* Deal with mergeability outside of error comparison function
* Make `better_version_exists` function pure (don't halt other versions
as a side effect).
* Tweak error comparison logic
Signed-off-by: Rick Winfrey <rewinfrey@github.com>
The previous approach to error recovery relied on special error-recovery
states in the parse table. For each token T, there was an error recovery
state in which the parser looked for *any* token that could follow T.
Unfortunately, sometimes the set of tokens that could follow T contained
conflicts. For example, in JS, the token '}' can be followed by the
open-ended 'template_chars' token, but also by ordinary tokens like
'identifier'. So with the old algorithm, when recovering from an
unexpected '}' token, the lexer had no way to distinguish identifiers
from template_chars.
This commit drops the error recovery states. Instead, when we encounter
an unexpected token T, we recover from the error by finding a previous
state S in the stack in which T would be valid, popping all of the nodes
after S, and wrapping them in an error.
This way, the lexer is always invoked in a normal parse state, in which
it is looking for a non-conflicting set of tokens. Eliminating the error
recovery states also shrinks the lex state machine significantly.
Signed-off-by: Rick Winfrey <rewinfrey@github.com>
We previously maintained a set of individual productions that were
involved in conflicts, but that was subtly incorrect because
we don't compare productions themselves when comparing parse items;
we only compare the parse items properties that could affect the
final reduce actions.
