DeCarabas/lrparsers
1
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

A cleaner API

Browse source 
I've been hacking on this in a different repository, so I thought I'd
bring it over here.
This commit is contained in:
John Doty 2024-04-21 08:07:59 -07:00
parent f656dbd8f3
commit 7c1d9b5f2b
8 changed files with 2902 additions and 2853 deletions
Download patch file Download diff file Expand all files Collapse all files

1
.gitignore vendored Normal file
View file

@ -0,0 +1 @@
__pycache__/

695
LICENSE.md
View file

@ -1,674 +1,21 @@
GNU GENERAL PUBLIC LICENSE
Version 3, 29 June 2007
Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
Preamble
The GNU General Public License is a free, copyleft license for
software and other kinds of works.
The licenses for most software and other practical works are designed
to take away your freedom to share and change the works. By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program--to make sure it remains free
software for all its users. We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors. You can apply it to
your programs, too.
When we speak of free software, we are referring to freedom, not
price. Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
free programs, and that you know you can do these things.
To protect your rights, we need to prevent others from denying you
these rights or asking you to surrender the rights. Therefore, you have
certain responsibilities if you distribute copies of the software, or if
you modify it: responsibilities to respect the freedom of others.
For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
freedoms that you received. You must make sure that they, too, receive
or can get the source code. And you must show them these terms so they
know their rights.
Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.
For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software. For both users' and
authors' sake, the GPL requires that modified versions be marked as
changed, so that their problems will not be attributed erroneously to
authors of previous versions.
Some devices are designed to deny users access to install or run
modified versions of the software inside them, although the manufacturer
can do so. This is fundamentally incompatible with the aim of
protecting users' freedom to change the software. The systematic
pattern of such abuse occurs in the area of products for individuals to
use, which is precisely where it is most unacceptable. Therefore, we
have designed this version of the GPL to prohibit the practice for those
products. If such problems arise substantially in other domains, we
stand ready to extend this provision to those domains in future versions
of the GPL, as needed to protect the freedom of users.
Finally, every program is threatened constantly by software patents.
States should not allow patents to restrict development and use of
software on general-purpose computers, but in those that do, we wish to
avoid the special danger that patents applied to a free program could
make it effectively proprietary. To prevent this, the GPL assures that
patents cannot be used to render the program non-free.
The precise terms and conditions for copying, distribution and
modification follow.
TERMS AND CONDITIONS
0. Definitions.
"This License" refers to version 3 of the GNU General Public License.
"Copyright" also means copyright-like laws that apply to other kinds of
works, such as semiconductor masks.
"The Program" refers to any copyrightable work licensed under this
License. Each licensee is addressed as "you". "Licensees" and
"recipients" may be individuals or organizations.
To "modify" a work means to copy from or adapt all or part of the work
in a fashion requiring copyright permission, other than the making of an
exact copy. The resulting work is called a "modified version" of the
earlier work or a work "based on" the earlier work.
A "covered work" means either the unmodified Program or a work based
on the Program.
To "propagate" a work means to do anything with it that, without
permission, would make you directly or secondarily liable for
infringement under applicable copyright law, except executing it on a
computer or modifying a private copy. Propagation includes copying,
distribution (with or without modification), making available to the
public, and in some countries other activities as well.
To "convey" a work means any kind of propagation that enables other
parties to make or receive copies. Mere interaction with a user through
a computer network, with no transfer of a copy, is not conveying.
An interactive user interface displays "Appropriate Legal Notices"
to the extent that it includes a convenient and prominently visible
feature that (1) displays an appropriate copyright notice, and (2)
tells the user that there is no warranty for the work (except to the
extent that warranties are provided), that licensees may convey the
work under this License, and how to view a copy of this License. If
the interface presents a list of user commands or options, such as a
menu, a prominent item in the list meets this criterion.
1. Source Code.
The "source code" for a work means the preferred form of the work
for making modifications to it. "Object code" means any non-source
form of a work.
A "Standard Interface" means an interface that either is an official
standard defined by a recognized standards body, or, in the case of
interfaces specified for a particular programming language, one that
is widely used among developers working in that language.
The "System Libraries" of an executable work include anything, other
than the work as a whole, that (a) is included in the normal form of
packaging a Major Component, but which is not part of that Major
Component, and (b) serves only to enable use of the work with that
Major Component, or to implement a Standard Interface for which an
implementation is available to the public in source code form. A
"Major Component", in this context, means a major essential component
(kernel, window system, and so on) of the specific operating system
(if any) on which the executable work runs, or a compiler used to
produce the work, or an object code interpreter used to run it.
The "Corresponding Source" for a work in object code form means all
the source code needed to generate, install, and (for an executable
work) run the object code and to modify the work, including scripts to
control those activities. However, it does not include the work's
System Libraries, or general-purpose tools or generally available free
programs which are used unmodified in performing those activities but
which are not part of the work. For example, Corresponding Source
includes interface definition files associated with source files for
the work, and the source code for shared libraries and dynamically
linked subprograms that the work is specifically designed to require,
such as by intimate data communication or control flow between those
subprograms and other parts of the work.
The Corresponding Source need not include anything that users
can regenerate automatically from other parts of the Corresponding
Source.
The Corresponding Source for a work in source code form is that
same work.
2. Basic Permissions.
All rights granted under this License are granted for the term of
copyright on the Program, and are irrevocable provided the stated
conditions are met. This License explicitly affirms your unlimited
permission to run the unmodified Program. The output from running a
covered work is covered by this License only if the output, given its
content, constitutes a covered work. This License acknowledges your
rights of fair use or other equivalent, as provided by copyright law.
You may make, run and propagate covered works that you do not
convey, without conditions so long as your license otherwise remains
in force. You may convey covered works to others for the sole purpose
of having them make modifications exclusively for you, or provide you
with facilities for running those works, provided that you comply with
the terms of this License in conveying all material for which you do
not control copyright. Those thus making or running the covered works
for you must do so exclusively on your behalf, under your direction
and control, on terms that prohibit them from making any copies of
your copyrighted material outside their relationship with you.
Conveying under any other circumstances is permitted solely under
the conditions stated below. Sublicensing is not allowed; section 10
makes it unnecessary.
3. Protecting Users' Legal Rights From Anti-Circumvention Law.
No covered work shall be deemed part of an effective technological
measure under any applicable law fulfilling obligations under article
11 of the WIPO copyright treaty adopted on 20 December 1996, or
similar laws prohibiting or restricting circumvention of such
measures.
When you convey a covered work, you waive any legal power to forbid
circumvention of technological measures to the extent such circumvention
is effected by exercising rights under this License with respect to
the covered work, and you disclaim any intention to limit operation or
modification of the work as a means of enforcing, against the work's
users, your or third parties' legal rights to forbid circumvention of
technological measures.
4. Conveying Verbatim Copies.
You may convey verbatim copies of the Program's source code as you
receive it, in any medium, provided that you conspicuously and
appropriately publish on each copy an appropriate copyright notice;
keep intact all notices stating that this License and any
non-permissive terms added in accord with section 7 apply to the code;
keep intact all notices of the absence of any warranty; and give all
recipients a copy of this License along with the Program.
You may charge any price or no price for each copy that you convey,
and you may offer support or warranty protection for a fee.
5. Conveying Modified Source Versions.
You may convey a work based on the Program, or the modifications to
produce it from the Program, in the form of source code under the
terms of section 4, provided that you also meet all of these conditions:
a) The work must carry prominent notices stating that you modified
it, and giving a relevant date.
b) The work must carry prominent notices stating that it is
released under this License and any conditions added under section
7. This requirement modifies the requirement in section 4 to
"keep intact all notices".
c) You must license the entire work, as a whole, under this
License to anyone who comes into possession of a copy. This
License will therefore apply, along with any applicable section 7
additional terms, to the whole of the work, and all its parts,
regardless of how they are packaged. This License gives no
permission to license the work in any other way, but it does not
invalidate such permission if you have separately received it.
d) If the work has interactive user interfaces, each must display
Appropriate Legal Notices; however, if the Program has interactive
interfaces that do not display Appropriate Legal Notices, your
work need not make them do so.
