roc/src/parse/parser.rs

use bumpalo::collections::vec::Vec;
use bumpalo::Bump;
use parse::ast::Attempting;
use region::Region;
use std::char;

// Strategy:
//
// 1. Let space parsers check indentation. They should expect indentation to only ever increase (right?) when
//    doing a many_whitespaces or many1_whitespaces. Multline strings can have separate whitespace parsers.
// 2. For any expression that has subexpressions (e.g. ifs, parens, operators) record their indentation levels
//    by doing .and(position()) followed by .and_then() which says "I can have a declaration inside me as
//    long as the entire decl is indented more than me."
// 3. Make an alternative to RangeStreamOnce where uncons_while barfs on \t (or maybe just do this in whitespaces?)

/// Struct which represents a position in a source file.
#[derive(Debug, Clone, PartialEq)]
pub struct State<'a> {
    /// The raw input string.
    pub input: &'a str,

    /// Current line of the input
    pub line: u32,
    /// Current column of the input
    pub column: u16,

    /// Current indentation level, in columns
    /// (so no indent is col 1 - this saves an arithmetic operation.)
    pub indent_col: u16,

    // true at the beginning of each line, then false after encountering
    // the first nonspace char on that line.
    pub is_indenting: bool,

    pub attempting: Attempting,
}

impl<'a> State<'a> {
    pub fn new(input: &'a str, attempting: Attempting) -> State<'a> {
        State {
            input,
            line: 0,
            column: 0,
            indent_col: 1,
            is_indenting: true,
            attempting,
        }
    }

    /// Increments the line, then resets column, indent_col, and is_indenting.
    /// This does *not* advance the input.
    pub fn newline(&self) -> Self {
        let line = self
            .line
            .checked_add(1)
            .unwrap_or_else(panic_max_line_count_exceeded);

        State {
            input: self.input,
            line,
            column: 0,
            indent_col: 1,
            is_indenting: true,
            attempting: self.attempting,
        }
    }

    /// Use advance_spaces to advance with indenting.
    /// This assumes we are *not* advancing with spaces, or at least that
    /// any spaces on the line were preceded by non-spaces - which would mean
    /// they weren't eligible to indent anyway.
    pub fn advance_without_indenting(&self, quantity: usize) -> Self {
        let column_usize = (self.column as usize)
            .checked_add(quantity)
            .unwrap_or_else(panic_max_line_length_exceeded);

        if column_usize > std::u16::MAX as usize {
            panic_max_line_length_exceeded();
        }

        State {
            input: &self.input[quantity..],
            line: self.line,
            column: column_usize as u16,
            indent_col: self.indent_col,
            // Once we hit a nonspace character, we are no longer indenting.
            is_indenting: false,
            attempting: self.attempting,
        }
    }
    /// Advance the parser while also indenting as appropriate.
    /// This assumes we are only advancing with spaces, since they can indent.
    pub fn advance_spaces(&self, spaces: usize) -> Self {
        // We'll cast this to u16 later.
        debug_assert!(spaces <= std::u16::MAX as usize);

        let column_usize = (self.column as usize)
            .checked_add(spaces)
            .unwrap_or_else(panic_max_line_length_exceeded);

        if column_usize > std::u16::MAX as usize {
            panic_max_line_length_exceeded();
        }

        // Spaces don't affect is_indenting; if we were previously indneting,
        // we still are, and if we already finished indenting, we're still done.
        let is_indenting = self.is_indenting;

        // If we're indenting, spaces indent us further.
        let indent_col = if is_indenting {
            // This doesn't need to be checked_add because it's always true that
            // indent_col <= col, so if this could possibly overflow, we would
            // already have panicked from the column calculation.
            //
            // Leaving a debug_assert! in case this invariant someday disappers.
            debug_assert!(std::u16::MAX - self.indent_col >= spaces as u16);

            self.indent_col + spaces as u16
        } else {
            self.indent_col
        };

