316 lines
11 KiB
Rust
316 lines
11 KiB
Rust
// Parsing automaton
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//
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// Copyright (C) 2014-2022 Ryan Specialty Group, LLC.
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//
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// This file is part of TAME.
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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//! State transitions for parser automata.
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use super::{ParseState, ParseStateResult, ParseStatus, Token};
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use std::{
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convert::Infallible,
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hint::unreachable_unchecked,
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ops::{ControlFlow, FromResidual},
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};
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#[cfg(doc)]
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use super::Parser;
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/// A state transition with associated data.
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///
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/// Conceptually,
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/// imagine the act of a state transition producing data.
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/// See [`Transition`] for convenience methods for producing this tuple.
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///
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/// Sometimes a parser is not able to complete the operation requested
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/// based on the provided input token.
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/// Since TAMER uses a streaming parsing framework that places strict
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/// limits on control flow,
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/// a single token can be returned as lookahead to indicate that the
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/// token could not be parsed yet and should be provided once again
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/// in place of the next token from the input stream.
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/// This allows,
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/// for example,
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/// for multiple data to be emitted in response to a single token.
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///
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/// This struct is opaque to ensure that critical invariants involving
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/// transitions and lookahead are properly upheld;
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/// callers must use the appropriate parsing APIs.
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#[derive(Debug, PartialEq)]
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pub struct TransitionResult<S: ParseState>(
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/// New parser state.
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pub(in super::super) Transition<S>,
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/// Result of the parsing operation.
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pub(in super::super) TransitionData<S>,
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);
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impl<S: ParseState> TransitionResult<S> {
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/// Indicate that this transition include a single token of lookahead,
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/// which should be provided back to the parser in place of the
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/// next token from the input stream.
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pub fn with_lookahead(self, lookahead: S::Token) -> Self {
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match self {
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Self(transition, TransitionData::Result(result, None)) => Self(
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transition,
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TransitionData::Result(result, Some(Lookahead(lookahead))),
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),
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// This represents a problem with the parser;
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// we should never specify a lookahead token more than once.
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// This could be enforced statically with the type system if
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// ever such a thing is deemed to be worth doing.
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Self(
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..,
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TransitionData::Result(_, Some(prev))
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| TransitionData::Dead(prev),
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) => {
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panic!("internal error: lookahead token overwrite: {prev:?}")
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}
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}
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}
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/// Possibly indicate that this transition includes a single token of
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/// lookahead.
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///
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/// If the argument is [`None`],
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/// this returns `self` unchanged.
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///
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/// This is useful when working with the output of other parsers.
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/// See [`with_lookahead`](TransitionResult::with_lookahead) for more
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/// information.
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pub(in super::super) fn maybe_with_lookahead(
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self,
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lookahead: Option<Lookahead<S::Token>>,
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) -> Self {
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match lookahead {
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Some(Lookahead(lookahead)) => self.with_lookahead(lookahead),
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None => self,
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}
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}
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}
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/// Token to use as a lookahead token in place of the next token from the
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/// input stream.
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#[derive(Debug, PartialEq)]
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pub struct Lookahead<T: Token>(pub(in super::super) T);
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/// Information about the state transition.
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///
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/// Note: Ideally a state wouldn't even be required for
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/// [`Dead`](TransitionData::Dead),
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/// but [`ParseState`] does not implement [`Default`] and [`Parser`]
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/// requires _some_ state exist.
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#[derive(Debug, PartialEq)]
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pub(in super::super) enum TransitionData<S: ParseState> {
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/// State transition was successful or not attempted,
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/// with an optional token of [`Lookahead`].
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///
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/// Note that a successful state transition _does not_ imply a
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/// successful [`ParseStateResult`]---the
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/// parser may choose to successfully transition into an error
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/// recovery state to accommodate future tokens.
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Result(ParseStateResult<S>, Option<Lookahead<S::Token>>),
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/// No valid state transition exists from the current state for the
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/// given input token,
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/// which is returned as a token of [`Lookahead`].
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///
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/// A dead state is an accepting state that has no state transition for
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/// the given token.
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/// This could simply mean that the parser has completed its job and
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/// that control must be returned to a parent context.
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/// Note that this differs from an error state,
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/// where a parser is unable to reach an accepting state because it
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/// received unexpected input.
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///
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/// Note that the parser may still choose to perform a state transition
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/// for the sake of error recovery,
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/// but note that the dead state is generally interpreted to mean
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/// "I have no further work that I am able to perform"
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/// and may lead to finalization of the parser.
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/// If a parser intends to do additional work,
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/// it should return an error instead via [`TransitionData::Result`].
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Dead(Lookahead<S::Token>),
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}
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/// A verb denoting a state transition.
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///
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/// This is typically instantiated directly by a [`ParseState`] to perform a
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/// state transition in [`ParseState::parse_token`].
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///
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/// This newtype was created to produce clear, self-documenting code;
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/// parsers can get confusing to read with all of the types involved,
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/// so this provides a mental synchronization point.
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///
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/// This also provides some convenience methods to help remove boilerplate
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/// and further improve code clarity.