A compilation of a covered work with other separate and independent
works, which are not by their nature extensions of the covered work,
and which are not combined with it such as to form a larger program,
in or on a volume of a storage or distribution medium, is called an
"aggregate" if the compilation and its resulting copyright are not
used to limit the access or legal rights of the compilation's users
beyond what the individual works permit. Inclusion of a covered work
in an aggregate does not cause this License to apply to the other
parts of the aggregate.
6. Conveying Non-Source Forms.
You may convey a covered work in object code form under the terms
of sections 4 and 5, provided that you also convey the
machine-readable Corresponding Source under the terms of this License,
in one of these ways:
a) Convey the object code in, or embodied in, a physical product
(including a physical distribution medium), accompanied by the
Corresponding Source fixed on a durable physical medium
customarily used for software interchange.
b) Convey the object code in, or embodied in, a physical product
(including a physical distribution medium), accompanied by a
written offer, valid for at least three years and valid for as
long as you offer spare parts or customer support for that product
model, to give anyone who possesses the object code either (1) a
copy of the Corresponding Source for all the software in the
product that is covered by this License, on a durable physical
medium customarily used for software interchange, for a price no
more than your reasonable cost of physically performing this
conveying of source, or (2) access to copy the
Corresponding Source from a network server at no charge.
c) Convey individual copies of the object code with a copy of the
written offer to provide the Corresponding Source. This
alternative is allowed only occasionally and noncommercially, and
only if you received the object code with such an offer, in accord
with subsection 6b.
d) Convey the object code by offering access from a designated
place (gratis or for a charge), and offer equivalent access to the
Corresponding Source in the same way through the same place at no
further charge. You need not require recipients to copy the
Corresponding Source along with the object code. If the place to
copy the object code is a network server, the Corresponding Source
may be on a different server (operated by you or a third party)
that supports equivalent copying facilities, provided you maintain
clear directions next to the object code saying where to find the
Corresponding Source. Regardless of what server hosts the
Corresponding Source, you remain obligated to ensure that it is
available for as long as needed to satisfy these requirements.
e) Convey the object code using peer-to-peer transmission, provided
you inform other peers where the object code and Corresponding
Source of the work are being offered to the general public at no
charge under subsection 6d.
A separable portion of the object code, whose source code is excluded
from the Corresponding Source as a System Library, need not be
included in conveying the object code work.
A "User Product" is either (1) a "consumer product", which means any
tangible personal property which is normally used for personal, family,
or household purposes, or (2) anything designed or sold for incorporation
into a dwelling. In determining whether a product is a consumer product,
doubtful cases shall be resolved in favor of coverage. For a particular
product received by a particular user, "normally used" refers to a
typical or common use of that class of product, regardless of the status
of the particular user or of the way in which the particular user
actually uses, or expects or is expected to use, the product. A product
is a consumer product regardless of whether the product has substantial
commercial, industrial or non-consumer uses, unless such uses represent
the only significant mode of use of the product.
"Installation Information" for a User Product means any methods,
procedures, authorization keys, or other information required to install
and execute modified versions of a covered work in that User Product from
a modified version of its Corresponding Source. The information must
suffice to ensure that the continued functioning of the modified object
code is in no case prevented or interfered with solely because
modification has been made.
If you convey an object code work under this section in, or with, or
specifically for use in, a User Product, and the conveying occurs as
part of a transaction in which the right of possession and use of the
User Product is transferred to the recipient in perpetuity or for a
fixed term (regardless of how the transaction is characterized), the
Corresponding Source conveyed under this section must be accompanied
by the Installation Information. But this requirement does not apply
if neither you nor any third party retains the ability to install
modified object code on the User Product (for example, the work has
been installed in ROM).
The requirement to provide Installation Information does not include a
requirement to continue to provide support service, warranty, or updates
for a work that has been modified or installed by the recipient, or for
the User Product in which it has been modified or installed. Access to a
network may be denied when the modification itself materially and
adversely affects the operation of the network or violates the rules and
protocols for communication across the network.
Corresponding Source conveyed, and Installation Information provided,
in accord with this section must be in a format that is publicly
documented (and with an implementation available to the public in
source code form), and must require no special password or key for
unpacking, reading or copying.
7. Additional Terms.
"Additional permissions" are terms that supplement the terms of this
License by making exceptions from one or more of its conditions.
Additional permissions that are applicable to the entire Program shall
be treated as though they were included in this License, to the extent
that they are valid under applicable law. If additional permissions
apply only to part of the Program, that part may be used separately
under those permissions, but the entire Program remains governed by
this License without regard to the additional permissions.
When you convey a copy of a covered work, you may at your option
remove any additional permissions from that copy, or from any part of
it. (Additional permissions may be written to require their own
removal in certain cases when you modify the work.) You may place
additional permissions on material, added by you to a covered work,
for which you have or can give appropriate copyright permission.
Notwithstanding any other provision of this License, for material you
add to a covered work, you may (if authorized by the copyright holders of
that material) supplement the terms of this License with terms:
a) Disclaiming warranty or limiting liability differently from the
terms of sections 15 and 16 of this License; or
b) Requiring preservation of specified reasonable legal notices or
author attributions in that material or in the Appropriate Legal
Notices displayed by works containing it; or
c) Prohibiting misrepresentation of the origin of that material, or
requiring that modified versions of such material be marked in
reasonable ways as different from the original version; or
d) Limiting the use for publicity purposes of names of licensors or
authors of the material; or
e) Declining to grant rights under trademark law for use of some
trade names, trademarks, or service marks; or
f) Requiring indemnification of licensors and authors of that
material by anyone who conveys the material (or modified versions of
it) with contractual assumptions of liability to the recipient, for
any liability that these contractual assumptions directly impose on
those licensors and authors.
All other non-permissive additional terms are considered "further
restrictions" within the meaning of section 10. If the Program as you
received it, or any part of it, contains a notice stating that it is
governed by this License along with a term that is a further
restriction, you may remove that term. If a license document contains
a further restriction but permits relicensing or conveying under this
License, you may add to a covered work material governed by the terms
of that license document, provided that the further restriction does
not survive such relicensing or conveying.
If you add terms to a covered work in accord with this section, you
must place, in the relevant source files, a statement of the
additional terms that apply to those files, or a notice indicating
where to find the applicable terms.
Additional terms, permissive or non-permissive, may be stated in the
form of a separately written license, or stated as exceptions;
the above requirements apply either way.
8. Termination.
You may not propagate or modify a covered work except as expressly
provided under this License. Any attempt otherwise to propagate or
modify it is void, and will automatically terminate your rights under
this License (including any patent licenses granted under the third
paragraph of section 11).
However, if you cease all violation of this License, then your
license from a particular copyright holder is reinstated (a)
provisionally, unless and until the copyright holder explicitly and
finally terminates your license, and (b) permanently, if the copyright
holder fails to notify you of the violation by some reasonable means
prior to 60 days after the cessation.
Moreover, your license from a particular copyright holder is
reinstated permanently if the copyright holder notifies you of the
violation by some reasonable means, this is the first time you have
received notice of violation of this License (for any work) from that
copyright holder, and you cure the violation prior to 30 days after
your receipt of the notice.
Termination of your rights under this section does not terminate the
licenses of parties who have received copies or rights from you under
this License. If your rights have been terminated and not permanently
reinstated, you do not qualify to receive new licenses for the same
material under section 10.
9. Acceptance Not Required for Having Copies.
You are not required to accept this License in order to receive or
run a copy of the Program. Ancillary propagation of a covered work
occurring solely as a consequence of using peer-to-peer transmission
to receive a copy likewise does not require acceptance. However,
nothing other than this License grants you permission to propagate or
modify any covered work. These actions infringe copyright if you do
not accept this License. Therefore, by modifying or propagating a
covered work, you indicate your acceptance of this License to do so.
