prattle 0.1.3

// parser.rs - MIT License
//  MIT License
//  Copyright (c) 2018 Tyler Laing (ZerothLaw)
// 
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//! # Parser
//! 
//! This is the core trait/implementation for a pratt parser. 
//! The basic algorithm is as follows:
//! > parse_expr(rbp: 0)
//! > left = null denotation of current token //ie, what is the context of this token on its own?
//! > while next token.rbp > rbp {
//! >   left = left denotation of next token //ie, what is the context of this token 
//! >                                        //combined with the context of the last node? 
//! > }
//! > return left;
//! 
//! The GeneralParser implementation here requires a provided ParserSpec and Lexer 
//! containing the tokens to be parsed. 

use std::collections::HashMap;
use std::marker::{Send, Sync};
use std::mem::{Discriminant, discriminant};

use prelude::*;
use types::*;

/// Parser trait. Theoretically, one could use different parser impls during parse of a 
/// language and so the syntax rules need to not be tied to any specific Parser impl. 
/// Hence why the ParserSpec uses closures with the signature ```&mut dyn Parser<T>```
pub trait Parser<T: Token + Send + Sync + 'static> {
    fn parse(&mut self) -> Result<Node<T>, ParseError<T>>;
    fn parse_expr(&mut self, rbp: PrecedenceLevel) -> Result<Node<T>, ParseError<T>>;
    fn next_binds_tighter_than(&mut self, rbp: PrecedenceLevel) -> bool;
    fn consume(&mut self, end_token: T) -> Result<(), ParseError<T>>;
}

/// General implementation of Parser trait. This implementation should work for any 
/// valid set of Syntax rules. 
/// A second generic, `L`, is added in order to allow us to decouple this impl from any specific 
/// Lexer impl. 
/// Here we just copy and consume the fields of the ParserSpec in order to set this up.
/// If we instead held a ParserSpec internally, we get borrow checker error messages. 
/// Consider: 
/// ```Rust
/// let null_info = self.spec.get_null(&tk);
/// ```
/// This inherently borrows self.spec, which then borrows self as an outcome. 
/// If instead you own the HashMaps, only those specific members are considered 
/// borrowed by borrowck. 
pub struct GeneralParser<T, L>
    where T: Token + Send + Sync + 'static, 
          L: Lexer<T>
{
    null_map: HashMap<Discriminant<T>, NullInfo<T>>, 
    left_map: HashMap<Discriminant<T>, LeftInfo<T>>,
    lexer: L, 
}

/// GeneralParser impl
/// Wraps trait methods to allow users to only need to import this, without 
/// the trait. Also offers a compile time check that GeneralParser still
/// impls Parser correctly. 
#[allow(dead_code)]
impl<T: Token + Send + Sync + 'static, L: Lexer<T>> GeneralParser<T, L> {
    pub fn new(spec: ParserSpec<T>, lexer: L) -> GeneralParser<T, L> {
        let (null_map, left_map) = spec.maps();
        GeneralParser {
            null_map: null_map,
            left_map: left_map,
            lexer: lexer
        }
    }

    fn parse(&mut self) -> Result<Node<T>, ParseError<T>> {
        self.parse_expr(PrecedenceLevel::Root)
    }

    fn parse_expr(&mut self, rbp: PrecedenceLevel) -> Result<Node<T>, ParseError<T>> {
        <Self as Parser<T>>::parse_expr(self, rbp)
    }

    fn next_binds_tighter_than(&mut self, rbp: PrecedenceLevel) -> bool {
        <Self as Parser<T>>::next_binds_tighter_than(self, rbp)
    }

    fn consume(&mut self, end_token: T) -> Result<(), ParseError<T>> {
        <Self as Parser<T>>::consume(self, end_token)
    }
}

impl<T: Token + Send + Sync + 'static, L: Lexer<T>> Parser<T> for GeneralParser<T, L> {
    fn parse(&mut self) -> Result<Node<T>, ParseError<T>> {
        self.parse_expr(PrecedenceLevel::Root)
    }

    fn parse_expr(&mut self, rbp: PrecedenceLevel) -> Result<Node<T>, ParseError<T>> {
        if let Some(tk) = self.lexer.peek() {
            self.lexer.next_token();
            let (lbp, func) = {
                let val = self.null_map.get(&discriminant(&tk));
                match val {
                    Some(val) => val.clone(), 
                    None => return Err(ParseError::MissingRule {token: tk.clone(), ty: "Null".into()})
                }
            };
            let mut left = func(self, tk, lbp)?;
            while self.next_binds_tighter_than(rbp) {
                let tk = self.lexer.next_token(); //implied that token exists
                let val = {
                    let v = self.left_map.get(&discriminant(&tk));
                    match v {
                        Some(val) => val.clone(), 
                        None => return Err(ParseError::MissingRule {token: tk.clone(), ty: "Left".into()})
                    }
                };
                let (lbp, _, func) = val;
                left = func(self, tk, lbp, left)?;
            }
            Ok(left)
        } else {
            Err(ParseError::Incomplete)
        }
    }

    fn next_binds_tighter_than(&mut self, rbp: PrecedenceLevel) -> bool {
        if let Some(tk) = self.lexer.peek() {
            if let Some((_, next_rbp, _)) = self.left_map.get(&discriminant(&tk)) {
                *next_rbp > rbp
            } else {
                false
            }
        } else {
            false
        }
    }

    fn consume(&mut self, end_token: T) -> Result<(), ParseError<T>> {
        if let Some(tk) = self.lexer.peek() {
            if tk == end_token {
                self.lexer.next_token();
                Ok(())
            } else {
                Err(ParseError::ConsumeFailed{expected: end_token, found: tk.clone()})
            }
        } else {
            Err(ParseError::Incomplete)
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use lexer::LexerVec;
    //Catch Send/Sync changes
    #[test]
    fn test_parser_send() {
        fn assert_send<T: Send>() {}
        assert_send::<GeneralParser<String, LexerVec<String>>>();
    }

    #[test]
    fn test_parser_sync() {
        fn assert_sync<T: Sync>() {}
        assert_sync::<GeneralParser<String, LexerVec<String>>>();
    }
}