        State {
            input: &self.input[spaces..],
            line: self.line,
            column: column_usize as u16,
            indent_col,
            is_indenting,
            attempting: self.attempting,
        }
    }
}

#[inline(never)]
fn panic_max_line_count_exceeded() -> u32 {
    panic!(
        "Maximum line count exceeded. Roc only supports compiling files with at most {} lines.",
        std::u32::MAX
    )
}

#[inline(never)]
fn panic_max_line_length_exceeded() -> usize {
    panic!(
"Maximum line length exceeded. Roc only supports compiling files whose lines each contain no more than {} characters.",
        std::u16::MAX
    )
}

#[test]
fn state_size() {
    // State should always be under 8 machine words, so it fits in a typical
    // cache line.
    assert!(std::mem::size_of::<State>() <= std::mem::size_of::<usize>() * 8);
}

pub type ParseResult<'a, Output> = Result<(State<'a>, Output), (State<'a>, Fail)>;

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum Fail {
    Unexpected(char, Region, Attempting),
    PredicateFailed(Attempting),
    LineTooLong(u32 /* which line was too long */),
    TooManyLines,
    Eof(Region, Attempting),
}

pub trait Parser<'a, Output> {
    fn parse(&self, &'a Bump, State<'a>) -> ParseResult<'a, Output>;
}

impl<'a, F, Output> Parser<'a, Output> for F
where
    F: Fn(&'a Bump, State<'a>) -> ParseResult<'a, Output>,
{
    fn parse(&self, arena: &'a Bump, state: State<'a>) -> ParseResult<'a, Output> {
        self(arena, state)
    }
}

pub fn map<'a, P, F, Before, After>(parser: P, transform: F) -> impl Parser<'a, After>
where
    P: Parser<'a, Before>,
    F: Fn(Before) -> After,
{
    move |arena, state| {
        parser
            .parse(arena, state)
            .map(|(next_state, output)| (next_state, transform(output)))
    }
}

pub fn attempt<'a, P, Val>(attempting: Attempting, parser: P) -> impl Parser<'a, Val>
where
    P: Parser<'a, Val>,
{
    move |arena, state| {
        parser.parse(
            arena,
            State {
                attempting,
                ..state
            },
        )
    }
}

pub fn one_or_more<'a, P, A>(parser: P) -> impl Parser<'a, Vec<'a, A>>
where
    P: Parser<'a, A>,
{
    move |arena, state| match parser.parse(arena, state) {
        Ok((next_state, first_output)) => {
            let mut state = next_state;
            let mut buf = Vec::with_capacity_in(1, arena);

            buf.push(first_output);

            loop {
                match parser.parse(arena, state) {
                    Ok((next_state, next_output)) => {
                        state = next_state;
                        buf.push(next_output);
                    }
                    Err((new_state, _)) => return Ok((new_state, buf)),
                }
            }
        }
        Err((new_state, _)) => {
            let attempting = new_state.attempting;

            Err(unexpected_eof(0, new_state, attempting))
        }
    }
}

pub fn unexpected_eof<'a>(
    chars_consumed: usize,
    state: State<'a>,
    attempting: Attempting,
) -> (State<'a>, Fail) {
    checked_unexpected(chars_consumed, state, |region| {
        Fail::Eof(region, attempting)
    })
}

pub fn unexpected<'a>(
    ch: char,
    chars_consumed: usize,
    state: State<'a>,
    attempting: Attempting,
) -> (State<'a>, Fail) {
    checked_unexpected(chars_consumed, state, |region| {
        Fail::Unexpected(ch, region, attempting)
    })
}

