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A Python library for parsing and tree transformation based on OMeta
License: Other
This project forked from peterderivaz/pymeta3
A Python library for parsing and tree transformation based on OMeta
License: Other
======== PyMeta3 ======== -------------------------------------------- A Pattern-Matching Language Based on Python -------------------------------------------- Summary ======= PyMeta is an implementation of OMeta, an object-oriented pattern-matching language developed by Alessandro Warth. PyMeta provides a compact syntax based on Parsing Expression Grammars (PEGs) for common lexing, parsing and tree-transforming activities in a way that's easy to reason about for Python programmers. History ======= The intriguing OMeta language was invented by Alessandro Warth. (Website http://tinlizzie.org/ometa/) This was ported to Python by Allen Short. (Called PyMeta) Some useful syntax updates were made by Waldemar Kornewald. (Called PyMeta2) This version contains some additional minor changes that I find useful: 1) Instead of writing token("(") you can write #( 2) You can also add comments in Python style via # This is a comment 3) You can match a given number of repetitions of a pattern, e.g. expr{4} This is especially useful when parsing binary formats where the number of entries in a list is given as part of the stream, e.g. num:x expr{x} How It Works ============ PyMeta compiles a grammar to a Python class, with the rules as methods. The rules specify parsing expressions, which consume input and return values if they succeed in matching. The grammar can either match a string, or a nested set of lists containing strings. Basic syntax ~~~~~~~~~~~~~~~~ ``foo = ....`` Define a rule named foo. ``expr1 expr2`` Match expr1, and then match expr2 if it succeeds, returning the value of expr2. Like Python's ``and``. ``expr1 | expr2`` Try to match expr1 --- if it fails, match expr2 instead. Like Python's ``or``. ``expr*`` Match expr zero or more times, returning a list of matches. ``expr+`` Match expr one or more times, returning a list of matches. ``expr{pythonExpression}`` Match expr pythonExpression times, returning a list of matches. ``expr?`` Try to match expr. Returns None if it fails to match. ``~expr`` Fail if the next item in the input matches expr. ``ruleName`` Call the rule ``ruleName``. ``ruleName(pythonExpression)`` Call the rule ``ruleName`` passing in the arguments in the Python expression. ``'x'`` Match the literal character 'x'. ``expr:name`` Bind the result of expr to the local variable ``name``. ``-> pythonExpression`` Evaluate the given Python expression and return its result. ``~~expr`` Lookahead and try to match expr, then rewind to the current position. ``#and`` Match optional whitespace followed by the sequence of characters. ``[ expr ]`` Matches a Python list that contains a pattern matching expr. ``?(pythonExpression)`` Match only if the Python expression evaluates to true. ``!(pythonExpression)`` Execute the Python expression if we reach this point. ``digit`` Built in rule to match a digit ``letterOrDigit`` Built in rule to match letters or digits ``anything`` Built in rule to match any single character ``spaces`` Built in rule to match any amount of whitespace ``apply(ruleExpr)`` Calls the rule ruleExpr. RuleExpr should be a Python expression that evaluates to a string naming the rule. Comments like Python comments are supported as well, starting with # and extending to the end of the line. Interface ========= The starting point for defining a new grammar is ``pymeta.grammar.OMeta.makeGrammar``, which takes a grammar definition and a dict of variable bindings for its embedded expressions and produces a Python class. Grammars can be subclassed as usual, and makeGrammar can be called on these classes to override rules and provide new ones. To invoke a grammar rule, call ``grammarObject.apply()`` with its name. Example Usage ============= >>> from pymeta3.grammar import OMeta >>> exampleGrammar = """ ones = '1' '1' -> 1 twos = '2' '2' -> 2 stuff = (ones | twos)+ """ >>> Example = OMeta.makeGrammar(exampleGrammar, {}) >>> g = Example("11221111") >>> result, error = g.apply("stuff") >>> result [1, 2, 1, 1]
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