10. Automatic Licensing of Downstream Recipients.
Each time you convey a covered work, the recipient automatically
receives a license from the original licensors, to run, modify and
propagate that work, subject to this License. You are not responsible
for enforcing compliance by third parties with this License.
An "entity transaction" is a transaction transferring control of an
organization, or substantially all assets of one, or subdividing an
organization, or merging organizations. If propagation of a covered
work results from an entity transaction, each party to that
transaction who receives a copy of the work also receives whatever
licenses to the work the party's predecessor in interest had or could
give under the previous paragraph, plus a right to possession of the
Corresponding Source of the work from the predecessor in interest, if
the predecessor has it or can get it with reasonable efforts.
You may not impose any further restrictions on the exercise of the
rights granted or affirmed under this License. For example, you may
not impose a license fee, royalty, or other charge for exercise of
rights granted under this License, and you may not initiate litigation
(including a cross-claim or counterclaim in a lawsuit) alleging that
any patent claim is infringed by making, using, selling, offering for
sale, or importing the Program or any portion of it.
11. Patents.
A "contributor" is a copyright holder who authorizes use under this
License of the Program or a work on which the Program is based. The
work thus licensed is called the contributor's "contributor version".
A contributor's "essential patent claims" are all patent claims
owned or controlled by the contributor, whether already acquired or
hereafter acquired, that would be infringed by some manner, permitted
by this License, of making, using, or selling its contributor version,
but do not include claims that would be infringed only as a
consequence of further modification of the contributor version. For
purposes of this definition, "control" includes the right to grant
patent sublicenses in a manner consistent with the requirements of
this License.
Each contributor grants you a non-exclusive, worldwide, royalty-free
patent license under the contributor's essential patent claims, to
make, use, sell, offer for sale, import and otherwise run, modify and
propagate the contents of its contributor version.
In the following three paragraphs, a "patent license" is any express
agreement or commitment, however denominated, not to enforce a patent
(such as an express permission to practice a patent or covenant not to
sue for patent infringement). To "grant" such a patent license to a
party means to make such an agreement or commitment not to enforce a
patent against the party.
If you convey a covered work, knowingly relying on a patent license,
and the Corresponding Source of the work is not available for anyone
to copy, free of charge and under the terms of this License, through a
publicly available network server or other readily accessible means,
then you must either (1) cause the Corresponding Source to be so
available, or (2) arrange to deprive yourself of the benefit of the
patent license for this particular work, or (3) arrange, in a manner
consistent with the requirements of this License, to extend the patent
license to downstream recipients. "Knowingly relying" means you have
actual knowledge that, but for the patent license, your conveying the
covered work in a country, or your recipient's use of the covered work
in a country, would infringe one or more identifiable patents in that
country that you have reason to believe are valid.
If, pursuant to or in connection with a single transaction or
arrangement, you convey, or propagate by procuring conveyance of, a
covered work, and grant a patent license to some of the parties
receiving the covered work authorizing them to use, propagate, modify
or convey a specific copy of the covered work, then the patent license
you grant is automatically extended to all recipients of the covered
work and works based on it.
A patent license is "discriminatory" if it does not include within
the scope of its coverage, prohibits the exercise of, or is
conditioned on the non-exercise of one or more of the rights that are
specifically granted under this License. You may not convey a covered
work if you are a party to an arrangement with a third party that is
in the business of distributing software, under which you make payment
to the third party based on the extent of your activity of conveying
the work, and under which the third party grants, to any of the
parties who would receive the covered work from you, a discriminatory
patent license (a) in connection with copies of the covered work
conveyed by you (or copies made from those copies), or (b) primarily
for and in connection with specific products or compilations that
contain the covered work, unless you entered into that arrangement,
or that patent license was granted, prior to 28 March 2007.
Nothing in this License shall be construed as excluding or limiting
any implied license or other defenses to infringement that may
otherwise be available to you under applicable patent law.
12. No Surrender of Others' Freedom.
If conditions are imposed on you (whether by court order, agreement or
otherwise) that contradict the conditions of this License, they do not
excuse you from the conditions of this License. If you cannot convey a
covered work so as to satisfy simultaneously your obligations under this
License and any other pertinent obligations, then as a consequence you may
not convey it at all. For example, if you agree to terms that obligate you
to collect a royalty for further conveying from those to whom you convey
the Program, the only way you could satisfy both those terms and this
License would be to refrain entirely from conveying the Program.
13. Use with the GNU Affero General Public License.
Notwithstanding any other provision of this License, you have
permission to link or combine any covered work with a work licensed
under version 3 of the GNU Affero General Public License into a single
combined work, and to convey the resulting work. The terms of this
License will continue to apply to the part which is the covered work,
but the special requirements of the GNU Affero General Public License,
section 13, concerning interaction through a network will apply to the
combination as such.
14. Revised Versions of this License.
The Free Software Foundation may publish revised and/or new versions of
the GNU General Public License from time to time. Such new versions will
be similar in spirit to the present version, but may differ in detail to
address new problems or concerns.
Each version is given a distinguishing version number. If the
Program specifies that a certain numbered version of the GNU General
Public License "or any later version" applies to it, you have the
option of following the terms and conditions either of that numbered
version or of any later version published by the Free Software
Foundation. If the Program does not specify a version number of the
GNU General Public License, you may choose any version ever published
by the Free Software Foundation.
If the Program specifies that a proxy can decide which future
versions of the GNU General Public License can be used, that proxy's
public statement of acceptance of a version permanently authorizes you
to choose that version for the Program.
Later license versions may give you additional or different
permissions. However, no additional obligations are imposed on any
author or copyright holder as a result of your choosing to follow a
later version.
15. Disclaimer of Warranty.
THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
16. Limitation of Liability.
IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
SUCH DAMAGES.
17. Interpretation of Sections 15 and 16.
If the disclaimer of warranty and limitation of liability provided
above cannot be given local legal effect according to their terms,
reviewing courts shall apply local law that most closely approximates
an absolute waiver of all civil liability in connection with the
Program, unless a warranty or assumption of liability accompanies a
copy of the Program in return for a fee.
END OF TERMS AND CONDITIONS
How to Apply These Terms to Your New Programs
If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least
the "copyright" line and a pointer to where the full notice is found.
{one line to give the program's name and a brief idea of what it does.}
Copyright (C) {year} {name of author}
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>.
Also add information on how to contact you by electronic and paper mail.
If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:
{project} Copyright (C) {year} {fullname}
This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate
parts of the General Public License. Of course, your program's commands
might be different; for a GUI interface, you would use an "about box".
You should also get your employer (if you work as a programmer) or school,
if any, to sign a "copyright disclaimer" for the program, if necessary.
For more information on this, and how to apply and follow the GNU GPL, see
<http://www.gnu.org/licenses/>.
The GNU General Public License does not permit incorporating your program
into proprietary programs. If your program is a subroutine library, you
may consider it more useful to permit linking proprietary applications with
the library. If this is what you want to do, use the GNU Lesser General
Public License instead of this License. But first, please read
<http://www.gnu.org/philosophy/why-not-lgpl.html>.
MIT License
Copyright (c) 2024 John Doty
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

134
README.md
View file

@ -1,18 +1,126 @@
# A collection of LR parser generators, from LR0 through LALR.
One day I read a tweet, asking for a tool which accepted a grammar and an
input file and which then produced simple parsed output, without any kind of
in-between. (There was other ranty stuff about how none of the existing tools
really worked, but that was beside the point.)
This is a small helper library to generate LR parser tables.
Upon reading the tweet, it occured to me that I didn't know how LR parsers
worked and how they were generated, except in the broadest of terms. Thus, I
set about writing this, learning as I went.
The primary inspiration for this library is tree-sitter, which also generates
LR parsers for grammars written in a turing-complete language. Like that, we
write grammars in a language, only we do it in Python instead of JavaScript.
This code is not written to be fast, or even efficient, although it runs its
test cases fast enough. It was instead written to be easy to follow along
with, so that when I forget how all this works I can come back to the code
and read along and learn all over again.
