Editing LR parser (section)

=== Shift and reduce actions ===

As with other shift-reduce parsers, an LR parser works by doing some combination of Shift steps and Reduce steps.  
* A '''Shift''' step advances in the input stream by one symbol.  That shifted symbol becomes a new single-node parse tree.
* A '''Reduce''' step applies a completed grammar rule to some of the recent parse trees, joining them together as one tree with a new root symbol.
If the input has no syntax errors, the parser continues with these steps until all of the input has been consumed and all of the parse trees have been reduced to a single tree representing an entire legal input.

LR parsers differ from other shift-reduce parsers in how they decide when to reduce, and how to pick between rules with similar endings.  But the final decisions and the sequence of shift or reduce steps are the same.

Much of the LR parser's efficiency is from being deterministic. To avoid guessing, the LR parser often looks ahead (rightwards) at the next scanned symbol, before deciding what to do with previously scanned symbols.  The lexical scanner works one or more symbols ahead of the parser.  The '''lookahead''' symbols are the 'right-hand context' for the parsing decision.<ref>
Engineering a Compiler (2nd edition), by Keith Cooper and Linda Torczon, [[Morgan Kaufmann]] 2011.</ref>