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Any context-free grammar can be transformed into an equivalent grammar that has no left recursion, but removal of left recursion does not always yield an LL(k) grammar. A predictive parser runs in linear time. Recursive descent with backtracking is a technique that determines which production to use by trying
In computer programming, a parser combinator is a higher-order function that accepts several parsers as input and returns a new parser as its output. In this context, a parser is a function accepting strings as input and returning some structure as output, typically a parse tree or a set of indices representing locations in the string where parsing stopped successfully.
The process of looking forward, failing, backing up, and then retrying the next alternative is known in parsing as backtracking, and it is primarily backtracking that presents opportunities for memoization in parsing. Consider a function RuleAcceptsSomeInput(Rule, Position, Input), where the parameters are as follows:
This technique essentially works by caching the results so that when the same inputs occur again, the cached result is simply returned, thus avoiding the time-consuming process of re-computing. [7] When using packrat parsing and memoization, it's noteworthy that the parsing function for each nonterminal is solely based on the input string.
LR parsing is not a useful technique for human languages with ambiguous grammars that depend on the interplay of words. Human languages are better handled by parsers like Generalized LR parser , the Earley parser , or the CYK algorithm that can simultaneously compute all possible parse trees in one pass.
LL grammars can alternatively be characterized as precisely those that can be parsed by a predictive parser – a recursive descent parser without backtracking – and these can be readily written by hand. This article is about the formal properties of LL grammars; for parsing, see LL parser or recursive descent parser.
To avoid guessing, the shift-reduce parser often looks ahead (to the right in left-to-right text) at the next scanned symbol before deciding what to do with previously scanned symbols. The lexical scanner works one symbol ahead of the rest of the parser. The lookahead symbol is also called the 'right-hand context' for each parsing decision ...
A simple tail recursive parser can be written much like a recursive descent parser. The typical algorithm for parsing a grammar like this using an abstract syntax tree is: Parse the next level of the grammar and get its output tree, designate it the first tree, F; While there is terminating token, T, that can be put as the parent of this node: