114 lines
3.7 KiB
Markdown
114 lines
3.7 KiB
Markdown
```
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title = "Part 10: Variable Declarations"
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tags = ["build a programming language", "rust"]
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date = "2021-05-09 19:14:42 -0400"
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slug = "variable-declarations"
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preamble = '<p style="font-style: italic;">This post is part of a <a href="/build-a-programming-language/" data-link="/build-a-programming-language/">series</a> about learning Rust and building a small programming language.</p><hr>'
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```
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Now that the parser can handle multiple statements and the usage of variables, let's add the ability to actually declare variables.
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<!-- excerpt-end -->
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First off, the lexer now lexes a couple new things: the `let` keyword and the equals sign.
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When the parser tries to parse a statement and sees a let token, it knows it's looking at a variable declaration. After the let token it expects to find a identifier (the variable name), an equals sign, and then an expression for the initial value of the variable. The variable name and the initial value expression then make up a `Declare` AST node.
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```rust
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fn parse_statement<'a, I: Iterator<Item = &'a Token>>(it: &mut Peekable<'a, I>) -> Option<Statement> {
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// ...
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let node = match token {
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Token::Let => {
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let name: String;
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if let Some(Token::Identifier(s)) = it.peek() {
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name = s.clone();
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it.next();
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} else {
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panic!("expected identifier after let");
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}
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expect_token!(it, Equals, "expected equals after identifier after let");
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let value = parse_expression(it).expect("initial value in let statement");
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Some(Statement::Declare { name, value })
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}
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// ...
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};
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// ...
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}
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```
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`expect_token!` is a simple macro I wrote to handle expecting to see a specific token in the stream and panicking if it's not there, since that's a pattern that was coming up frequently:
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```rust
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macro_rules! expect_token {
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($stream:ident, $token:ident, $msg:expr) => {
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if let Some(Token::$token) = $stream.peek() {
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$stream.next();
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} else {
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panic!($msg);
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}
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};
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}
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```
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Next, to actually evaluate variable declarations, the evaulator needs to have some concept of a context. Right now, every expression can be evaluated without any external state. But, when a variable is declared, we want it to be accessible later on in the code, so there needs to be somewhere to store that information.
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For now, the only information we need is the map of variable names to their values.
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```rust
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struct Context {
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variables: HashMap<String, Value>,
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}
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```
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There are also a few methods for `Context`, one to construct a new context and one to declare a variable with an initial value.
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```rust
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impl Context {
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fn new() -> Self {
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Self {
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variables: HashMap::new(),
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}
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}
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fn declare_variable(&mut self, name: &str, value: Value) {
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if self.variables.contains_key(name) {
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panic!("cannot re-declare variable {}", name);
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} else {
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self.variables.insert(name.into(), value);
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}
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}
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}
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```
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Every `eval_` function has also changed to take a reference to the current context[^1] and the main `eval` function creates a context before evaluating each statement.
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[^1]: For now a simple mutable reference is fine, because there's only ever one context: the global one. But, in the future, this will need to be something a bit more complicated.
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With that, declaration statements can be evaluated just by calling the `declare_variable` method on the context:
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```rust
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fn eval_declare_variable(name: &str, value: &Node, context: &mut Context) {
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let val = eval_expr(value, context);
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context.declare_variable(name, val);
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}
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```
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And we can actually set and read variables now[^2]:
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```rust
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fn main() {
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let code = "let foo = 1; dbg(foo)";
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let tokens = tokenize(&code);
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let statements = parse(&tokens);
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eval(&statements);
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}
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```
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```sh
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$ cargo run
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Integer(1)
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```
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[^2]: The `dbg` function is a builtin I added that prints out the Rust version of the `Value` it's passed.
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