Make the macro parser theory description more accurate
This commit is contained in:
Piotr Czarnecki
parent
6270257f4e
commit
346717686b
@ -1,4 +1,4 @@
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// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
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// Copyright 2012-2017 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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@ -8,18 +8,19 @@
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! This is an Earley-like parser, without support for in-grammar nonterminals,
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//! only by calling out to the main rust parser for named nonterminals (which it
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//! commits to fully when it hits one in a grammar). This means that there are no
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//! completer or predictor rules, and therefore no need to store one column per
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//! token: instead, there's a set of current Earley items and a set of next
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//! ones. Instead of NTs, we have a special case for Kleene star. The big-O, in
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//! pathological cases, is worse than traditional Earley parsing, but it's an
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//! easier fit for Macro-by-Example-style rules, and I think the overhead is
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//! lower. (In order to prevent the pathological case, we'd need to lazily
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//! construct the resulting `NamedMatch`es at the very end. It'd be a pain,
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//! and require more memory to keep around old items, but it would also save
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//! overhead)
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//! This is an NFA-based parser, which calls out to the main rust parser for named nonterminals
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//! (which it commits to fully when it hits one in a grammar). There's a set of current NFA threads
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//! and a set of next ones. Instead of NTs, we have a special case for Kleene star. The big-O, in
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//! pathological cases, is worse than traditional use of NFA or Earley parsing, but it's an easier
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//! fit for Macro-by-Example-style rules.
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//!
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//! (In order to prevent the pathological case, we'd need to lazily construct the resulting
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//! `NamedMatch`es at the very end. It'd be a pain, and require more memory to keep around old
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//! items, but it would also save overhead)
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//!
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//! We don't say this parser uses the Earley algorithm, because it's unnecessarily innacurate.
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//! The macro parser restricts itself to the features of finite state automata. Earley parsers
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//! can be described as an extension of NFAs with completion rules, prediction rules, and recursion.
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//!
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//! Quick intro to how the parser works:
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//!
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@ -27,14 +28,15 @@
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//! dot. For example `· a $( a )* a b` is a position, as is `a $( · a )* a b`.
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//!
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//! The parser walks through the input a character at a time, maintaining a list
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//! of items consistent with the current position in the input string: `cur_eis`.
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//! of threads consistent with the current position in the input string: `cur_items`.
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//!
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//! As it processes them, it fills up `eof_eis` with items that would be valid if
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//! the macro invocation is now over, `bb_eis` with items that are waiting on
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//! a Rust nonterminal like `$e:expr`, and `next_eis` with items that are waiting
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//! As it processes them, it fills up `eof_items` with threads that would be valid if
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//! the macro invocation is now over, `bb_items` with threads that are waiting on
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//! a Rust nonterminal like `$e:expr`, and `next_items` with threads that are waiting
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//! on a particular token. Most of the logic concerns moving the · through the
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//! repetitions indicated by Kleene stars. It only advances or calls out to the
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//! real Rust parser when no `cur_eis` items remain
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//! repetitions indicated by Kleene stars. The rules for moving the · without
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//! consuming any input are called epsilon transitions. It only advances or calls
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//! out to the real Rust parser when no `cur_items` threads remain.
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//!
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//! Example:
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//!
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@ -42,7 +44,7 @@
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//! Start parsing a a a a b against [· a $( a )* a b].
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//!
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//! Remaining input: a a a a b
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//! next_eis: [· a $( a )* a b]
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//! next: [· a $( a )* a b]
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//!
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//! - - - Advance over an a. - - -
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//!
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@ -54,23 +56,23 @@
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//! - - - Advance over an a. - - -
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//!
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//! Remaining input: a a b
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//! cur: [a $( a · )* a b] next: [a $( a )* a · b]
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//! Finish/Repeat (first item)
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over an a. - - - (this looks exactly like the last step)
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//!
