rlox/src/middleend/visit_ast.rs

//! A reusable, read-only AST visitor with default traversal.
//!
//! The traversal is split into two layers so that many different static
//! analyses can share a single definition of "how to walk the tree":
//!
//! * The `walk_*` free functions contain the canonical recursion. For each
//!   node they call back into the visitor on every child. This is the only
//!   place that needs to know the shape of the AST.
//! * The [`Visitor`] trait's `visit_*` methods are the overridable hooks. Each
//!   one defaults to calling the matching `walk_*` function, so a visitor that
//!   overrides nothing still performs a complete traversal.
//!
//! To implement an analysis, implement [`Visitor`] and override only the hooks
//! you care about. Inside an override, call the corresponding `walk_*` function
//! whenever you want the default "descend into children" behaviour to happen.
//! Accumulate results in your own struct fields (errors, scope tables, type
//! information, ...) rather than through the return value, which is only used
//! to short-circuit on error.
//!
//! # Example
//!
//! ```ignore
//! struct IdentifierCounter { count: usize }
//!
//! impl Visitor for IdentifierCounter {
//!     fn visit_expr(&mut self, expr: &AstNode) -> LoxResult<()> {
//!         if let Expr::Identifier { .. } = &expr.node {
//!             self.count += 1;
//!         }
//!         walk_expr(self, expr) // keep descending into children
//!     }
//! }
//! ```

use crate::common::{
    ast::{AstNode, Expr},
    base_value::{BaseValue, LoxFunction},
    lox_result::LoxResult,
};

/// A read-only visitor over the AST.
///
/// Every hook has a default implementation that performs the standard
/// recursive traversal, so implementors only override the cases they need.
pub trait Visitor: Sized {
    /// Visit an expression node. Defaults to [`walk_expr`].
    fn visit_expr(&mut self, expr: &AstNode) -> LoxResult<()> {
        walk_expr(self, expr)
    }

    /// Visit a function literal (parameters, optional guard and body).
    ///
    /// Defaults to [`walk_function`], which walks the guard (if any) and the
    /// body. Override this to manage a parameter scope before descending.
    fn visit_function(&mut self, function: &LoxFunction) -> LoxResult<()> {
        walk_function(self, function)
    }
}

/// Recurse into the children of `expr`, calling back into `visitor`.
pub fn walk_expr<V: Visitor>(visitor: &mut V, expr: &AstNode) -> LoxResult<()> {
    match &expr.node {
        // A function literal carries an entire sub-tree (its body), so it is
        // not a leaf: hand it to the dedicated function hook.
        Expr::Literal { value } => {
            if let BaseValue::Function(function) = &**value {
                visitor.visit_function(function)?;
            }
            Ok(())
        }
        Expr::Identifier { .. } => Ok(()),
        Expr::Binary { left, right, .. } => {
            visitor.visit_expr(left)?;
            visitor.visit_expr(right)
        }
        Expr::Unary { operand, .. } => visitor.visit_expr(operand),
        Expr::Assign { value, .. } => visitor.visit_expr(value),
        Expr::Grouping { expression } => visitor.visit_expr(expression),
        Expr::Call {
            callee, arguments, ..
        } => {
            visitor.visit_expr(callee)?;
            for argument in arguments.iter() {
                visitor.visit_expr(argument)?;
            }
            Ok(())
        }

        Expr::Print { expression, .. } => visitor.visit_expr(expression),
        Expr::VarDeclaration { initializer, .. } => {
            if let Some(initializer) = initializer {
                visitor.visit_expr(initializer)?;
            }
            Ok(())
        }
        Expr::Return { expression, .. } => visitor.visit_expr(expression),
        Expr::Block { statements, .. } => {
            for statement in statements.iter() {
                visitor.visit_expr(statement)?;
            }
            Ok(())
        }
        Expr::If {
            condition,
            then_branch,
            elif_branches,
            else_branch,
            ..
        } => {
            visitor.visit_expr(condition)?;
            visitor.visit_expr(then_branch)?;
            for (elif_condition, elif_body) in elif_branches.iter() {
                visitor.visit_expr(elif_condition)?;
                visitor.visit_expr(elif_body)?;
            }
            if let Some(else_body) = else_branch {
                visitor.visit_expr(else_body)?;
            }
            Ok(())
        }
        Expr::While {
            condition, body, ..
        } => {
            visitor.visit_expr(condition)?;
            visitor.visit_expr(body)
        }
        Expr::For {
            variable,
            condition,
            increment,
            body,
            ..
        } => {
            visitor.visit_expr(variable)?;
            visitor.visit_expr(condition)?;
            visitor.visit_expr(increment)?;
            visitor.visit_expr(body)
        }
    }
}

/// Walk the guard (if present) and body of a function literal.
pub fn walk_function<V: Visitor>(visitor: &mut V, function: &LoxFunction) -> LoxResult<()> {
    if let Some(guard) = &function.guard {
        visitor.visit_expr(guard)?;
    }
    visitor.visit_expr(&function.body)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::common::lox_result::runtime_error;
    use crate::frontend::lexer::Lexer;
    use crate::frontend::parser::Parser;

    fn parse(src: &str) -> Vec<AstNode> {
        let tokens = Lexer::new(src.to_string(), 0)
            .scans_tokens()
            .expect("source should lex");
        Parser::new(tokens).parse().expect("source should parse")
    }

    /// A visitor that relies entirely on the default traversal and just counts
    /// how many nodes it sees.
    #[derive(Default)]
    struct Counter {
        nodes: usize,
    }

    impl Visitor for Counter {
        fn visit_expr(&mut self, node: &AstNode) -> LoxResult<()> {
            self.nodes += 1;
            walk_expr(self, node)
        }
    }

    fn count(src: &str) -> Counter {
        let mut counter = Counter::default();
        for stmt in parse(src).iter() {
            counter.visit_expr(stmt).unwrap();
        }
        counter
    }

    #[test]
    fn counts_every_node_via_default_traversal() {
        // 1 + 2 * 3  =>  Binary(+){ Literal, Binary(*){ Literal, Literal } }
        let counter = count("1 + 2 * 3;");
        assert_eq!(counter.nodes, 5);
    }

    #[test]
    fn descends_into_function_bodies() {
        // The function body must be traversed through `visit_function`, so the
        // `return a;` inside it should contribute to the counts.
        let counter = count("f :: fn (a): Number do return a; end;");
        // VarDeclaration + Function literal + Block + Return + identifier `a`
        assert_eq!(counter.nodes, 5);
    }

    /// A visitor that aborts as soon as it sees an identifier, used to check
    /// that errors short-circuit the traversal.
    struct FailOnIdentifier;

    impl Visitor for FailOnIdentifier {
        fn visit_expr(&mut self, expr: &AstNode) -> LoxResult<()> {
            if let Expr::Identifier { .. } = &expr.node {
                return runtime_error(expr.source_slice.clone(), "found an identifier");
            }
            walk_expr(self, expr)
        }
    }

    #[test]
    fn errors_propagate_through_traversal() {
        let stmts = parse("var x: Int = 1; x;");
        let mut visitor = FailOnIdentifier;
        let mut result = Ok(());
        for stmt in stmts.iter() {
            result = visitor.visit_expr(stmt);
            if result.is_err() {
                break;
            }
        }
        assert!(result.is_err());
    }
}