Parser

Vamos montar um parser de descida recursiva: percorrer a gramática manualmente e erguer a AST.

No começo o parser só guarda o fonte, o léxico e o token atual.

rust

pub struct Parser<'a> {
    /// Source Code
    source: &'a str,

    lexer: Lexer<'a>,

    /// Current Token consumed from the lexer
    cur_token: Token,

    /// The end range of the previous token
    prev_token_end: usize,
}

impl<'a> Parser<'a> {
    pub fn new(source: &'a str) -> Self {
        Self {
            source,
            lexer: Lexer::new(source),
            cur_token: Token::default(),
        }
    }

    pub fn parse(&mut self) -> Program<'a> {
        Ok(Program {
            node: Node {
                start: 0,
                end: self.source.len(),
            }
            body: vec![]
        })
    }
}

Helper functions

cur_token é o último token vindo do léxico. Helpers deixam o código mais limpo:

rust

impl<'a> Parser<'a> {
    fn start_node(&self) -> Node {
        let token = self.cur_token();
        Node::new(token.start, 0)
    }

    fn finish_node(&self, node: Node) -> Node {
        Node::new(node.start, self.prev_token_end)
    }

    fn cur_token(&self) -> &Token {
        &self.cur_token
    }

    fn cur_kind(&self) -> Kind {
        self.cur_token.kind
    }

    /// Checks if the current index has token `Kind`
    fn at(&self, kind: Kind) -> bool {
        self.cur_kind() == kind
    }

    /// Advance if we are at `Kind`
    fn bump(&mut self, kind: Kind) {
        if self.at(kind) {
            self.advance();
        }
    }

    /// Advance any token
    fn bump_any(&mut self) {
        self.advance();
    }

    /// Advance and return true if we are at `Kind`, return false otherwise
    fn eat(&mut self, kind: Kind) -> bool {
        if self.at(kind) {
            self.advance();
            return true;
        }
        false
    }

    /// Move to the next token
    fn advance(&mut self) {
        let token = self.lexer.next_token();
        self.prev_token_end = self.cur_token.end;
        self.cur_token = token;
    }
}

Parse Functions

DebuggerStatement é o statement mais simples — parseie e monte um Program.

rust

impl<'a> Parser<'a> {
    pub fn parse(&mut self) -> Program {
        let stmt = self.parse_debugger_statement();
        let body = vec![stmt];
        Program {
            node: Node {
                start: 0,
                end: self.source.len(),
            }
            body,
        }
    }

    fn parse_debugger_statement(&mut self) -> Statement {
        let node = self.start_node();
        // NOTE: the token returned from the lexer is `Kind::Debugger`, we'll fix this later.
        self.bump_any();
        Statement::DebuggerStatement {
            node: self.finish_node(node),
        }
    }
}

Demais parse functions reutilizam esses helpers; exemplo de while no swc:

rust

// https://github.com/swc-project/swc/blob/554b459e26b24202f66c3c58a110b3f26bbd13cd/crates/swc_ecma_parser/src/parser/stmt.rs#L952-L970

fn parse_while_stmt(&mut self) -> PResult<Stmt> {
    let start = cur_pos!(self);

    assert_and_bump!(self, "while");

    expect!(self, '(');
    let test = self.include_in_expr(true).parse_expr()?;
    expect!(self, ')');

    let ctx = Context {
        is_break_allowed: true,
        is_continue_allowed: true,
        ..self.ctx()
    };
    let body = self.with_ctx(ctx).parse_stmt(false).map(Box::new)?;

    let span = span!(self, start);
    Ok(Stmt::While(WhileStmt { span, test, body }))
}

Parsing Expressions

Expressões são profundas e recursivas — compridas demais estouram a pilha (ex.: teste de stress do TypeScript).

Para fugir da recursão, use Pratt Parsing — tutorial excelente aqui (matklad/Rust Analyzer) e exemplo Rust na Rome.

Lists

Muitos nós são listas separadas por pontuação: [a, b, c], {a, b, c}.

