Dealing with Errors

Quoting from the Dragon Book

Most programming language specifications do not describe how a compiler should respond to errors; error handling is left to the compiler designer. Planning the error handling right from the start can both simplify the structure of a compiler and improve its handling of errors.

A fully recoverable parser can construct an AST no matter what we throw at it. For tools such as linter or formatter, one would wish for a fully recoverable parser so we can act on part of the program.

A panicking parser will abort if there is any grammar mismatch, and a partially recoverable parser will recover from deterministic grammars.

For example, given a grammatically incorrect while statement while true {}, we know it is missing round brackets, and the only punctuation it can have are round brackets, so we can still return a valid AST and indicate its missing brackets.

Most JavaScript parsers out there are partially recoverable, so we'll do the same and build a partially recoverable parser.

INFO

The Biome parser is a fully recoverable parser.

Rust has the Result type for returning and propagating errors. In conjunction with the ? syntax, the parse functions will remain simple and clean.

It is common to wrap the Result type so we can replace the error later:

pub type Result<T> = std::result::Result<T, ()>;

Our parse functions will return a Result, for example:

pub fn parse_binding_pattern(&mut self, ctx: Context) -> Result<BindingPattern<'a>> {
    match self.cur_kind() {
        Kind::LCurly => self.parse_object_binding_pattern(ctx),
        Kind::LBrack => self.parse_array_binding_pattern(ctx),
        kind if kind.is_binding_identifier() => {
          // ... code omitted
        }
        _ => Err(()), 
    }
}

We can add an expect function for returning an error if the current token does not match the grammar:

/// Expect a `Kind` or return error
pub fn expect(&mut self, kind: Kind) -> Result<()> {
    if !self.at(kind) {
        return Err(())
    }
    self.advance();
    Ok(())
}

And use it as such:

pub fn parse_paren_expression(&mut self, ctx: Context) -> Result<Expression> {
    self.expect(Kind::LParen)?;
    let expression = self.parse_expression(ctx)?;
    self.expect(Kind::RParen)?;
    Ok(expression)
}

INFO

For completeness, the lexer function read_next_token should also return Result when an unexpected char is found when lexing.

The Error Trait

To return specific errors, we need to fill in the Err part of Result:

pub type Result<T> = std::result::Result<T, SyntaxError>;
                                            ^^^^^^^^^^^
#[derive(Debug)]
pub enum SyntaxError {
    UnexpectedToken(String),
    AutoSemicolonInsertion(String),
    UnterminatedMultiLineComment(String),
}

We call it SyntaxError because all "early error"s defined in the grammar section of the ECMAScript specification are syntax errors.

To make this a proper Error, it needs to implement the Error Trait. For cleaner code, we can use macros from the thiserror crate:

#[derive(Debug, Error)]
pub enum SyntaxError {
    #[error("Unexpected Token")]
    UnexpectedToken,

    #[error("Expected a semicolon or an implicit semicolon after a statement, but found none")]
    AutoSemicolonInsertion,

    #[error("Unterminated multi-line comment")]
    UnterminatedMultiLineComment,
}

We can then add an expect helper function for throwing an error if the token does not match:

/// Expect a `Kind` or return error
pub fn expect(&mut self, kind: Kind) -> Result<()> {
    if self.at(kind) {
        return Err(SyntaxError::UnexpectedToken);
    }
    self.advance(kind);
    Ok(())
}

The parse_debugger_statement can now use the expect function for proper error management:

fn parse_debugger_statement(&mut self) -> Result<Statement> {
    let node = self.start_node();
    self.expect(Kind::Debugger)?;
    Ok(Statement::DebuggerStatement {
        node: self.finish_node(node),
    })
}

Notice the ? after the expect, it is a syntactic sugar called the "question mark operator" for making the function return early if the expect function returns a Err.

Fancy Error Report

miette is one of the nicest error reporting crate out there, it provides a fancy colored output

Add miette to your Cargo.toml

[dependencies]
miette = { version = "5", features = ["fancy"] }

We can wrap our Error with miette and not modify the Result type defined in our parser:

pub fn main() -> Result<()> {
    let source_code = "".to_string();
    let file_path = "test.js".to_string();
    let mut parser = Parser::new(&source_code);
    parser.parse().map_err(|error| {
        miette::Error::new(error).with_source_code(miette::NamedSource::new(file_path, source_code))
    })
}