pest/pratt_parser.rs
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// pest. The Elegant Parser
// Copyright (c) 2018 DragoČ™ Tiselice
//
// Licensed under the Apache License, Version 2.0
// <LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0> or the MIT
// license <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. All files in the project carrying such notice may not be copied,
// modified, or distributed except according to those terms.
//! Constructs useful in prefix, postfix, and infix operator parsing with the
//! Pratt parsing method.
use core::iter::Peekable;
use core::marker::PhantomData;
use core::ops::BitOr;
use alloc::boxed::Box;
use alloc::collections::BTreeMap;
use crate::iterators::Pair;
use crate::RuleType;
/// Associativity of an infix binary operator, used by [`Op::infix(Assoc)`].
///
/// [`Op::infix(Assoc)`]: struct.Op.html
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum Assoc {
/// Left operator associativity. Evaluate expressions from left-to-right.
Left,
/// Right operator associativity. Evaluate expressions from right-to-left.
Right,
}
type Prec = u32;
const PREC_STEP: Prec = 10;
/// An operator that corresponds to a rule.
pub struct Op<R: RuleType> {
rule: R,
affix: Affix,
next: Option<Box<Op<R>>>,
}
enum Affix {
Prefix,
Postfix,
Infix(Assoc),
}
impl<R: RuleType> Op<R> {
/// Defines `rule` as a prefix unary operator.
pub fn prefix(rule: R) -> Self {
Self {
rule,
affix: Affix::Prefix,
next: None,
}
}
/// Defines `rule` as a postfix unary operator.
pub fn postfix(rule: R) -> Self {
Self {
rule,
affix: Affix::Postfix,
next: None,
}
}
/// Defines `rule` as an infix binary operator with associativity `assoc`.
pub fn infix(rule: R, assoc: Assoc) -> Self {
Self {
rule,
affix: Affix::Infix(assoc),
next: None,
}
}
}
impl<R: RuleType> BitOr for Op<R> {
type Output = Self;
fn bitor(mut self, rhs: Self) -> Self {
fn assign_next<R: RuleType>(op: &mut Op<R>, next: Op<R>) {
if let Some(ref mut child) = op.next {
assign_next(child, next);
} else {
op.next = Some(Box::new(next));
}
}
assign_next(&mut self, rhs);
self
}
}
/// Struct containing operators and precedences, which can perform [Pratt parsing][1] 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.
///
/// ```pest
/// 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.
///
/// ```
/// # use pest::pratt_parser::{Assoc, Op, PrattParser};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { program, expr, int, add, mul, sub, div, pow, fac, neg }
/// 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::prefix(Rule::neg))
/// .op(Op::postfix(Rule::fac));
/// ```
///
/// To parse an expression, call the [`map_primary`], [`map_prefix`], [`map_postfix`],
/// [`map_infix`] and [`parse`] methods as follows:
///
/// ```
/// # use pest::{iterators::Pairs, pratt_parser::PrattParser};
/// # #[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
/// # enum Rule { program, expr, int, add, mul, sub, div, pow, fac, neg }
/// 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)
/// }
/// ```
///
/// Note that [`map_prefix`], [`map_postfix`] and [`map_infix`] only need to be specified if the
/// grammar contains the corresponding operators.
///
/// [1]: https://en.wikipedia.org/wiki/Pratt_parser
/// [`Pairs`]: ../iterators/struct.Pairs.html
/// [`PrattParser`]: struct.PrattParser.html
/// [`map_primary`]: struct.PrattParser.html#method.map_primary
/// [`map_prefix`]: struct.PrattParserMap.html#method.map_prefix
/// [`map_postfix`]: struct.PrattParserMap.html#method.map_postfix
/// [`map_infix`]: struct.PrattParserMap.html#method.map_infix
/// [`parse`]: struct.PrattParserMap.html#method.parse
/// [`op`]: struct.PrattParserMap.html#method.op
pub struct PrattParser<R: RuleType> {
prec: Prec,
ops: BTreeMap<R, (Affix, Prec)>,
has_prefix: bool,
has_postfix: bool,
has_infix: bool,
}
impl<R: RuleType> Default for PrattParser<R> {
fn default() -> Self {
Self::new()
}
}
impl<R: RuleType> PrattParser<R> {
/// Instantiate a new `PrattParser`.
pub fn new() -> Self {
Self {
prec: PREC_STEP,
ops: BTreeMap::new(),
has_prefix: false,
has_postfix: false,
has_infix: false,
}
}
/// Add `op` to `PrattParser`.
pub fn op(mut self, op: Op<R>) -> Self {
self.prec += PREC_STEP;
let mut iter = Some(op);
while let Some(Op { rule, affix, next }) = iter.take() {
match affix {
Affix::Prefix => self.has_prefix = true,
Affix::Postfix => self.has_postfix = true,
Affix::Infix(_) => self.has_infix = true,
}
self.ops.insert(rule, (affix, self.prec));
iter = next.map(|op| *op);
}
self
}
/// Maps primary expressions with a closure `primary`.
pub fn map_primary<'pratt, 'a, 'i, X, T>(
&'pratt self,
primary: X,
) -> PrattParserMap<'pratt, 'a, 'i, R, X, T>
where
X: FnMut(Pair<'i, R>) -> T,
R: 'pratt,
{
PrattParserMap {
pratt: self,
primary,
prefix: None,
postfix: None,
infix: None,
phantom: PhantomData,
}
}
}
type PrefixFn<'a, 'i, R, T> = Box<dyn FnMut(Pair<'i, R>, T) -> T + 'a>;
type PostfixFn<'a, 'i, R, T> = Box<dyn FnMut(T, Pair<'i, R>) -> T + 'a>;
type InfixFn<'a, 'i, R, T> = Box<dyn FnMut(T, Pair<'i, R>, T) -> T + 'a>;
/// Product of calling [`map_primary`] on [`PrattParser`], defines how expressions should
/// be mapped.
