hat-peg 0.4.1.dev20210407

Hat PEG parser

This library is part of Hat Open project - open-source framework of tools and libraries for developing applications used for remote monitoring, control and management of intelligent electronic devices such as IoT devices, PLCs, industrial automation or home automation systems.

Development of Hat Open and associated repositories is sponsored by Končar - Power Plant and Electric Traction Engineering Inc. (Končar KET - https://www.koncar-ket.hr).

For more information see:

• hat-peg documentation - https://core.hat-open.com/docs/libraries/peg.html

• Hat Core homepage - https://core.hat-open.com

• Hat Core git repository - https://github.com/hat-open/hat-core.git

Warning

This project is currently in state of active development. Features, functionality and API are unstable.

About

Implementation of PEG parser as described in paper “Parsing Expression Grammars: A Recognition-Based Syntactic Foundation” (Bryan Ford, 2004). PEG’s grammar itself can be defined by PEG grammar:

# Hierarchical syntax
Grammar    <- Spacing Definition+ EndOfFile
Definition <- Identifier LEFTARROW Expression
Expression <- Sequence (SLASH Sequence)*
Sequence   <- Prefix*
Prefix     <- (AND / NOT)? Suffix
Suffix     <- Primary (QUESTION / STAR / PLUS)?
Primary    <- Identifier !LEFTARROW
            / OPEN Expression CLOSE
            / Literal
            / Class
            / DOT

# Lexical syntax
Identifier <- IdentStart IdentCont* Spacing
IdentStart <- [a-zA-Z_]
IdentCont  <- IdentStart / [0-9]
Literal    <- ['] (!['] Char)* ['] Spacing
            / ["] (!["] Char)* ["] Spacing
Class      <- '[' (!']' Range)* ']' Spacing
Range      <- Char '-' Char / Char
Char       <- '\\' [nrt'"\[\]\\]
            / '\\' 'x' Hex Hex
            / '\\' 'u' Hex Hex Hex Hex
            / !'\\' .
Hex        <- [0-9a-fA-F]

LEFTARROW  <- '<-' Spacing
SLASH      <- '/' Spacing
AND        <- '&' Spacing
NOT        <- '!' Spacing
QUESTION   <- '?' Spacing
STAR       <- '*' Spacing
PLUS       <- '+' Spacing
OPEN       <- '(' Spacing
CLOSE      <- ')' Spacing
DOT        <- '.' Spacing

Spacing    <- (Space / Comment)*
Comment    <- '#' (!EndOfLine .)* EndOfLine
Space      <- ' ' / '\t' / EndOfLine
EndOfLine  <- '\r\n' / '\n' / '\r'
EndOfFile  <- !.

Example usage of PEG parser:

import functools
import hat.peg

grammar = hat.peg.Grammar(r'''
    Expr    <- Sum
    Sum     <- Product (('+' / '-') Product)*
    Product <- Value (('*' / '/') Value)*
    Value   <- Spacing ([0-9]+ / '(' Expr ')') Spacing
    Spacing <- ' '*
''', 'Expr')

ast = grammar.parse('1 + 2 * 3 + (4 - 5) * 6 + 7')

result = hat.peg.walk_ast(ast, {
    'Expr': lambda n, c: c[0],
    'Sum': lambda n, c: functools.reduce(
        (lambda acc, x: acc + x[1] if x[0] == '+' else acc - x[1]),
        zip(c[1::2], c[2::2]),
        c[0]),
    'Product': lambda n, c: functools.reduce(
        (lambda acc, x: acc * x[1] if x[0] == '*' else acc / x[1]),
        zip(c[1::2], c[2::2]),
        c[0]),
    'Value': lambda n, c: (c[2] if c[1] == '(' else
                           int(''.join(c[1:-1])))})

assert result == 8