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#[derive(Debug, PartialEq, Eq)]
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pub struct Transition<S: ParseState>(pub S);
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impl<S: ParseState> Transition<S> {
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/// A state transition with corresponding data.
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///
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/// This allows [`ParseState::parse_token`] to emit a parsed object and
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/// corresponds to [`ParseStatus::Object`].
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pub fn ok<T>(self, obj: T) -> TransitionResult<S>
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where
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T: Into<ParseStatus<S>>,
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{
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TransitionResult(self, TransitionData::Result(Ok(obj.into()), None))
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}
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/// A transition with corresponding error.
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///
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/// This indicates a parsing failure.
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/// The state ought to be suitable for error recovery.
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pub fn err<E: Into<S::Error>>(self, err: E) -> TransitionResult<S> {
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TransitionResult(self, TransitionData::Result(Err(err.into()), None))
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}
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/// A state transition with corresponding [`Result`].
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///
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/// This translates the provided [`Result`] in a manner equivalent to
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/// [`Transition::ok`] and [`Transition::err`].
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pub fn result<T, E>(self, result: Result<T, E>) -> TransitionResult<S>
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where
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T: Into<ParseStatus<S>>,
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E: Into<S::Error>,
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{
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TransitionResult(
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self,
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TransitionData::Result(
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result.map(Into::into).map_err(Into::into),
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None,
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),
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)
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}
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/// A state transition indicating that more data is needed before an
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/// object can be emitted.
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///
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/// This corresponds to [`ParseStatus::Incomplete`].
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pub fn incomplete(self) -> TransitionResult<S> {
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TransitionResult(
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self,
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TransitionData::Result(Ok(ParseStatus::Incomplete), None),
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)
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}
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/// A state transition could not be performed and parsing will not
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/// continue.
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///
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/// A dead state represents an _accepting state_ that has no edge to
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/// another state for the given `tok`.
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/// Rather than throw an error,
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/// a parser uses this status to indicate that it has completed
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/// parsing and that the token should be utilized elsewhere;
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/// the provided token will be used as a token of [`Lookahead`].
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///
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/// If a parser is not prepared to be finalized and needs to yield an
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/// object first,
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/// use [`Transition::result`] or other methods along with a token
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/// of [`Lookahead`].
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pub fn dead(self, tok: S::Token) -> TransitionResult<S> {
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TransitionResult(self, TransitionData::Dead(Lookahead(tok)))
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}
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}
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impl<S: ParseState> FromResidual<(Transition<S>, ParseStateResult<S>)>
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for TransitionResult<S>
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{
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fn from_residual(residual: (Transition<S>, ParseStateResult<S>)) -> Self {
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match residual {
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(st, result) => Self(st, TransitionData::Result(result, None)),
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}
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}
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}
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impl<S: ParseState> FromResidual<Result<Infallible, TransitionResult<S>>>
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for TransitionResult<S>
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{
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fn from_residual(
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residual: Result<Infallible, TransitionResult<S>>,
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) -> Self {
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match residual {
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Err(e) => e,
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// SAFETY: This match arm doesn't seem to be required in
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// core::result::Result's FromResidual implementation,
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// but as of 1.61 nightly it is here.
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// Since this is Infallable,
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// it cannot occur.
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Ok(_) => unsafe { unreachable_unchecked() },
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}
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}
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}
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impl<S: ParseState> FromResidual<ControlFlow<TransitionResult<S>, Infallible>>
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for TransitionResult<S>
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{
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fn from_residual(
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residual: ControlFlow<TransitionResult<S>, Infallible>,
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) -> Self {
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match residual {
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ControlFlow::Break(result) => result,
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// SAFETY: Infallible, so cannot hit.
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ControlFlow::Continue(_) => unsafe { unreachable_unchecked() },
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}
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}
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}
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/// An object able to be used as data for a state [`Transition`].
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///
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/// This flips the usual order of things:
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/// rather than using a method of [`Transition`] to provide data,
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/// this starts with the data and produces a transition from it.
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/// This is sometimes necessary to satisfy ownership/borrowing rules.
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///
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/// This trait simply removes boilerplate associated with storing
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/// intermediate values and translating into the resulting type.
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pub trait Transitionable<S: ParseState> {
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/// Perform a state transition to `S` using [`Self`] as the associated
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/// data.
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///
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/// This may be necessary to satisfy ownership/borrowing rules when
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/// state data from `S` is used to compute [`Self`].
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fn transition(self, to: S) -> TransitionResult<S>;
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}
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impl<S, E> Transitionable<S> for Result<ParseStatus<S>, E>
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where
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S: ParseState,
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<S as ParseState>::Error: From<E>,
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{
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fn transition(self, to: S) -> TransitionResult<S> {
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Transition(to).result(self)
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}
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}
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impl<S, E> Transitionable<S> for Result<(), E>
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where
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S: ParseState,
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<S as ParseState>::Error: From<E>,
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{
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fn transition(self, to: S) -> TransitionResult<S> {
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Transition(to).result(self.map(|_| ParseStatus::Incomplete))
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}
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}
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impl<S: ParseState> Transitionable<S> for ParseStatus<S> {
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fn transition(self, to: S) -> TransitionResult<S> {
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Transition(to).ok(self)
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}
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}
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