Struct pest::pratt_parser::PrattParser

source ·

pub struct PrattParser<R: RuleType> { /* private fields */ }

Expand description

Struct containing operators and precedences, which can perform Pratt parsing on primary, prefix, postfix and infix expressions over Pairs. The tokens in Pairs should alternate in the order: prefix* ~ primary ~ postfix* ~ (infix ~ prefix* ~ primary ~ postfix*)*

Panics

Panics will occur when:

pairs is empty
The tokens in pairs does not alternate in the expected order.
No map_* function is specified for a certain kind of operator encountered in pairs.

Example

The following pest grammar defines a calculator which can be used for Pratt parsing.

WHITESPACE   =  _{ " " | "\t" | NEWLINE }
  
program      =   { SOI ~ expr ~ EOI }
  expr       =   { prefix* ~ primary ~ postfix* ~ (infix ~ prefix* ~ primary ~ postfix* )* }
    infix    =  _{ add | sub | mul | div | pow }
      add    =   { "+" } // Addition
      sub    =   { "-" } // Subtraction
      mul    =   { "*" } // Multiplication
      div    =   { "/" } // Division
      pow    =   { "^" } // Exponentiation
    prefix   =  _{ neg }
      neg    =   { "-" } // Negation
    postfix  =  _{ fac }
      fac    =   { "!" } // Factorial
    primary  =  _{ int | "(" ~ expr ~ ")" }
      int    =  @{ (ASCII_NONZERO_DIGIT ~ ASCII_DIGIT+ | ASCII_DIGIT) }

Below is a PrattParser that is able to parse an expr in the above grammar. The order of precedence corresponds to the order in which op is called. Thus, mul will have higher precedence than add. Operators can also be chained with | to give them equal precedence.

let pratt =
    PrattParser::new()
        .op(Op::infix(Rule::add, Assoc::Left) | Op::infix(Rule::sub, Assoc::Left))
        .op(Op::infix(Rule::mul, Assoc::Left) | Op::infix(Rule::div, Assoc::Left))
        .op(Op::infix(Rule::pow, Assoc::Right))
        .op(Op::postfix(Rule::fac))
        .op(Op::prefix(Rule::neg));

To parse an expression, call the map_primary, map_prefix, map_postfix, map_infix and parse methods as follows:

fn parse_expr(pairs: Pairs<Rule>, pratt: &PrattParser<Rule>) -> i32 {
    pratt
        .map_primary(|primary| match primary.as_rule() {
            Rule::int  => primary.as_str().parse().unwrap(),
            Rule::expr => parse_expr(primary.into_inner(), pratt), // from "(" ~ expr ~ ")"
            _          => unreachable!(),
        })
        .map_prefix(|op, rhs| match op.as_rule() {
            Rule::neg  => -rhs,
            _          => unreachable!(),
        })
        .map_postfix(|lhs, op| match op.as_rule() {
            Rule::fac  => (1..lhs+1).product(),
            _          => unreachable!(),
        })
        .map_infix(|lhs, op, rhs| match op.as_rule() {
            Rule::add  => lhs + rhs,
            Rule::sub  => lhs - rhs,
            Rule::mul  => lhs * rhs,
            Rule::div  => lhs / rhs,
            Rule::pow  => (1..rhs+1).map(|_| lhs).product(),
            _          => unreachable!(),
        })
        .parse(pairs)
}