Question: Writing a Ternary Operator Parser about chumsky HOT 5 CLOSED

Ah sorry, I misunderstood. This works now! Undersleeping is not recommended for anyone writing C parsers 😄

from chumsky.

There's a neat trick you can do for ternary operators: imagine that the ? and the : are actually just parentheses:

expr ( expr ) expr

As you can see, the inner part (( expr )) behaves a lot like a regular binary operator (syntax-wise, anyway). So, you can just apply the same reasoning as you would to a binary operator:

let ternary = expr.foldl(
    just('?').ignore_then(expr).then_ignore(':').then(expr).repeated(),
    |cond, (a, b)| Expr::Ternary(cond, a, b),
);

or, if using the pratt combinator,

atom.pratt((
    ...
    infix(right(_), just('?').ignore_then(expr).then_ignore(':'), |cond, a, b| Expr::Ternary(cond, a, b)),
    ...
));

from chumsky.

But then how would I define a comma separated value parser -

expr, expr, expr, expr

where the , operator would have lower precedence than the ternary operator? Also how does this method ensure that the precedence of the :? Operators is less than the assignment operators?

from chumsky.

Also using the following -

recursive(|expr| {
    // ...
    let ternary = expr
            .clone()
            .foldl_with(
                just(Token::Question)
                    .ignore_then(expr.clone())
                    .then_ignore(just(Token::Colon))
                    .then(expr)
                    .repeated(),
                |cond, (a, b), e| {
                    (
                        Expr::Ternary(Box::new(cond), Box::new(a), Box::new(b)),
                        e.span(),
                    )
                },
            )
            .boxed();

    ternary.labelled("expression")
})

Leads to a stack overflow

from chumsky.

But then how would I define a comma separated value parser ... where the , operator would have lower precedence than the ternary operator?

When I say 'think of ? and : as parentheses', I don't mean "imagine the overall operator as having the same precedence as parentheses". Both of the code solutions I provided can be slot into a precedence hierarchy anywhere you want.

from chumsky.