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In formal language theory, the left corner of a production rule in a context-free grammar is the left-most symbol on the right side of the rule. [1] For example, in the rule A→Xα, X is the left corner. The left corner table associates to a symbol all possible left corners for that symbol, and the left corners of those symbols, etc. Given the ...
In computer science, a left corner parser is a type of chart parser used for parsing context-free grammars.It combines the top-down and bottom-up approaches of parsing. The name derives from the use of the left corner of the grammar's production rules.
Left corner parsing is a hybrid method that works bottom-up along the left edges of each subtree, and top-down on the rest of the parse tree. If a language grammar has multiple rules that may start with the same leftmost symbols but have different endings, then that grammar can be efficiently handled by a deterministic bottom-up parse but ...
The multiple valid parse trees are computed simultaneously, without backtracking. GLR is sometimes helpful for computer languages that are not easily described by a conflict-free LALR(1) grammar. LC Left corner parsers use LR bottom-up techniques for recognizing the left end of alternative grammar rules. When the alternatives have been narrowed ...
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An LL(1) grammar with symbols that have only the empty derivation may or may not be LALR(1). [9] LL grammars cannot have rules containing left recursion. [10] Each LL(k) grammar that is ε-free can be transformed into an equivalent LL(k) grammar in Greibach normal form (which by definition does not have rules with left recursion). [11]
Top-down parsing can be viewed as an attempt to find left-most derivations of an input-stream by searching for parse trees using a top-down expansion of the given formal grammar rules. Tokens are consumed from left to right. Inclusive choice is used to accommodate ambiguity by expanding all alternative right-hand-sides of grammar rules. [18]
A GLR parser (generalized left-to-right rightmost derivation parser) is an extension of an LR parser algorithm to handle non-deterministic and ambiguous grammars. [1] The theoretical foundation was provided in a 1974 paper [ 2 ] by Bernard Lang (along with other general context-free parsers such as GLL).