Add Part 12: Typed Variables
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```
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metadata.title = "Part 12: Typed Variables"
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metadata.tags = ["build a programming language", "rust"]
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metadata.date = "2022-05-25 16:38:42 -0400"
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metadata.shortDesc = ""
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metadata.slug = "typed-variables"
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metadata.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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Hi. It's been a while. Though the pace of blog posts fell off a cliff last year[^1], I've continued working on my toy programming language on and off.
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[^1]: During and after WWDC21, basically all of my non-work programming energy shifted onto iOS apps, and then never shifted back. I do recognize the irony of resuming mere weeks before WWDC22.
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<!-- excerpt-end -->
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## Part 1: Type Theory is for Chumps
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I spent a while thinking about what I wanted the type system to look like—I do want some level of static typing, I know that much—but it got to the point where I was tired of thinking about it and just wanted to get back to writing code. So, lo and behold, the world's simplest type system:
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```rust
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#[derive(Debug, PartialEq, Clone, Copy)]
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enum Type {
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Integer,
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Boolean,
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String,
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}
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impl Type {
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fn is_assignable_to(&self, other: &Type) -> bool {
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self == other
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}
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}
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```
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Then, in the `Context`, rather than variables just being a map of names to `Value`s, the map now stores `VariableDecl`s:
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```rust
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struct VariableDecl {
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variable_type: Type,
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value: Value,
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}
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```
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So variable declaration and lookup now goes through a simple helper in the function that creates the `VariableDecl`.
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For now, types at variable declarations are optional at parse time since I haven't touched type inference yet and I didn't want to go back and update a bunch of unit tests. They are, however, inferred at evaluation time, if one wasn't specified.
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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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let mut variable_type = None;
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if let Some(Token::Colon) = it.peek() {
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it.next();
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variable_type = Some(parse_type().expect("type after colon in variable declaration"));
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}
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expect_token!(it, Equals, "equals in variable declaration");
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let value = parse_expression(it).expect("initial value in variable declaration");
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Some(Statement::Declare {
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name,
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variable_type,
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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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```
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The `parse_type` function is super simple, so I won't go over it—it just converts a the tokens for string/int/bool into their respective `Type`s. I call `expect` on the result of that type and then again wrap it in a `Some`, which seems redundant, because if whatever followed the colon wasn't a type, there's a syntax error and I don't want to continue.
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Actually evaluating the variable declaration is still pretty straightforward, though it now checks that the type the initialization expression evaluated to matches the declared type:
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```rust
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fn eval_declare_variable(
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name: &str,
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mutable: bool,
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variable_type: &Option<Type>,
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value: &Node,
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context: &ContextRef,
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) {
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let val = eval_expr(value, context);
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let variable_type = match variable_type {
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Some(declared) => {
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assert!(
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val.value_type().is_assignable_to(declared),
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"variable value type is not assignable to declared type"
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);
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*declared
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}
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None => val.value_type(),
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};
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context
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.borrow_mut()
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.declare_variable(name, mutable, variable_type, val);
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}
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```
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## Part 2: Variable Variables
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The other bit I added was mutable variables, so that I could write a small program that did something non-trivial.
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To do this, I changed the `VariableDecl` struct I showed above to hold a `ValueStorage` rather than a `Value` directly.
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`ValueStorage` is an enum with variants for mutable and immutable variables. Immutables variables simply own their `Value`. Mutable ones, though, wrap it in a `RefCell` so that it can be mutated.
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```rust
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enum ValueStorage {
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Immutable(Value),
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Mutable(RefCell<Value>),
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}
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```
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Setting the value is straightforward, but getting them is a bit annoying because `Value` isn't `Copy`, since it may own a string. So, there are a couple of helper functions: one to access the borrowed value and one to clone it.
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```rust
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impl ValueStorage {
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fn set(&self, value: Value) {
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match self {
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ValueStorage::Immutable(_) => panic!("cannot set immutable variable"),
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ValueStorage::Mutable(cell) => {
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*cell.borrow_mut() = value;
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}
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}
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}
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fn with_value<R, F: FnOnce(&Value) -> R>(&self, f: F) -> R {
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match self {
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ValueStorage::Immutable(val) => f(&val),
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ValueStorage::Mutable(cell) => f(&cell.borrow()),
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}
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}
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fn clone_value(&self) -> Value {
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self.with_value(|v| v.clone())
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}
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}
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```
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This works, but isn't ideal. At some point, the complex `Value` types should probably changed to reference-counted so, even if they're still not copy-able, cloning doesn't always involve an allocation.
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Lexing and parsing I won't go into detail on, since it's trivial. There's a new for `var` and whether a declaration starts with that or `let` controls the mutability.
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Setting variables isn't complicated either: when parsing a statement, if there's an equals sign after an identifier, that turns into a `SetVariable` which is evaluated simply by calling the aforementioned `set` function on the `ValueStorage` for that variable.
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And with that, I can write a little fibonacci program:
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```txt
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$ cat fib.toy
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var a = 0
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var b = 1
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var i = 0
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while (i < 10) {
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print("iteration: " + toString(i) + ", a: " + toString(a));
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let tmp = a
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a = b
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b = tmp + a
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i = i + 1
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}
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$ cargo r -- fib.toy
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iteration: 0, a: 0
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iteration: 1, a: 1
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iteration: 2, a: 1
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iteration: 3, a: 2
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iteration: 4, a: 3
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iteration: 5, a: 5
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iteration: 6, a: 8
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iteration: 7, a: 13
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iteration: 8, a: 21
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iteration: 9, a: 34
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```
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I also added a small CLI using [`structopt`](https://lib.rs/structopt) so I didn't have to keep writing code inside a string in `main.rs`.
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