenthesis with an extra space before and after it. For example,</p>
<pre><code class="language-Lisp">(define sqr (* x x))
</code></pre>
<p>will get converted to</p>
<pre><code class="language-Lisp">( define sqr ( * x x ) )
</code></pre>
<p>With this simple trick, the process of tokenization involves splitting the Lisp program with whitespace. </p>
<pre><code class="language-Rust"> let program2 = program.replace("(", " ( ")
.replace(")", " ) ");
let words = program2.split_whitespace();
</code></pre>
<p>Once the words for the program are obtained, they can be converted into tokens using Rust's pattern matching as follows</p>
<pre><code class="language-Rust">let mut tokens: Vec<Token> = Vec::new();
for word in words {
match word {
"(" => tokens.push(Token::LParen),
")" => tokens.push(Token::RParen),
_ => {
let i = word.parse::<i64>();
if i.is_ok() {
tokens.push(Token::Integer(
i.unwrap()));
} else {
tokens.push(Token::Symbol(
word.to_string()));
}
}
}
}
</code></pre>
<p>At this point, we have a vector of tokens for the entire Lisp program. Note that a vector in Rust is a stack, hence the tokens are stored in the vector in the reverse order as shown below with an example. </p>
<p><img src="images/token_stack.png" alt="List Recursion" /></p>
<h2 id="testing"><a class="header" href="#testing">Testing</a></h2>
<p>The lexer code can be unit tested as shown below</p>
<pre><code>let tokens = tokenize("(+ 1 2)");
assert_eq!(
tokens,
vec![
Token::LParen,
Token::Symbol("+".to_string()),
Token::Integer(1),
Token::Integer(2),
Token::RParen,
]
);
</code></pre>
<p>To cement your understanding of the Lexing process please go through the remaining tests in <strong>lexer.rs</strong></p>
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