Why Python? Because Python 3 is widely pre-installed on MacOS and Linux. This
library requires nothing more than the basic standard library, and not even a
new version of it. Therefore, it turns out to be a pretty light dependency for
a rust or C++ or something kind of project. (Tree-sitter, on the other hand,
requires node, which is a far less stable and available runtime in 2024.)
The parser tables can really be used to power anything. I prefer to make
concrete syntax trees (again, see tree-sitter), and there is no facility at all
for actions or custom ASTs or whatnot. Any such processing needs to be done by
the thing that processes the tables.
## Making Grammars
To get started, create a grammar that derives from the `Grammar` class. Create
one method per nonterminal, decorated with the `rule` decorator. Here's an
example:
PLUS = Token('+')
LPAREN = Token('(')
RPAREN = Token(')')
ID = Token('id')
class SimpleGrammar(Grammar):
@rule
def expression(self):
return seq(self.expression, PLUS, self.term) | self.term
@rule
def term(self):
return seq(LPAREN, self.expression, RPAREN) | ID
## Using grammars
TODO
## Representation Choices
The SimpleGrammar class might seem a little verbose compared to a dense
structure like:
grammar_simple = [
('E', ['E', '+', 'T']),
('E', ['T']),
('T', ['(', 'E', ')']),
('T', ['id']),
]
or
grammar_simple = {
'E': [
['E', '+', 'T'],
['T'],
],
'T': [
['(', 'E', ')'],
['id'],
],
}
The advantage that the class has over a table like this is that you get to have
all of your Python tools help you make sure your grammar is good, if you want
them. e.g., if you're working with an LSP or something, the members give you
autocomplete and jump-to-definition and possibly even type-checking.
At the very least, if you mis-type the name of a nonterminal, or forget to
implement it, we will immediately raise an error that *INCLUDES THE LOCATION IN
THE SOURCE WHERE THE ERROR WAS MADE.* With tables, we can tell you that you
made a mistake but it's up to you to figure out where you did it.
### Aside: What about a custom DSL/EBNF like thing?
Yeah, OK, there's a rich history of writing your grammar in a domain-specific
language. YACC did it, ANTLR does it, GRMTools.... just about everybody except
Tree-Sitter does this.
But look, I've got several reasons for not doing it.
First, I'm lazy, and don't want to write yet another parser for my parser. What
tools should I use to write my parser generator parser? I guess I don't have my
parser generator parser yet, so probably a hand-written top down parser? Some
other python parser generator? Ugh!
As an add-on to that, if I make my own format then I need to make tooling for
*that* too: syntax highlighters, jump to definition, the works. Yuck. An
existing language, and a format that builds on an existing language, gets me the
tooling that comes along with that language. If you can leverage that
effictively (and I think I have) then you start way ahead in terms of tooling.
Second, this whole thing is supposed to be easy to include in an existing
project, and adding a custom compiler doesn't seem to be that. Adding two python
files seems to be about the right speed.
Thirdly, and this is just hypothetical, it's probably pretty easy to write your
own tooling around a grammar if it's already in Python. If you want to make
railroad diagrams or EBNF pictures or whatever, all the productions are already
right there in data structures for you to process. I've tried to keep them
accessible and at least somewhat easy to work with. There's nothing that says a
DSL-based system *has* to produce unusable intermediate data- certainly there
are some tools that *try*- but with this approach the accessibility and the
ergonomics of the tool go hand in hand.
## Some History
The first version of this code was written as an idle exercise to learn how LR
parser table generation even worked. It was... very simple, fairly easy to
follow, and just *incredibly* slow. Like, mind-bogglingly slow. Unusably slow
for anything but the most trivial grammar.
As a result, when I decided I wanted to use it for a larger grammar, I found that
I just couldn't. So this has been hacked and significantly improved from that
version, now capable of building tables for nontrivial grammars. It could still
be a lot faster, but it meets my needs for now.
(BTW, the notes I read to learn how all this works are at
http://dragonbook.stanford.edu/lecture-notes/Stanford-CS143/. Specifically,
@ -20,7 +128,5 @@ I started with handout 8, 'Bottom-up-parsing', and went from there. (I did
eventually have to backtrack a little into handout 7, since that's where
First() and Follow() are covered.)
Enjoy!
doty
2016-12-09
May 2024

622
grammar.py
View file

@ -1,90 +1,5 @@
import parser_faster
import sys
import typing
from parser_faster import Assoc
class Token:
value: str
def __init__(self, value):
self.value = sys.intern(value)
Symbol = Token | str
def desugar(
grammar: dict[str, list[list[Symbol]]],
precedence: list[typing.Tuple[Assoc, list[Symbol]]],
):
nonterminal_refs = set()
nonterminals = set()
terminals = set()
result: list[typing.Tuple[str, list[str]]] = []
for (k, v) in grammar.items():
nonterminals.add(k)
for rule in v:
assert isinstance(rule, list)
result_rule: list[str] = []
for symbol in rule:
if isinstance(symbol, Token):
result_rule.append(symbol.value)
terminals.add(symbol.value)
else:
result_rule.append(symbol)
nonterminal_refs.add(symbol)
result.append((k, result_rule))
unknown_rules = nonterminal_refs - nonterminals
if len(unknown_rules) > 0:
undefined = "\n ".join(unknown_rules)
raise Exception(f"The following rules are not defined:\n {undefined}")
overlap_rules = nonterminals & terminals
if len(overlap_rules) > 0:
overlap = "\n ".join(overlap_rules)
raise Exception(f"The following symbols are both tokens and rules:\n {overlap}")
result_precedence = {
(symbol.value if isinstance(symbol, Token) else symbol):(associativity, precedence + 1)
for precedence, (associativity, symbols) in enumerate(precedence)
for symbol in symbols
}
return result, result_precedence
def dump_yacc(grammar):
tokens = set()
for rules in grammar.values():
for rule in rules:
for symbol in rule:
if symbol.startswith("token:"):
symbol = symbol[6:].upper()
tokens.add(symbol)
for token in sorted(tokens):
print(f"%token {token}")
print()
print("%%")
for name, rules in grammar.items():
print(f"{name} : ", end='');
for i,rule in enumerate(rules):
if i != 0:
print(f"{' ' * len(name)} | ", end='')
parts = []
for symbol in rule:
if symbol.startswith("token:"):
symbol = symbol[6:].upper()
parts.append(symbol)
print(' '.join(parts))
print()
print("%%")
# This is an example grammar.
from parser import Assoc, Grammar, Nothing, Token, rule, seq
ARROW = Token("Arrow")
AS = Token("As")
@ -136,7 +51,9 @@ LSQUARE = Token("LeftBracket")
RSQUARE = Token("RightBracket")
# fmt: off
class FineGrammar(Grammar):
def __init__(self):