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//! Remaining input: a b
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//! cur: [a $( a · )* a b] next: [a $( a )* a · b]
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//! Finish/Repeat (first item)
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over an a. - - - (this looks exactly like the last step)
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//!
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//! Remaining input: b
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//! cur: [a $( a · )* a b] next: [a $( a )* a · b]
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//! Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b]
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//! cur: [a $( a · )* a b] [a $( a )* a · b]
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//! Follow epsilon transition: Finish/Repeat (first item)
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//! next: [a $( a )* · a b] [a $( · a )* a b] [a $( a )* a · b]
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//!
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//! - - - Advance over a b. - - -
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//!
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@ -289,94 +291,94 @@ fn create_matches(len: usize) -> Vec<Rc<Vec<NamedMatch>>> {
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}
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fn inner_parse_loop(sess: &ParseSess,
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cur_eis: &mut SmallVector<Box<MatcherPos>>,
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next_eis: &mut Vec<Box<MatcherPos>>,
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eof_eis: &mut SmallVector<Box<MatcherPos>>,
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bb_eis: &mut SmallVector<Box<MatcherPos>>,
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cur_items: &mut SmallVector<Box<MatcherPos>>,
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next_items: &mut Vec<Box<MatcherPos>>,
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eof_items: &mut SmallVector<Box<MatcherPos>>,
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bb_items: &mut SmallVector<Box<MatcherPos>>,
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token: &Token,
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span: syntax_pos::Span)
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-> ParseResult<()> {
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while let Some(mut ei) = cur_eis.pop() {
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while let Some(mut item) = cur_items.pop() {
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// When unzipped trees end, remove them
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while ei.idx >= ei.top_elts.len() {
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match ei.stack.pop() {
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while item.idx >= item.top_elts.len() {
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match item.stack.pop() {
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Some(MatcherTtFrame { elts, idx }) => {
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ei.top_elts = elts;
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ei.idx = idx + 1;
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item.top_elts = elts;
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item.idx = idx + 1;
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}
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None => break
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}
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}
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let idx = ei.idx;
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let len = ei.top_elts.len();
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let idx = item.idx;
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let len = item.top_elts.len();
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// at end of sequence
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if idx >= len {
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// We are repeating iff there is a parent
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if ei.up.is_some() {
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if item.up.is_some() {
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// Disregarding the separator, add the "up" case to the tokens that should be
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// examined.
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// (remove this condition to make trailing seps ok)
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if idx == len {
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let mut new_pos = ei.up.clone().unwrap();
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let mut new_pos = item.up.clone().unwrap();
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// update matches (the MBE "parse tree") by appending
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// each tree as a subtree.
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// Only touch the binders we have actually bound
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for idx in ei.match_lo..ei.match_hi {
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let sub = ei.matches[idx].clone();
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new_pos.push_match(idx, MatchedSeq(sub, Span { lo: ei.sp_lo, ..span }));
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for idx in item.match_lo..item.match_hi {
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let sub = item.matches[idx].clone();
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new_pos.push_match(idx, MatchedSeq(sub, Span { lo: item.sp_lo, ..span }));
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}
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new_pos.match_cur = ei.match_hi;
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new_pos.match_cur = item.match_hi;
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new_pos.idx += 1;
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cur_eis.push(new_pos);
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cur_items.push(new_pos);
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}
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// Check if we need a separator
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if idx == len && ei.sep.is_some() {
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if idx == len && item.sep.is_some() {
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// We have a separator, and it is the current token.
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if ei.sep.as_ref().map(|sep| token_name_eq(token, sep)).unwrap_or(false) {
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ei.idx += 1;
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next_eis.push(ei);
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if item.sep.as_ref().map(|sep| token_name_eq(token, sep)).unwrap_or(false) {
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item.idx += 1;
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next_items.push(item);
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}
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} else { // we don't need a separator
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ei.match_cur = ei.match_lo;
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ei.idx = 0;
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cur_eis.push(ei);
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item.match_cur = item.match_lo;
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item.idx = 0;
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cur_items.push(item);
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}
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} else {
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// We aren't repeating, so we must be potentially at the end of the input.
