parsing binary expressions in rust

Parsing binary expressions in Rust can be achieved using the Pratt Parsing algorithm. In this algorithm, we create an Abstract Syntax Tree (AST) and assign a precedence value to each operator. The algorithm recursively generates the AST nodes, with each operator node containing references to its left and right operands.

To begin the implementation, we can create an enum for the different types of tokens - identifiers, literals, and operators. Here is an example enum:


main.rs
#[derive(Debug, Clone, PartialEq)]
pub enum Token {
    Identifier(String),
    Literal(Literal),
    Operator(Operator),
    EndOfFile,
}
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Next, we can define the Operator struct to hold information about the operator, such as its symbol and precedence value:


main.rs
#[derive(Debug, Clone, PartialEq)]
pub struct Operator {
    pub symbol: String,
    pub precedence: u8,
}
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We can then define a function to parse the binary expression:


main.rs
fn parse_binary_expression(
    tokens: &Vec<Token>,
    min_precedence: u8,
    index: &mut usize,
) -> Result<Expression, Error> {
    let mut left_expression = match tokens.get(*index).unwrap() {
        Token::Identifier(_) => parse_identifier(tokens, index),
        Token::Literal(_) => parse_literal(tokens, index),
        Token::Operator(op) if op.symbol == "(" => parse_parentheses(tokens, index),
        Token::Operator(op) => {
            return Err(Error::new(
                ErrorType::Parsing,
                format!("Expected an operand, got operator '{}'", op.symbol),
            ));
        }
        _ => {
            return Err(Error::new(
                ErrorType::Parsing,
                "Expected an operand or parentheses, but found nothing".to_string(),
            ));
        }
    }?;

    loop {
        let token = match tokens.get(*index) {
            Some(token) => token,
            None => break,
        };

        let operator = match token {
            Token::Operator(operator) => operator,
            _ => break,
        };

        if operator.precedence < min_precedence {
            break;
        }

        *index += 1;

        let right_expression = parse_binary_expression(tokens, operator.precedence + 1, index)?;

        left_expression = Expression::OperatorNode {
            operator: operator.clone(),
            left: Box::new(left_expression),
            right: Box::new(right_expression),
        };
    }

    Ok(left_expression)
}
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In this function, we pass in the tokens vector, the min_precedence value, and a mutable index value to keep track of our position in the vector of tokens.

We first get the left expression, which can be an identifier, literal, or a nested expression within parentheses. We then loop through the remaining tokens to check for operators with a precedence value greater than or equal to min_precedence. If an appropriate operator is found, we recursively call the parse_binary_expression function to generate the right expression node, which is then used to create an operator node. The loop continues until there are no more applicable operators, and the left expression node is returned.

With this implementation, we can successfully parse binary expressions in Rust using the Pratt Parsing algorithm.

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