Editing LR parser (section)

==== A note about LR(0) versus SLR and LALR parsing ====

Only step 4 of the above procedure produces reduce actions, and so all reduce actions must occupy an entire table row, causing the reduction to occur regardless of the next symbol in the input stream.  This is why these are LR(0) parse tables: they don't do any lookahead (that is, they look ahead zero symbols) before deciding which reduction to perform.  A grammar that needs lookahead to disambiguate reductions would require a parse table row containing different reduce actions in different columns, and the above procedure is not capable of creating such rows.
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Refinements to the '''LR'''(0) table construction procedure (such as [[Simple LR parser|SLR]] and [[LALR parser|LALR]]) are capable of constructing reduce actions that do not occupy entire rows.  Therefore, they are capable of parsing more grammars than LR(0) parsers.