The steps that are performed when parsing the expression 3 + 5 * (10 - 20) using the grammar defined above. The details of the algorithm canīe found in a compiler textbook, but the following example illustrates Stack and looking at the stack and the next input token for patterns LR parsing is commonly implemented by shifting grammar symbols onto a Is triggered and the grammar symbols on right hand side are replacedīy the grammar symbol on the left-hand-side. Right-hand-side is found in the input, the appropriate action method The right-hand-side of various grammar rules. LR parsing is a bottom up technique that tries to recognize SLY uses a parsing technique known as LR-parsing or shift-reduce Specification for the operation of a simple calculator like this: ForĮxample, given the expression grammar above, you might write the On those values when different grammar rules are recognized. Values can be attached each symbol and operations carried out Translation, the symbols in the grammar become a kind of The semantics of what happens during parsing is often specified usingĪ technique known as syntax directed translation. In thisĬase, multiplication and division have higher precedence than addition
The operator precedence rules for the different operations. Grammar into different levels (e.g., expr and term) encodes Identifiers such as term and factor refer to grammar rulesĬomprised of a collection of terminals and other rules. are known as terminals and correspond to raw input tokens. In the grammar, symbols such as NUMBER, +, -, *, and You can also add error handling.Įxpr : expr + term | expr - term | term term : term * factor | term / factor | factor factor : NUMBER | ( expr ) Of writing separate regular expression rules. Certain features such asĬharacter literals are there mainly for convenience, saving you the trouble You can optionally attach actions thatĮxecute when certain patterns are encountered. It might take a bit to digest, but all of theĮssential parts of writing a lexer are there. This is always required tokens = ', lineno = 7, index = 88 ) # calclex.py from sly import Lexer class CalcLexer ( Lexer ): # Set of token names. Used as a reference for SLY as the concepts are virtually identical. O’Reilly’s “LexĪnd Yacc” by John Levine may also be handy. Techniques, and Tools”, by Aho, Sethi, and Ullman. Want to consult an introductory text such as “Compilers: Principles, If you are unfamiliar with these topics, you will probably Parsing theory, syntax directed translation, and the use of compilerĬonstruction tools such as lex and yacc in other programming The rest of this document assumes that you are somewhat familiar with Instead, you will find a bare-bones, yetįully capable library for writing parsers in Python.
Nor should you view itĪs a parsing framework. The way of bells and whistles (e.g., automatic construction ofĪbstract syntax trees, tree traversal, etc.).
It should also be noted that SLY does not provide much in Most of theįeatures available in lex and yacc are also available in SLY. It is looselyīased on the traditional compiler construction tools lex and yaccĪnd implements the same LALR(1) parsing algorithm. SLY is library for writing parsers and compilers.