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eof_eis.push(ei);
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eof_items.push(item);
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}
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} else {
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match ei.top_elts.get_tt(idx) {
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match item.top_elts.get_tt(idx) {
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/* need to descend into sequence */
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TokenTree::Sequence(sp, seq) => {
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if seq.op == quoted::KleeneOp::ZeroOrMore {
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// Examine the case where there are 0 matches of this sequence
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let mut new_ei = ei.clone();
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new_ei.match_cur += seq.num_captures;
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new_ei.idx += 1;
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for idx in ei.match_cur..ei.match_cur + seq.num_captures {
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new_ei.push_match(idx, MatchedSeq(Rc::new(vec![]), sp));
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let mut new_item = item.clone();
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new_item.match_cur += seq.num_captures;
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new_item.idx += 1;
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for idx in item.match_cur..item.match_cur + seq.num_captures {
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new_item.push_match(idx, MatchedSeq(Rc::new(vec![]), sp));
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}
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cur_eis.push(new_ei);
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cur_items.push(new_item);
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}
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// Examine the case where there is at least one match of this sequence
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let matches = create_matches(ei.matches.len());
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cur_eis.push(Box::new(MatcherPos {
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let matches = create_matches(item.matches.len());
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cur_items.push(Box::new(MatcherPos {
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stack: vec![],
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sep: seq.separator.clone(),
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idx: 0,
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matches: matches,
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match_lo: ei.match_cur,
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match_cur: ei.match_cur,
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match_hi: ei.match_cur + seq.num_captures,
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up: Some(ei),
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match_lo: item.match_cur,
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match_cur: item.match_cur,
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match_hi: item.match_cur + seq.num_captures,
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up: Some(item),
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sp_lo: sp.lo,
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top_elts: Tt(TokenTree::Sequence(sp, seq)),
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}));
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@ -390,22 +392,22 @@ fn inner_parse_loop(sess: &ParseSess,
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// Built-in nonterminals never start with these tokens,
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// so we can eliminate them from consideration.
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if may_begin_with(&*id.name.as_str(), token) {
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bb_eis.push(ei);
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bb_items.push(item);
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}
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}
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seq @ TokenTree::Delimited(..) | seq @ TokenTree::Token(_, DocComment(..)) => {
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let lower_elts = mem::replace(&mut ei.top_elts, Tt(seq));
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let idx = ei.idx;
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ei.stack.push(MatcherTtFrame {
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let lower_elts = mem::replace(&mut item.top_elts, Tt(seq));
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let idx = item.idx;
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item.stack.push(MatcherTtFrame {
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elts: lower_elts,
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idx: idx,
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});
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ei.idx = 0;
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cur_eis.push(ei);
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item.idx = 0;
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cur_items.push(item);
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}
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TokenTree::Token(_, ref t) if token_name_eq(t, token) => {
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ei.idx += 1;
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next_eis.push(ei);
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item.idx += 1;
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next_items.push(item);
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}
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TokenTree::Token(..) | TokenTree::MetaVar(..) => {}
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}
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@ -422,38 +424,38 @@ pub fn parse(sess: &ParseSess,
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recurse_into_modules: bool)
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-> NamedParseResult {
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let mut parser = Parser::new(sess, tts, directory, recurse_into_modules, true);
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let mut cur_eis = SmallVector::one(initial_matcher_pos(ms.to_owned(), parser.span.lo));
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let mut next_eis = Vec::new(); // or proceed normally
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let mut cur_items = SmallVector::one(initial_matcher_pos(ms.to_owned(), parser.span.lo));
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let mut next_items = Vec::new(); // or proceed normally
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loop {
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let mut bb_eis = SmallVector::new(); // black-box parsed by parser.rs
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let mut eof_eis = SmallVector::new();
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assert!(next_eis.is_empty());
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let mut bb_items = SmallVector::new(); // black-box parsed by parser.rs
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let mut eof_items = SmallVector::new();
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assert!(next_items.is_empty());
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match inner_parse_loop(sess, &mut cur_eis, &mut next_eis, &mut eof_eis, &mut bb_eis,
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match inner_parse_loop(sess, &mut cur_items, &mut next_items, &mut eof_items, &mut bb_items,
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&parser.token, parser.span) {
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Success(_) => {},
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Failure(sp, tok) => return Failure(sp, tok),
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Error(sp, msg) => return Error(sp, msg),
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}
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// inner parse loop handled all cur_eis, so it's empty
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assert!(cur_eis.is_empty());
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// inner parse loop handled all cur_items, so it's empty
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assert!(cur_items.is_empty());
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/* error messages here could be improved with links to orig. rules */
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if token_name_eq(&parser.token, &token::Eof) {
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if eof_eis.len() == 1 {
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let matches = eof_eis[0].matches.iter_mut().map(|mut dv| {
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if eof_items.len() == 1 {
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let matches = eof_items[0].matches.iter_mut().map(|mut dv| {
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Rc::make_mut(dv).pop().unwrap()
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});
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return nameize(sess, ms, matches);
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} else if eof_eis.len() > 1 {
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} else if eof_items.len() > 1 {
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return Error(parser.span, "ambiguity: multiple successful parses".to_string());
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} else {
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return Failure(parser.span, token::Eof);
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}
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} else if (!bb_eis.is_empty() && !next_eis.is_empty()) || bb_eis.len() > 1 {
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let nts = bb_eis.iter().map(|ei| match ei.top_elts.get_tt(ei.idx) {
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} else if (!bb_items.is_empty() && !next_items.is_empty()) || bb_items.len() > 1 {
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let nts = bb_items.iter().map(|item| match item.top_elts.get_tt(item.idx) {
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TokenTree::MetaVarDecl(_, bind, name) => {
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format!("{} ('{}')", name, bind)
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}
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@ -462,33 +464,33 @@ pub fn parse(sess: &ParseSess,
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return Error(parser.span, format!(
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"local ambiguity: multiple parsing options: {}",
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match next_eis.len() {
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match next_items.len() {
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0 => format!("built-in NTs {}.", nts),
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1 => format!("built-in NTs {} or 1 other option.", nts),
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n => format!("built-in NTs {} or {} other options.", nts, n),
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}
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));
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} else if bb_eis.is_empty() && next_eis.is_empty() {
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} else if bb_items.is_empty() && next_items.is_empty() {
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return Failure(parser.span, parser.token);
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} else if !next_eis.is_empty() {
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} else if !next_items.is_empty() {
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/* Now process the next token */
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cur_eis.extend(next_eis.drain(..));
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cur_items.extend(next_items.drain(..));
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parser.bump();
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} else /* bb_eis.len() == 1 */ {
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let mut ei = bb_eis.pop().unwrap();
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if let TokenTree::MetaVarDecl(span, _, ident) = ei.top_elts.get_tt(ei.idx) {
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let match_cur = ei.match_cur;
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ei.push_match(match_cur,
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} else /* bb_items.len() == 1 */ {
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let mut item = bb_items.pop().unwrap();
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if let TokenTree::MetaVarDecl(span, _, ident) = item.top_elts.get_tt(item.idx) {
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let match_cur = item.match_cur;
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item.push_match(match_cur,
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MatchedNonterminal(Rc::new(parse_nt(&mut parser, span, &ident.name.as_str()))));
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ei.idx += 1;
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ei.match_cur += 1;
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item.idx += 1;
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item.match_cur += 1;
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} else {
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unreachable!()
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}
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cur_eis.push(ei);
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cur_items.push(item);
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}
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assert!(!cur_eis.is_empty());
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assert!(!cur_items.is_empty());
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}
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}
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