///
/// [`map_primary`]: struct.PrattParser.html#method.map_primary
/// [`PrattParser`]: struct.PrattParser.html
pub struct PrattParserMap<'pratt, 'a, 'i, R, F, T>
where
R: RuleType,
F: FnMut(Pair<'i, R>) -> T,
{
pratt: &'pratt PrattParser<R>,
primary: F,
prefix: Option<PrefixFn<'a, 'i, R, T>>,
postfix: Option<PostfixFn<'a, 'i, R, T>>,
infix: Option<InfixFn<'a, 'i, R, T>>,
phantom: PhantomData<T>,
}
impl<'pratt, 'a, 'i, R, F, T> PrattParserMap<'pratt, 'a, 'i, R, F, T>
where
R: RuleType + 'pratt,
F: FnMut(Pair<'i, R>) -> T,
{
/// Maps prefix operators with closure `prefix`.
pub fn map_prefix<X>(mut self, prefix: X) -> Self
where
X: FnMut(Pair<'i, R>, T) -> T + 'a,
{
self.prefix = Some(Box::new(prefix));
self
}
/// Maps postfix operators with closure `postfix`.
pub fn map_postfix<X>(mut self, postfix: X) -> Self
where
X: FnMut(T, Pair<'i, R>) -> T + 'a,
{
self.postfix = Some(Box::new(postfix));
self
}
/// Maps infix operators with a closure `infix`.
pub fn map_infix<X>(mut self, infix: X) -> Self
where
X: FnMut(T, Pair<'i, R>, T) -> T + 'a,
{
self.infix = Some(Box::new(infix));
self
}
/// The last method to call on the provided pairs to execute the Pratt
/// parser (previously defined using [`map_primary`], [`map_prefix`], [`map_postfix`],
/// and [`map_infix`] methods).
///
/// [`map_primary`]: struct.PrattParser.html#method.map_primary
/// [`map_prefix`]: struct.PrattParserMap.html#method.map_prefix
/// [`map_postfix`]: struct.PrattParserMap.html#method.map_postfix
/// [`map_infix`]: struct.PrattParserMap.html#method.map_infix
pub fn parse<P: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: P) -> T {
self.expr(&mut pairs.peekable(), 0)
}
fn expr<P: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<P>, rbp: Prec) -> T {
let mut lhs = self.nud(pairs);
while rbp < self.lbp(pairs) {
lhs = self.led(pairs, lhs);
}
lhs
}
/// Null-Denotation
///
/// "the action that should happen when the symbol is encountered
/// as start of an expression (most notably, prefix operators)
fn nud<P: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<P>) -> T {
let pair = pairs.next().expect("Pratt parsing expects non-empty Pairs");
match self.pratt.ops.get(&pair.as_rule()) {
Some((Affix::Prefix, prec)) => {
let rhs = self.expr(pairs, *prec - 1);
match self.prefix.as_mut() {
Some(prefix) => prefix(pair, rhs),
None => panic!("Could not map {}, no `.map_prefix(...)` specified", pair),
}
}
None => (self.primary)(pair),
_ => panic!("Expected prefix or primary expression, found {}", pair),
}
}
/// Left-Denotation
///
/// "the action that should happen when the symbol is encountered
/// after the start of an expression (most notably, infix and postfix operators)"
fn led<P: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<P>, lhs: T) -> T {
let pair = pairs.next().unwrap();
match self.pratt.ops.get(&pair.as_rule()) {
Some((Affix::Infix(assoc), prec)) => {
let rhs = match *assoc {
Assoc::Left => self.expr(pairs, *prec),
Assoc::Right => self.expr(pairs, *prec - 1),
};
match self.infix.as_mut() {
Some(infix) => infix(lhs, pair, rhs),
None => panic!("Could not map {}, no `.map_infix(...)` specified", pair),
}
}
Some((Affix::Postfix, _)) => match self.postfix.as_mut() {
Some(postfix) => postfix(lhs, pair),
None => panic!("Could not map {}, no `.map_postfix(...)` specified", pair),
},
_ => panic!("Expected postfix or infix expression, found {}", pair),
}
}
/// Left-Binding-Power
///
/// "describes the symbol's precedence in infix form (most notably, operator precedence)"
fn lbp<P: Iterator<Item = Pair<'i, R>>>(&mut self, pairs: &mut Peekable<P>) -> Prec {
match pairs.peek() {
Some(pair) => match self.pratt.ops.get(&pair.as_rule()) {
Some((_, prec)) => *prec,
None => panic!("Expected operator, found {}", pair),
},
None => 0,
}
}
}