/// Check for line overflow, then compute a new Region based on chars_consumed
/// and provide it as a way to construct a Problem.
/// If maximum line length was exceeded, return a Problem indicating as much.
#[inline(always)]
fn checked_unexpected<'a, F>(
    chars_consumed: usize,
    state: State<'a>,
    problem_from_region: F,
) -> (State<'a>, Fail)
where
    F: FnOnce(Region) -> Fail,
{
    match (state.column as usize).checked_add(chars_consumed) {
        Some(end_col) if end_col <= std::u16::MAX as usize => {
            let region = Region {
                start_col: state.column,
                end_col: end_col as u16,
                start_line: state.line,
                end_line: state.line,
            };

            (state, problem_from_region(region))
        }
        _ => {
            let line = state.line;

            (state, Fail::LineTooLong(line))
        }
    }
}

/// A string with no newlines in it.
pub fn string<'a>(string: &'static str) -> impl Parser<'a, ()> {
    // We can't have newlines because we don't attempt to advance the row
    // in the state, only the column.
    debug_assert!(!string.contains("\n"));

    move |_arena: &'a Bump, state: State<'a>| {
        let input = state.input;
        let len = string.len();

        match input.get(0..len) {
            Some(next_str) if next_str == string => Ok((state.advance_without_indenting(len), ())),
            _ => Err(unexpected_eof(len, state, Attempting::Keyword)),
        }
    }
}

pub fn satisfies<'a, P, A, F>(parser: P, predicate: F) -> impl Parser<'a, A>
where
    P: Parser<'a, A>,
    F: Fn(&A) -> bool,
{
    move |arena: &'a Bump, state: State<'a>| {
        if let Ok((next_state, output)) = parser.parse(arena, state.clone()) {
            if predicate(&output) {
                return Ok((next_state, output));
            }
        }

        let fail = Fail::PredicateFailed(state.attempting);
        Err((state, fail))
    }
}

// pub fn any<'a>(
//     _arena: &'a Bump,
//     state: State<'a>,
//     attempting: Attempting,
// ) -> ParseResult<'a, char> {
//     let input = state.input;

//     match input.chars().next() {
//         Some(ch) => {
//             let len = ch.len_utf8();
//             let mut new_state = State {
//                 input: &input[len..],

//                 ..state.clone()
//             };

//             if ch == '\n' {
//                 new_state.line = new_state.line + 1;
//                 new_state.column = 0;
//             }

//             Ok((new_state, ch))
//         }
//         _ => Err((state.clone(), attempting)),
//     }
// }

// fn whitespace<'a>() -> impl Parser<'a, char> {
//     // TODO advance the state appropriately, in terms of line, col, indenting, etc.
//     satisfies(any, |ch| ch.is_whitespace())
// }

// pub fn one_of2<'a, P1, P2, A>(p1: P1, p2: P2) -> impl Parser<'a, A>
// where
//     P1: Parser<'a, A>,
//     P2: Parser<'a, A>,
// {
//     move |arena: &'a Bump, state: State<'a>, attempting| {
//         if let Ok((next_state, output)) = p1.parse(arena, state, attempting) {
//             Ok((next_state, output))
//         } else if let Ok((next_state, output)) = p2.parse(arena, state, attempting) {
//             Ok((next_state, output))
//         } else {
//             Err((state, attempting))
//         }
//     }
// }

// pub fn one_of3<'a, P1, P2, P3, A>(p1: P1, p2: P2, p3: P3) -> impl Parser<'a, A>
// where
//     P1: Parser<'a, A>,
//     P2: Parser<'a, A>,
//     P3: Parser<'a, A>,
// {
//     move |arena: &'a Bump, state: State<'a>, attempting| {
//         if let Ok((next_state, output)) = p1.parse(arena, state, attempting) {
//             Ok((next_state, output))
//         } else if let Ok((next_state, output)) = p2.parse(arena, state, attempting) {
//             Ok((next_state, output))
//         } else if let Ok((next_state, output)) = p3.parse(arena, state, attempting) {
//             Ok((next_state, output))
//         } else {
//             Err((state, attempting))
//         }
//     }
// }