5.2.4 LR-Family Members

In Section 5.2.2, LR(0) states were created: no lookahead was used to create them. We did, however, consider the next input symbol(one symbol lookahead) when creating the table (see SLR table construction algorithm). If no lookahead is used to create the table, then the parser would be called an LR(0) parser. Unfortunately, LR(0) parsers don't recognize the constructs one finds in typical programming languages. If we consider the next possible symbol for each of the items in a state, as well as for creating the table, we would have an LR(1) parser.

LR(1) tables for typical programming languages are massive. SLR(1) parsers recognize many, but not all, of the constructs in typical programming languages.

There is another type of parser which recognizes almost as many constructs as an LR(1) parser. This is called a LALR(1) parser and is constructed by first constructing the LR(1) items and states and them merging many of them. Whenever two states are the same except for the lookahead symbol, they are merged. The first LA stands for Lookahead token is added to the item.

It is important to note that the same driver is used to parse. It is the table generation that is different.