Skip to content

docs: clarify ignored terminals with LALR #1597

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Draft
Mirochill wants to merge 1 commit into
base: master
Choose a base branch
from
+4 −0
Draft

docs: clarify ignored terminals with LALR #1597

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
4 changes: 4 additions & 0 deletions docs/grammar.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -272,6 +272,10 @@ Using the `%ignore` directive results in a cleaner grammar.

It's especially important for the LALR(1) algorithm, because adding whitespace (or comments, or other extraneous elements) explicitly in the grammar, harms its predictive abilities, which are based on a lookahead of 1.

When using LALR, ignored terminals are skipped by the lexer before parsing.
Because of that, ignored terminals should not be referenced from LALR rules when their position matters.
Earley can still use such a reference to require an ignored terminal at a specific position, but that behavior is currently a missing feature in LALR.

**Syntax:**
```html
%ignore <TERMINAL>
Expand Down