82 lines
2.9 KiB
Markdown
82 lines
2.9 KiB
Markdown
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```
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title = "Part 3: Basic Evaluation"
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tags = ["build a programming language", "rust"]
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date = "2021-04-15 17:00:42 -0400"
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short_desc = "A bad calculator."
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slug = "evaluation"
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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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Last time I said operator precedence was going to be next. Well, if you've read the title, you know that's not the case. I decided I really wanted to see this actually run[^1] some code[^2], so let's do that.
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[^1]: evaluate
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[^2]: a single expression
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<!-- excerpt-end -->
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First, there needs to be something to actually store values during the evaluation process. For this, I used yet another enum. It only has one case for now because we can currently only lex and parse integer values and one arithmetic operator.
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```rust
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enum Value {
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Integer(i64),
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}
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```
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There's also a helper function to extract the underlying integer from a value in places where we're certain it's going to be an integer:
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```rust
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impl Value {
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fn expect_integer(&self, &msg) -> i64 {
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match self {
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Value::Integer(n) => *n,
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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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The compiler warns about the unreachable match arm, but it'll be useful once there are more types of values. (Once again, actual error reporting will wait.)
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The actual evaulation starts in the `eval` function which takes a reference to the node to evaluate and returns a `Value` representing its result.
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For integer nodes, the value of the AST node is wrapped in a Value and returned directly. For binary operator (i.e. addition) nodes the left- and right-hand values are extracted and another function is called to perform the operation.
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```rust
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fn eval(node: &Node) -> Value {
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match node {
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Node::Integer(n) => Value::Integer(*n),
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Node::BinaryOp { left, right } => eval_binary_op(left, right),
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}
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}
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```
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This `eval_binary_op` function takes each of the nodes and calls `eval` with it. By doing this, it recurses through the the AST evaluating each node in a depth-first manner. It then turns each value into an integer (panicking if either isn't what it expects) and returns a new Value with the values added together.
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```rust
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fn eval_binary_op(left: &Node, right: &Node) -> Value {
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let left = eval(left).expect_integer("left hand side of binary operator must be an integer");
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let right = eval(right).expect_integer("right hand side of binary operator must be an integer");
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Value::Integer(left + right)
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}
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```
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And with that surpisingly small amount of code, I've got a very dumb calculator that can perform arbitrary additions:
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```rust
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fn main() {
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let tokens = tokenize("1 + 2 + 3");
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if let Some(node) = parse(tokens) {
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println!("result: {:?}", eval(&node));
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}
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}
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```
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```sh
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$ cargo run
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result: Integer(6)
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```
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Next time, I'll add some more operators and actually get around to operator precedence.
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