O código se repete; dá para aplicar template method com traits.

rust

// https://github.com/rome/tools/blob/85ddb4b2c622cac9638d5230dcefb6cf571677f8/crates/rome_js_parser/src/parser/parse_lists.rs#L131-L157

fn parse_list(&mut self, p: &mut Parser) -> CompletedMarker {
    let elements = self.start_list(p);
    let mut progress = ParserProgress::default();
    let mut first = true;
    while !p.at(JsSyntaxKind::EOF) && !self.is_at_list_end(p) {
        if first {
            first = false;
        } else {
            self.expect_separator(p);

            if self.allow_trailing_separating_element() && self.is_at_list_end(p) {
                break;
            }
        }

        progress.assert_progressing(p);

        let parsed_element = self.parse_element(p);

        if parsed_element.is_absent() && p.at(self.separating_element_kind()) {
            // a missing element
            continue;
        } else if self.recover(p, parsed_element).is_err() {
            break;
        }
    }
    self.finish_list(p, elements)
}

Isso também evita laços infinitos via progress.assert_progressing(p);.

Detalhes por lista, ex. elementos de array:

rust

// https://github.com/rome/tools/blob/85ddb4b2c622cac9638d5230dcefb6cf571677f8/crates/rome_js_parser/src/syntax/expr.rs#L1543-L1580

struct ArrayElementsList;

impl ParseSeparatedList for ArrayElementsList {
    fn parse_element(&mut self, p: &mut Parser) -> ParsedSyntax {
        match p.cur() {
            T![...] => parse_spread_element(p, ExpressionContext::default()),
            T![,] => Present(p.start().complete(p, JS_ARRAY_HOLE)),
            _ => parse_assignment_expression_or_higher(p, ExpressionContext::default()),
        }
    }

    fn is_at_list_end(&self, p: &mut Parser) -> bool {
        p.at(T![']'])
    }

    fn recover(&mut self, p: &mut Parser, parsed_element: ParsedSyntax) -> RecoveryResult {
        parsed_element.or_recover(
            p,
            &ParseRecovery::new(
                JS_UNKNOWN_EXPRESSION,
                EXPR_RECOVERY_SET.union(token_set!(T![']'])),
            ),
            js_parse_error::expected_array_element,
        )
    }

    fn list_kind() -> JsSyntaxKind {
        JS_ARRAY_ELEMENT_LIST
    }

    fn separating_element_kind(&mut self) -> JsSyntaxKind {
        T![,]
    }

    fn allow_trailing_separating_element(&self) -> bool {
        true
    }
}

Cover grammar

Explorado na cover grammar: às vezes precisamos converter Expression em BindingIdentifier. Em linguagens dinâmicas basta “trocar o tipo” do nó:

javascript

https://github.com/acornjs/acorn/blob/11735729c4ebe590e406f952059813f250a4cbd1/acorn/src/lval.js#L11-L26

Em Rust fazemos transformação estrutura-a-estrutura; um trait deixa bonito:

rust

pub trait CoverGrammar<'a, T>: Sized {
    fn cover(value: T, p: &mut Parser<'a>) -> Result<Self>;
}

O trait recebe T e produz Self:

rust

impl<'a> CoverGrammar<'a, Expression<'a>> for BindingPattern<'a> {
    fn cover(expr: Expression<'a>, p: &mut Parser<'a>) -> Result<Self> {
        match expr {
            Expression::Identifier(ident) => Self::cover(ident.unbox(), p),
            Expression::ObjectExpression(expr) => Self::cover(expr.unbox(), p),
            Expression::ArrayExpression(expr) => Self::cover(expr.unbox(), p),
            _ => Err(()),
        }
    }
}

impl<'a> CoverGrammar<'a, ObjectExpression<'a>> for BindingPattern<'a> {
    fn cover(obj_expr: ObjectExpression<'a>, p: &mut Parser<'a>) -> Result<Self> {
        ...
        BindingIdentifier::ObjectPattern(ObjectPattern { .. })
    }
}

impl<'a> CoverGrammar<'a, ArrayExpression<'a>> for BindingPattern<'a> {
    fn cover(expr: ArrayExpression<'a>, p: &mut Parser<'a>) -> Result<Self> {
        ...
        BindingIdentifier::ArrayPattern(ArrayPattern { .. })
    }
}

Onde precisar de BindingPattern a partir de Expression, chame BindingPattern::cover(expression).