super().__init__(
precedence=[
(Assoc.RIGHT, [EQUAL]),
(Assoc.LEFT, [OR]),
@ -146,280 +63,327 @@ precedence = [
(Assoc.LEFT, [LESS, GREATER, GREATEREQUAL, LESSEQUAL]),
(Assoc.LEFT, [PLUS, MINUS]),
(Assoc.LEFT, [STAR, SLASH]),
(Assoc.LEFT, ["PrimaryExpression"]),
(Assoc.LEFT, [self.primary_expression]),
(Assoc.LEFT, [LPAREN]),
(Assoc.LEFT, [DOT]),
# If there's a confusion about whether to make an IF statement or an
# expression, prefer the statement.
(Assoc.NONE, ["IfStatement"]),
#
# If there's a confusion about whether to make an IF
# statement or an expression, prefer the statement.
#
(Assoc.NONE, [self.if_statement]),
]
)
grammar = {
"File": [
["FileStatementList"],
],
"FileStatementList": [
["FileStatement"],
["FileStatement", "FileStatementList"],
],
"FileStatement": [
["ImportStatement"],
["ClassDeclaration"],
["ExportStatement"],
["Statement"],
],
@rule
def file(self):
return self.file_statement_list
"ImportStatement": [
[IMPORT, STRING, AS, IDENTIFIER, SEMICOLON],
],
@rule
def file_statement_list(self):
return self.file_statement | (self.file_statement_list + self.file_statement)
# Classes
"ClassDeclaration": [
[CLASS, IDENTIFIER, "ClassBody"],
],
"ClassBody": [
[LCURLY, RCURLY],
[LCURLY, "ClassMembers", RCURLY],
],
"ClassMembers": [
["ClassMember"],
["ClassMembers", "ClassMember"],
],
"ClassMember": [
["FieldDeclaration"],
["FunctionDeclaration"],
],
"FieldDeclaration": [
[IDENTIFIER, COLON, "TypeExpression", SEMICOLON],
],
@rule
def file_statement(self):
return (
self.import_statement | self.class_declaration | self.export_statement | self.statement
)
@rule
def import_statement(self):
return seq(IMPORT, STRING, AS, IDENTIFIER, SEMICOLON)
@rule
def class_declaration(self):
return seq(CLASS, IDENTIFIER, self.class_body)
@rule
def class_body(self):
return seq(LCURLY, RCURLY) | seq(LCURLY, self.class_members, RCURLY)
@rule
def class_members(self):
return self.class_member | seq(self.class_members, self.class_member)
@rule
def class_member(self):
return self.field_declaration | self.function_declaration
@rule
def field_declaration(self):
return seq(IDENTIFIER, COLON, self.type_expression, SEMICOLON)
# Types
"TypeExpression": [
["AlternateType"],
["TypeIdentifier"],
],
"AlternateType": [
["TypeExpression", BAR, "TypeIdentifier"],
],
"TypeIdentifier": [
[IDENTIFIER],
],
@rule
def type_expression(self):
return self.alternate_type | self.type_identifier
"ExportStatement": [
[EXPORT, "ClassDeclaration"],
[EXPORT, "FunctionDeclaration"],
[EXPORT, "LetStatement"],
[EXPORT, "ExportList", SEMICOLON],
],
"ExportList": [
[],
[IDENTIFIER],
[IDENTIFIER, COMMA, "ExportList"],
],
@rule
def alternate_type(self):
return seq(self.type_expression, BAR, self.type_identifier)
@rule
def type_identifier(self):
return IDENTIFIER
@rule
def export_statement(self):
return (
seq(EXPORT, self.class_declaration)
| seq(EXPORT, self.function_declaration)
| seq(EXPORT, self.let_statement)
| seq(EXPORT, self.export_list, SEMICOLON)
)
@rule
def export_list(self):
return Nothing | IDENTIFIER | seq(IDENTIFIER, COMMA, self.export_list)
# Functions
"FunctionDeclaration": [
[FUN, IDENTIFIER, "FunctionParameters", "Block"],
[FUN, IDENTIFIER, "FunctionParameters", ARROW, "TypeExpression", "Block"],
],
"FunctionParameters": [
[LPAREN, RPAREN],
[LPAREN, "FirstParameter", RPAREN],
[LPAREN, "FirstParameter", COMMA, "ParameterList", RPAREN],
],
"FirstParameter": [
[SELF],
["Parameter"],
],
"ParameterList": [
[],
["Parameter"],
["Parameter", COMMA, "ParameterList"],
],
"Parameter": [
[IDENTIFIER, COLON, "TypeExpression"],
],
@rule
def function_declaration(self):
return seq(FUN, IDENTIFIER, self.function_parameters, self.block) | seq(
FUN, IDENTIFIER, self.function_parameters, ARROW, self.type_expression, self.block
)
@rule
def function_parameters(self):
return (
seq(LPAREN, RPAREN)
| seq(LPAREN, self.first_parameter, RPAREN)
| seq(LPAREN, self.first_parameter, COMMA, self.parameter_list, RPAREN)
)
@rule
def first_parameter(self):
return SELF | self.parameter
@rule
def parameter_list(self):
return Nothing | self.parameter | seq(self.parameter, COMMA, self.parameter_list)
@rule
def parameter(self):
return seq(IDENTIFIER, COLON, self.type_expression)
# Block
"Block": [
[LCURLY, RCURLY],
[LCURLY, "StatementList", RCURLY],
[LCURLY, "StatementList", "Expression", RCURLY],
],
"StatementList": [
["Statement"],
["StatementList", "Statement"],
],
@rule
def block(self):
return (
seq(LCURLY, RCURLY)
| seq(LCURLY, self.statement_list, RCURLY)
| seq(LCURLY, self.statement_list, self.expression, RCURLY)
)
"Statement": [
["FunctionDeclaration"],
["LetStatement"],
["ReturnStatement"],
["ForStatement"],
["IfStatement"],
["WhileStatement"],
["ExpressionStatement"],
],
@rule
def statement_list(self):
return self.statement | seq(self.statement_list, self.statement)
"LetStatement": [
[LET, IDENTIFIER, EQUAL, "Expression", SEMICOLON],
],
@rule
def statement(self):
return (
self.function_declaration
| self.let_statement
| self.return_statement
| self.for_statement
| self.if_statement
| self.while_statement
| self.expression_statement
)
"ReturnStatement": [
[RETURN, "Expression", SEMICOLON],
],
@rule
def let_statement(self):
return seq(LET, IDENTIFIER, EQUAL, self.expression, SEMICOLON)
"ForStatement": [
[FOR, "IteratorVariable", IN, "Expression", "Block"],
],
"IteratorVariable": [[IDENTIFIER]],
@rule
def return_statement(self):
return seq(RETURN, self.expression, SEMICOLON)
"IfStatement": [["ConditionalExpression"]],
@rule
def for_statement(self):
return seq(FOR, self.iterator_variable, IN, self.expression, self.block)
"WhileStatement": [
[WHILE, "Expression", "Block"],
],
@rule
def iterator_variable(self):
return IDENTIFIER
"ExpressionStatement": [
["Expression", SEMICOLON],
],
@rule
def if_statement(self):
return self.conditional_expression
@rule
def while_statement(self):
return seq(WHILE, self.expression, self.block)
@rule
def expression_statement(self):
return seq(self.expression, SEMICOLON)
# Expressions
"Expression": [["AssignmentExpression"]],
@rule
def expression(self):
return self.assignment_expression
"AssignmentExpression": [
["OrExpression", EQUAL, "AssignmentExpression"],
["OrExpression"],
],
"OrExpression": [
["OrExpression", OR, "IsExpression"],
["IsExpression"],
],
"IsExpression": [
["IsExpression", IS, "Pattern"],
["AndExpression"],
],
"AndExpression": [
["AndExpression", AND, "EqualityExpression"],
["EqualityExpression"],
],
"EqualityExpression": [
["EqualityExpression", EQUALEQUAL, "RelationExpression"],
["EqualityExpression", BANGEQUAL, "RelationExpression"],
["RelationExpression"],
],
"RelationExpression": [
["RelationExpression", LESS, "AdditiveExpression"],
["RelationExpression", LESSEQUAL, "AdditiveExpression"],
["RelationExpression", GREATER, "AdditiveExpression"],
["RelationExpression", GREATEREQUAL, "AdditiveExpression"],
["AdditiveExpression"],
],
"AdditiveExpression": [
["AdditiveExpression", PLUS, "MultiplicationExpression"],
["AdditiveExpression", MINUS, "MultiplicationExpression"],
["MultiplicationExpression"],
],
"MultiplicationExpression": [
["MultiplicationExpression", STAR, "PrimaryExpression"],
["MultiplicationExpression", SLASH, "PrimaryExpression"],
["PrimaryExpression"],
],
"PrimaryExpression": [
[IDENTIFIER],
[SELF],
[NUMBER],
[STRING],
[TRUE],
[FALSE],
[BANG, "PrimaryExpression"],
[MINUS, "PrimaryExpression"],
@rule
def assignment_expression(self):
