| Andrew Cooke | Contents | Latest | RSS | Twitter | Previous | Next


Welcome to my blog, which was once a mailing list of the same name and is still generated by mail. Please reply via the "comment" links.

Always interested in offers/projects/new ideas. Eclectic experience in fields like: numerical computing; Python web; Java enterprise; functional languages; GPGPU; SQL databases; etc. Based in Santiago, Chile; telecommute worldwide. CV; email.

Personal Projects

Lepl parser for Python.

Colorless Green.

Photography around Santiago.

SVG experiment.

Professional Portfolio

Calibration of seismometers.

Data access via web services.

Cache rewrite.

Extending OpenSSH.

C-ORM: docs, API.

Last 100 entries

Calling C From Fortran 95; Bjork DJ Set; Z3 Example With Python; Week 1; Useful Guide To Starting With IJulia; UK Election + Media; Review: Reinventing Organizations; Inline Assembly With Julia / LLVM; Against the definition of types; Dumb Crypto Paper; The Search For Quasi-Periodicity...; Is There An Alternative To Processing?; CARDIAC (CARDboard Illustrative Aid to Computation); The Bolivian Case Against Chile At The Hague; Clear, Cogent Economic Arguments For Immigration; A Program To Say If I Am Working; Decent Cards For Ill People; New Photo; Luksic And Barrick Gold; President Bachelet's Speech; Baltimore Primer; libxml2 Parsing Stream; configure.ac Recipe For Library Path; The Davalos Affair For Idiots; Not The Onion: Google Fireside Chat w Kissinger; Bicycle Wheels, Inertia, and Energy; Another Tax Fraud; Google's Borg; A Verion That Redirects To Local HTTP Server; Spanish Accents For Idiots; Aluminium Cans; Advice on Spray Painting; Female View of Online Chat From a Male; UX Reading List; S4 Subgroups - Geometric Interpretation; Fucking Email; The SQM Affair For Idiots; Using Kolmogorov Complexity; Oblique Strategies in bash; Curses Tools; Markov Chain Monte Carlo Without all the Bullshit; Email Para Matias Godoy Mercado; The Penta Affair For Idiots; Example Code To Create numpy Array in C; Good Article on Bias in Graphic Design (NYTimes); Do You Backup github?; Data Mining Books; SimpleDateFormat should be synchronized; British Words; Chinese Govt Intercepts External Web To DDOS github; Numbering Permutations; Teenage Engineering - Low Price Synths; GCHQ Can Do Whatever It Wants; Dublinesque; A Cryptographic SAT Solver; Security Challenges; Word Lists for Crosswords; 3D Printing and Speaker Design; Searchable Snowden Archive; XCode Backdoored; Derived Apps Have Malware (CIA); Rowhammer - Hacking Software Via Hardware (DRAM) Bugs; Immutable SQL Database (Kinda); Tor GPS Tracker; That PyCon Dongle Mess...; ASCII Fluid Dynamics; Brandalism; Table of Shifter, Cassette and Derailleur Compatability; Lenovo Demonstrates How Bad HTTPS Is; Telegraph Owned by HSBC; Smaptop - Sunrise (Music); Equation Group (NSA); UK Torture in NI; And - A Natural Extension To Regexps; This Is The Future Of Religion; The Shazam (Music Matching) Algorithm; Tributes To Lesbian Community From AIDS Survivors; Nice Rust Summary; List of Good Fiction Books; Constructing JSON From Postgres (Part 2); Constructing JSON From Postgres (Part 1); Postgres in Docker; Why Poor Places Are More Diverse; Smart Writing on Graceland; Satire in France; Free Speech in France; MTB Cornering - Where Should We Point Our Thrusters?; Secure Secure Shell; Java Generics over Primitives; 2014 (Charlie Brooker); How I am 7; Neural Nets Applied to Go; Programming, Business, Social Contracts; Distributed Systems for Fun and Profit; XML and Scheme; Internet Radio Stations (Curated List); Solid Data About Placebos; Half of Americans Think Climate Change Is a Sign of the Apocalypse; Saturday Surf Sessions With Juvenile Delinquents; Ssh, tty, stdout and stderr; Feathers falling in a vacuum; Santiago 30m Bike Route

© 2006-2015 Andrew Cooke (site) / post authors (content).

Error Handling in Recursive Descent Parsers with Backtracking

From: "andrew cooke" <andrew@...>

Date: Wed, 21 Jan 2009 09:31:23 -0300 (CLST)

It seems that the best approach is often construct an AST node that
corresponds to an error, and then later walk the tree and raise the error.
 This allows the error to be given a full context (in other words, it may
be discarded if that path subsequently fails and their is backtracking; if
it had been raised immediately the parser would never have reached a point
where it could discard the error).

Here's some example code:

from logging import basicConfig, DEBUG, INFO
from unittest import TestCase

from lepl.match import *
from lepl.node import Node, make_error, Error, throw


class Term(Node): pass
class Factor(Node): pass
class Expression(Node): pass

expr    = Delayed()
number  = Digit()[1:,...]                          > 'number'
badChar = AnyBut(Space() | Digit() | '(')[1:,...]

with Separator(r'\s*'):

    unopen   = number ** make_error('no ( before {stream_out}') & ')'
    unclosed = ('(' & expr & Eos())
                      ** make_error('no ) for {stream_in}')

    term    = Or(
        (number | '(' & expr & ')')               > Term,
        badChar                                   ^ 'unexpected text:
        unopen                                    >> throw,
        unclosed                                  >> throw
    muldiv  = Any('*/')                           > 'operator'
    factor  = (term & (muldiv & term)[:])         > Factor
    addsub  = Any('+-')                           > 'operator'
    expr   += (factor & (addsub & factor)[:])     > Expression
    line    = Empty() & Trace(expr) & Eos()

parser = line.parse_string

parser('1 + 2 * (3 + 4 - 5')[0]
#  File "<string>", line 1
#    1 + 2 * (3 + 4 - 5
#            ^
#lepl.node.Error: no ) for '(3 + 4...'

parser('1 + 2 * 3 + 4 - 5)')[0]
#  File "<string>", line 1
#    1 + 2 * 3 + 4 - 5)
#                    ^
#lepl.node.Error: no ( before ')'

parser('1 + 2 * (3 + four - 5)')[0]
#  File "<string>", line 1
#    1 + 2 * (3 + four - 5)
#                 ^
#lepl.node.Error: unexpected text: four

parser('1 + 2 ** (3 + 4 - 5)')[0]
#  File "<string>", line 1
#    1 + 2 ** (3 + 4 - 5)
#           ^
#lepl.node.Error: unexpected text: *

Comment on this post