TypeScript

Terminou JavaScript e quer TypeScript? Má notícia: não há spec separada; boa notícia: o parser está num arquivo só 🙃.

JSX vs TSX

javascript

let foo = <string> bar;

Em tsx isso é erro (JSX sem fechar); em ts é VariableDeclaration com TSTypeAssertion.

Lookahead

Em alguns pontos o parser precisa enxergar mais de um token.

TSIndexSignature

Dois jeitos ao parsear TSIndexSignature:

typescript

type A = { readonly [a: number]: string }
           ^__________________________^ TSIndexSignature

type B = { [a]: string }
           ^_________^ TSPropertySignature

No primeiro {, convém bisbilhotar cinco tokens (readonly, [, a, :, number) para distinguir TSIndexSignature de TSPropertySignature.

Para isso vale um buffer de tokens no léxico.

Arrow expressions

Como na cover grammar, depois de SequenceExpression e => transformamos expressões em BindingPattern.

TypeScript complica porque dentro de () há sintaxe extra — há muitos casos:

typescript

(<x>a, b as c, d!);
(a?: b = {} as c!) => {};

Vale estudar o fonte oficial. Trecho útil:

typescript

function tryParseParenthesizedArrowFunctionExpression(
  allowReturnTypeInArrowFunction: boolean,
): Expression | undefined {
  const triState = isParenthesizedArrowFunctionExpression();
  if (triState === Tristate.False) {
    // It's definitely not a parenthesized arrow function expression.
    return undefined;
  }

  // If we definitely have an arrow function, then we can just parse one, not requiring a
  // following => or { token. Otherwise, we *might* have an arrow function.  Try to parse
  // it out, but don't allow any ambiguity, and return 'undefined' if this could be an
  // expression instead.
  return triState === Tristate.True
    ? parseParenthesizedArrowFunctionExpression(
        /*allowAmbiguity*/ true,
        /*allowReturnTypeInArrowFunction*/ true,
      )
    : tryParse(() =>
        parsePossibleParenthesizedArrowFunctionExpression(allowReturnTypeInArrowFunction),
      );
}

//  True        -> We definitely expect a parenthesized arrow function here.
//  False       -> There *cannot* be a parenthesized arrow function here.
//  Unknown     -> There *might* be a parenthesized arrow function here.
//                 Speculatively look ahead to be sure, and rollback if not.
function isParenthesizedArrowFunctionExpression(): Tristate {
  if (
    token() === SyntaxKind.OpenParenToken ||
    token() === SyntaxKind.LessThanToken ||
    token() === SyntaxKind.AsyncKeyword
  ) {
    return lookAhead(isParenthesizedArrowFunctionExpressionWorker);
  }

  if (token() === SyntaxKind.EqualsGreaterThanToken) {
    // ERROR RECOVERY TWEAK:
    // If we see a standalone => try to parse it as an arrow function expression as that's
    // likely what the user intended to write.
    return Tristate.True;
  }
  // Definitely not a parenthesized arrow function.
  return Tristate.False;
}

Em resumo: lookahead (caminho rápido) + backtracking para setas.

Parser ​

Helper functions ​

Parse Functions ​

Parsing Expressions ​

Lists ​

Cover grammar ​

TypeScript ​

JSX vs TSX ​

Lookahead ​

TSIndexSignature ​

Arrow expressions ​

Parser

Helper functions

Parse Functions

Parsing Expressions

Lists

Cover grammar

TypeScript

JSX vs TSX

Lookahead

TSIndexSignature

Arrow expressions