return seq(self.or_expression, EQUAL, self.assignment_expression) | self.or_expression
["Block"],
["ConditionalExpression"],
["ListConstructorExpression"],
["ObjectConstructorExpression"],
["MatchExpression"],
@rule
def or_expression(self):
return seq(self.or_expression, OR, self.is_expression) | self.is_expression
["PrimaryExpression", LPAREN, "ExpressionList", RPAREN],
["PrimaryExpression", DOT, IDENTIFIER],
@rule
def is_expression(self):
return seq(self.is_expression, IS, self.pattern) | self.and_expression
[LPAREN, "Expression", RPAREN],
],
@rule
def and_expression(self):
return seq(self.and_expression, AND, self.equality_expression) | self.equality_expression
"ConditionalExpression": [
[IF, "Expression", "Block"],
[IF, "Expression", "Block", ELSE, "ConditionalExpression"],
[IF, "Expression", "Block", ELSE, "Block"],
],
@rule
def equality_expression(self):
return (
seq(self.equality_expression, EQUALEQUAL, self.relation_expression)
| seq(self.equality_expression, BANGEQUAL, self.relation_expression)
| self.relation_expression
)
"ListConstructorExpression": [
[LSQUARE, RSQUARE],
[LSQUARE, "ExpressionList", RSQUARE],
],
@rule
def relation_expression(self):
return (
seq(self.relation_expression, LESS, self.additive_expression)
| seq(self.relation_expression, LESSEQUAL, self.additive_expression)
| seq(self.relation_expression, GREATER, self.additive_expression)
| seq(self.relation_expression, GREATEREQUAL, self.additive_expression)
)
"ExpressionList": [
["Expression"],
["Expression", COMMA],
["Expression", COMMA, "ExpressionList"],
],
@rule
def additive_expression(self):
return (
seq(self.additive_expression, PLUS, self.multiplication_expression)
| seq(self.additive_expression, MINUS, self.multiplication_expression)
| self.multiplication_expression
)
# Match Expression
"MatchExpression": [
[MATCH, "MatchBody"],
],
"MatchBody": [
[LCURLY, RCURLY],
[LCURLY, "MatchArms", RCURLY],
],
"MatchArms": [
["MatchArm"],
["MatchArm", COMMA],
["MatchArm", COMMA, "MatchArms"],
],
"MatchArm": [
["Pattern", ARROW, "Expression"],
],
@rule
def multiplication_expression(self):
return (
seq(self.multiplication_expression, STAR, self.primary_expression)
| seq(self.multiplication_expression, SLASH, self.primary_expression)
| self.primary_expression
)
# Pattern
"Pattern": [
["VariableBinding", "PatternCore", AND, "AndExpression"],
["VariableBinding", "PatternCore"],
["PatternCore", AND, "AndExpression"],
["PatternCore"],
],
"PatternCore": [
["TypeExpression"],
["WildcardPattern"],
],
"WildcardPattern": [[UNDERSCORE]],
"VariableBinding": [[IDENTIFIER, COLON]],
@rule
def primary_expression(self):
return (
IDENTIFIER
| SELF
| NUMBER
| STRING
| TRUE
| FALSE
| seq(BANG, self.primary_expression)
| seq(MINUS, self.primary_expression)
| self.block
| self.conditional_expression
| self.list_constructor_expression
| self.object_constructor_expression
| self.match_expression
| seq(self.primary_expression, LPAREN, self.expression_list, RPAREN)
| seq(self.primary_expression, DOT, IDENTIFIER)
| seq(LPAREN, self.expression, RPAREN)
)
# Object Constructor
"ObjectConstructorExpression": [
[NEW, "TypeIdentifier", "FieldList"],
],
"FieldList": [
[LCURLY, RCURLY],
[LCURLY, "FieldValues", RCURLY],
],
"FieldValues": [
["FieldValue"],
["FieldValue", COMMA],
["FieldValue", COMMA, "FieldValues"],
],
"FieldValue": [
[IDENTIFIER],
[IDENTIFIER, COLON, "Expression"],
],
}
# fmt: on
@rule
def conditional_expression(self):
return (
seq(IF, self.expression, self.block)
| seq(IF, self.expression, self.block, ELSE, self.conditional_expression)
| seq(IF, self.expression, self.block, ELSE, self.block)
)
@rule
def list_constructor_expression(self):
return seq(LSQUARE, RSQUARE) | seq(LSQUARE, self.expression_list, RSQUARE)
@rule
def expression_list(self):
return (
self.expression
| seq(self.expression, COMMA)
| seq(self.expression, COMMA, self.expression_list)
)
@rule
def match_expression(self):
return seq(MATCH, self.match_body)
@rule
def match_body(self):
return seq(LCURLY, RCURLY) | seq(LCURLY, self.match_arms, RCURLY)
@rule
def match_arms(self):
return (
self.match_arm
| seq(self.match_arm, COMMA)
| seq(self.match_arm, COMMA, self.match_arms)
)
@rule
def match_arm(self):
return seq(self.pattern, ARROW, self.expression)
@rule
def pattern(self):
return (
seq(self.variable_binding, self.pattern_core, AND, self.and_expression)
| seq(self.variable_binding, self.pattern_core)
| seq(self.pattern_core, AND, self.and_expression)
| self.pattern_core
)
@rule
def pattern_core(self):
return self.type_expression | self.wildcard_pattern
@rule
def wildcard_pattern(self):
return UNDERSCORE
@rule
def variable_binding(self):
return seq(IDENTIFIER, COLON)
@rule
def object_constructor_expression(self):
return seq(NEW, self.type_identifier, self.field_list)
@rule
def field_list(self):
return seq(LCURLY, RCURLY) | seq(LCURLY, self.field_values, RCURLY)
@rule
def field_values(self):
return (
self.field_value
| seq(self.field_value, COMMA)
| seq(self.field_value, COMMA, self.field_values)
)
@rule
def field_value(self):
return IDENTIFIER | seq(IDENTIFIER, COLON, self.expression)
grammar = FineGrammar()
table = grammar.build_table(start="file")
print(f"{len(table)} states")
average_entries = sum(len(row) for row in table) / len(table)
max_entries = max(len(row) for row in table)
print(f"{average_entries} average, {max_entries} max")
# dump_yacc(grammar)
grammar, precedence = desugar(grammar, precedence)
gen = parser_faster.GenerateLR1("File", grammar, precedence=precedence)
gen.gen_table()
# print(parser_faster.format_table(gen, table))
# print()
# tree = parse(table, ["id", "+", "(", "id", "[", "id", "]", ")"])

853
historical/parser.py Normal file
View file

@ -0,0 +1,853 @@
"""A collection of LR parser generators, from LR0 through LALR.
One day I read a tweet, asking for a tool which accepted a grammar and an
input file and which then produced simple parsed output, without any kind of
in-between. (There was other ranty stuff about how none of the existing tools
really worked, but that was beside the point.)
Upon reading the tweet, it occured to me that I didn't know how LR parsers
worked and how they were generated, except in the broadest of terms. Thus, I
set about writing this, learning as I went.
This code is not written to be fast, or even efficient, although it runs its
test cases fast enough. It was instead written to be easy to follow along
with, so that when I forget how all this works I can come back to the code
and read along and learn all over again.
(BTW, the notes I read to learn how all this works are at
http://dragonbook.stanford.edu/lecture-notes/Stanford-CS143/. Specifically,
I started with handout 8, 'Bottom-up-parsing', and went from there. (I did
eventually have to backtrack a little into handout 7, since that's where
First() and Follow() are covered.)
Enjoy!
doty
2016-12-09
"""
from collections import namedtuple
###############################################################################
# LR0
#
# We start with LR0 parsers, because they form the basis of everything else.
###############################################################################
class Configuration(namedtuple("Configuration", ["name", "symbols", "position", "lookahead"])):
"""A rule being tracked in a state.
(Note: technically, lookahead isn't used until we get to LR(1) parsers,
but if left at its default it's harmless. Ignore it until you get to
the part about LR(1).)
