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Pattern-matching language based on OMeta

Project description


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.

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.

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.
Match expr zero or more times, returning a list of matches.
Match expr one or more times, returning a list of matches.
Try to match expr. Returns None if it fails to match.
Fail if the next item in the input matches expr.
Call the rule ruleName.
Match the literal character ‘x’.
Bind the result of expr to the local variable name.
=> pythonExpression
Evaluate the given Python expression and return its result.

Comments like Python comments are supported as well, starting with # and extending to the end of the line.


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 pymeta.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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