"""
__slots__ = ()
@classmethod
def from_rule(cls, rule, lookahead=()):
return Configuration(
name=rule[0],
symbols=rule[1],
position=0,
lookahead=lookahead,
)
@property
def at_end(self):
return self.position == len(self.symbols)
@property
def next(self):
return self.symbols[self.position] if not self.at_end else None
@property
def rest(self):
return self.symbols[(self.position + 1) :]
def at_symbol(self, symbol):
return self.next == symbol
def replace(self, **kwargs):
return self._replace(**kwargs)
def __str__(self):
la = ", " + str(self.lookahead) if self.lookahead != () else ""
return "{name} -> {bits}{lookahead}".format(
name=self.name,
bits=" ".join(
["* " + sym if i == self.position else sym for i, sym in enumerate(self.symbols)]
)
+ (" *" if self.at_end else ""),
lookahead=la,
)
class GenerateLR0(object):
"""Generate parser tables for an LR0 parser.
The input grammars are of the form:
grammar_simple = [
('E', ['E', '+', 'T']),
('E', ['T']),
('T', ['(', 'E', ')']),
('T', ['id']),
]
Which is to say, they are a list of productions. Each production is a
tuple where the first element of the tuple is the name of the
non-terminal being added, and the second elment of the tuple is the
list of terminals and non-terminals that make up the production.
There is currently no support for custom actions or alternation or
anything like that. If you want alternations that you'll have to lower
the grammar by hand into the simpler form first.
Don't name anything with double-underscores; those are reserved for
the generator. Don't add '$' either, as it is reserved to mean
end-of-stream. Use an empty list to indicate nullability, that is:
('O', []),
means that O can be matched with nothing.
Implementation notes:
- This is implemented in the dumbest way possible, in order to be the
most understandable it can be. I built this to learn, and I want to
make sure I can keep learning with it.
- We tend to use tuples everywhere. This is because tuples can be
compared for equality and put into tables and all that jazz. They might
be a little bit slower in places but like I said, this is for
learning. (Also, if we need this to run faster we can probably go a
long way by memoizing results, which is much easier if we have tuples
everywhere.)
"""
def __init__(self, start, grammar):
"""Initialize the parser generator with the specified grammar and
start symbol.
"""
# We always store the "augmented" grammar, which contains an initial
# production for the start state. grammar[0] is always the start
# rule, and in the set of states and table and whatever the first
# element is always the starting state/position.
self.grammar = [("__start", [start])] + grammar
self.nonterminals = {rule[0] for rule in grammar}
self.terminals = {
sym for name, symbols in grammar for sym in symbols if sym not in self.nonterminals
}
self.alphabet = self.terminals | self.nonterminals
# Check to make sure they didn't use anything that will give us
# heartburn later.
reserved = [a for a in self.alphabet if a.startswith("__") or a == "$"]
if reserved:
raise ValueError(
"Can't use {symbols} in grammars, {what} reserved.".format(
symbols=" or ".join(reserved),
what="it's" if len(reserved) == 1 else "they're",
)
)
self.terminals.add("$")
self.alphabet.add("$")
def gen_closure_next(self, config):
"""Return the next set of configurations in the closure for
config.
If the position for config is just before a non-terminal, then the
next set of configurations is configurations for all of the
productions for that non-terminal, with the position at the
beginning. (If the position for config is just before a terminal,
or at the end of the production, then the next set is empty.)
"""
if config.at_end:
return ()
else:
return tuple(
Configuration.from_rule(rule) for rule in self.grammar if rule[0] == config.next
)
def gen_closure(self, config, closure):
"""Compute the closure for the specified config and unify it with the
existing closure.
If the provided config is already in the closure then nothing is
done. (We assume that the closure of the config is *also* already in
the closure.)
"""
if config in closure:
return closure
else:
new_closure = tuple(closure) + (config,)
for next_config in self.gen_closure_next(config):
new_closure = self.gen_closure(next_config, new_closure)
return new_closure
def gen_successor(self, config_set, symbol):
"""Compute the successor state for the given config set and the
given symbol.
The successor represents the next state of the parser after seeing
the symbol.
"""
seeds = [
config.replace(position=config.position + 1)
for config in config_set
if config.at_symbol(symbol)
]
closure = ()
for seed in seeds:
closure = self.gen_closure(seed, closure)
return closure
def gen_all_successors(self, config_set):
"""Return all of the non-empty successors for the given config set."""
next = []
for symbol in self.alphabet:
successor = self.gen_successor(config_set, symbol)
if len(successor) > 0:
next.append(successor)
return tuple(next)
def gen_sets(self, config_set, F):
"""Recursively generate all configuration sets starting from the
provided set, and merge them with the provided set 'F'.
"""
if config_set in F:
return F
else:
new_F = F + (config_set,)
for successor in self.gen_all_successors(config_set):
new_F = self.gen_sets(successor, new_F)
return new_F
def gen_all_sets(self):
"""Generate all of the configuration sets for the grammar."""
initial_set = self.gen_closure(
Configuration.from_rule(self.grammar[0]),
(),
)
return self.gen_sets(initial_set, ())
def find_set_index(self, sets, set):
"""Find the specified set in the set of sets, and return the
index, or None if it is not found.
"""
for i, s in enumerate(sets):
if s == set:
return i
return None
def gen_reduce_set(self, config):
"""Return the set of symbols that indicate we should reduce the given
configuration.
In an LR0 parser, this is just the set of all terminals."""
return self.terminals
def gen_table(self):
"""Generate the parse table.
The parse table is a list of states. The first state in the list is
the starting state. Each state is a dictionary that maps a symbol to an
action. Each action is a tuple. The first element of the tuple is a
string describing what to do:
- 'shift': The second element of the tuple is the state
number. Consume the input and push that state onto the stack.
- 'reduce': The second element is the name of the non-terminal being
reduced, and the third element is the number of states to remove
from the stack. Don't consume the input; just remove the specified
number of things from the stack, and then consult the table again,
this time using the new top-of-stack as the current state and the
name of the non-terminal to find out what to do.
- 'goto': The second element is the state number to push onto the
stack. In the literature, these entries are treated distinctly from
the actions, but we mix them here because they never overlap with the
other actions. (These are always associated with non-terminals, and
the other actions are always associated with terminals.)
- 'accept': Accept the result of the parse, it worked.
Anything missing from the row indicates an error.
"""
action_table = []
config_sets = self.gen_all_sets()
for config_set in config_sets:
actions = {}
# Actions
for config in config_set:
if config.at_end:
if config.name != "__start":
for a in self.gen_reduce_set(config):
self.set_table_action(
actions,
a,
("reduce", config.name, len(config.symbols)),
config,
)
else:
self.set_table_action(
actions,
"$",
("accept",),
config,
)
else:
if config.next in self.terminals:
successor = self.gen_successor(config_set, config.next)
index = self.find_set_index(config_sets, successor)
self.set_table_action(
actions,
config.next,
("shift", index),
config,
)
# Gotos
for symbol in self.nonterminals:
successor = self.gen_successor(config_set, symbol)
index = self.find_set_index(config_sets, successor)
if index is not None:
self.set_table_action(
actions,
symbol,
("goto", index),
None,
)
# set_table_action stores the configs that generated the actions in
# the table, for diagnostic purposes. This filters them out again
# so that the parser has something clean to work with.
actions = {k: self.get_table_action(actions, k) for k in actions}
action_table.append(actions)
return action_table
def set_table_action(self, row, symbol, action, config):
"""Set the action for 'symbol' in the table row to 'action'.
This is destructive; it changes the table. It raises an error if
there is already an action for the symbol in the row.
"""
existing, existing_config = row.get(symbol, (None, None))
if existing is not None and existing != action:
config_old = str(existing_config)
config_new = str(config)
max_len = max(len(config_old), len(config_new)) + 1
error = (
"Conflicting actions for token '{symbol}':\n"
" {config_old: <{max_len}}: {old}\n"
" {config_new: <{max_len}}: {new}\n".format(
config_old=config_old,
config_new=config_new,
max_len=max_len,
old=existing,
new=action,
symbol=symbol,
)
)
raise ValueError(error)
row[symbol] = (action, config)
def get_table_action(self, row, symbol):
return row[symbol][0]
def parse(table, input, trace=False):
"""Parse the input with the generated parsing table and return the
concrete syntax tree.
The parsing table can be generated by GenerateLR0.gen_table() or by any
of the other generators below. The parsing mechanism never changes, only
the table generation mechanism.
input is a list of tokens. Don't stick an end-of-stream marker, I'll stick
one on for you.
"""
assert "$" not in input
input = input + ["$"]
input_index = 0
# Our stack is a stack of tuples, where the first entry is the state number
# and the second entry is the 'value' that was generated when the state was
# pushed.
stack = [(0, None)]
while True:
current_state = stack[-1][0]
current_token = input[input_index]
action = table[current_state].get(current_token, ("error",))
if trace:
print(
"{stack: <20} {input: <50} {action: <5}".format(
stack=repr([s[0] for s in stack]),
input=repr(input[input_index:]),
action=repr(action),
)
)
if action[0] == "accept":
return stack[-1][1]
elif action[0] == "reduce":
name = action[1]
size = action[2]
value = (name, tuple(s[1] for s in stack[-size:]))
stack = stack[:-size]
goto = table[stack[-1][0]].get(name, ("error",))
assert goto[0] == "goto" # Corrupt table?
stack.append((goto[1], value))
elif action[0] == "shift":
stack.append((action[1], (current_token, ())))
input_index += 1
elif action[0] == "error":
raise ValueError(
"Syntax error: unexpected symbol {sym}".format(
sym=current_token,
),
)
###############################################################################
# SLR(1)
###############################################################################
class GenerateSLR1(GenerateLR0):
"""Generate parse tables for SLR1 grammars.
SLR1 parsers can recognize more than LR0 parsers, because they have a
little bit more information: instead of generating reduce actions for a
production on all possible inputs, as LR0 parsers do, they generate
reduce actions only for inputs that are in the 'follow' set of the
non-terminal.
That means SLR1 parsers need to know how to generate 'follow(A)', which
means they need to know how to generate 'first(A)', which is most of the
code in this class.
"""
def gen_first_symbol(self, symbol, visited):
"""Compute the first set for a single symbol.
If a symbol can be empty, then the set contains epsilon, which we
represent as python's `None`.
The first set is the set of tokens that can appear as the first token
for a given symbol. (Obviously, if the symbol is itself a token, then
this is trivial.)
'visited' is a set of already visited symbols, to stop infinite
recursion on left-recursive grammars. That means that sometimes this
function can return an empty tuple. Don't confuse that with a tuple
containing epsilon: that's a tuple containing `None`, not an empty
tuple.
"""
if symbol in self.terminals:
return (symbol,)
elif symbol in visited:
return ()
else:
assert symbol in self.nonterminals
visited.add(symbol)
# All the firsts from all the productions.
firsts = [
self.gen_first(rule[1], visited) for rule in self.grammar if rule[0] == symbol
]
result = ()
for fs in firsts:
result = result + tuple(f for f in fs if f not in result)
return tuple(sorted(result))
def gen_first(self, symbols, visited=None):
"""Compute the first set for a sequence of symbols.
The first set is the set of tokens that can appear as the first token
for this sequence of symbols. The interesting wrinkle in computing the
first set for a sequence of symbols is that we keep computing the first
sets so long as epsilon appears in the set. i.e., if we are computing
for ['A', 'B', 'C'] and the first set of 'A' contains epsilon, then the
first set for the *sequence* also contains the first set of ['B', 'C'],
since 'A' could be missing entirely.
An epsilon in the result is indicated by 'None'. There will always be
at least one element in the result.
The 'visited' parameter, if not None, is a set of symbols that are
already in the process of being evaluated, to deal with left-recursive
grammars. (See gen_first_symbol for more.)
"""
if len(symbols) == 0:
return (None,) # Epsilon.
else:
if visited is None:
visited = set()
result = self.gen_first_symbol(symbols[0], visited)
if None in result:
result = tuple(s for s in result if s is not None)
result = result + self.gen_first(symbols[1:], visited)
result = tuple(sorted(set(result)))
return result
def gen_follow(self, symbol, visited=None):
"""Generate the follow set for the given nonterminal.
The follow set for a nonterminal is the set of terminals that can
follow the nonterminal in a valid sentence. The resulting set never
contains epsilon and is never empty, since we should always at least
ground out at '$', which is the end-of-stream marker.
"""
if symbol == "__start":
return tuple("$")
assert symbol in self.nonterminals
# Deal with left-recursion.
if visited is None:
visited = set()
if symbol in visited:
return ()
visited.add(symbol)
follow = ()
for production in self.grammar:
for index, prod_symbol in enumerate(production[1]):
if prod_symbol != symbol:
continue
first = self.gen_first(production[1][index + 1 :])
follow = follow + tuple(f for f in first if f is not None)
if None in first:
follow = follow + self.gen_follow(production[0], visited)
assert None not in follow # Should always ground out at __start
return follow
def gen_reduce_set(self, config):
"""Return the set of symbols that indicate we should reduce the given
config.
In an SLR1 parser, this is the follow set of the config nonterminal."""
return self.gen_follow(config.name)
class GenerateLR1(GenerateSLR1):
"""Generate parse tables for LR1, or "canonical LR" grammars.
LR1 parsers can recognize more than SLR parsers. Like SLR parsers, they
are choosier about when they reduce. But unlike SLR parsers, they specify
the terminals on which they reduce by carrying a 'lookahead' terminal in
the configuration. The lookahead of a configuration is computed as the
closure of a configuration set is computed, so see gen_closure_next for
details. (Except for the start configuration, which has '$' as its
lookahead.)
"""
def gen_reduce_set(self, config):
"""Return the set of symbols that indicate we should reduce the given
config.
In an LR1 parser, this is the lookahead of the configuration."""
return config.lookahead
def gen_closure_next(self, config):
"""Return the next set of configurations in the closure for
config.
In LR1 parsers, we must compute the lookahead for the configurations
we're adding to the closure. The lookahead for the new configurations
is the first() of the rest of this config's production. If that
contains epsilon, then the lookahead *also* contains the lookahead we
already have. (This lookahead was presumably generated by the same
process, so in some sense it is a 'parent' lookahead, or a lookahead
from an upstream production in the grammar.)
(See the documentation in GenerateLR0 for more information on how
this function fits into the whole process.)
"""
if config.at_end:
return ()
else:
next = []
for rule in self.grammar:
if rule[0] != config.next:
continue
# N.B.: We can't just append config.lookahead to config.rest
# and compute first(), because lookahead is a *set*. So
# in this case we just say if 'first' contains epsilon,
# then we need to remove the epsilon and union with the
# existing lookahead.
lookahead = self.gen_first(config.rest)
if None in lookahead:
lookahead = tuple(l for l in lookahead if l is not None)
lookahead = lookahead + config.lookahead
lookahead = tuple(sorted(set(lookahead)))
next.append(Configuration.from_rule(rule, lookahead=lookahead))
return tuple(next)
def gen_all_sets(self):
"""Generate all of the configuration sets for the grammar.
In LR1 parsers, we must remember to set the lookahead of the start
symbol to '$'.
"""
initial_set = self.gen_closure(
Configuration.from_rule(self.grammar[0], lookahead=("$",)),
(),
)
return self.gen_sets(initial_set, ())
class GenerateLALR(GenerateLR1):
"""Generate tables for LALR.
LALR is smaller than LR(1) but bigger than SLR(1). It works by generating
the LR(1) configuration sets, but merging configuration sets which are
equal in everything but their lookaheads. This works in that it doesn't
generate any shift/reduce conflicts that weren't already in the LR(1)
grammar. It can, however, introduce new reduce/reduce conflicts, because
it does lose information. The advantage is that the number of parser
states is much much smaller in LALR than in LR(1).
(Note that because we use immutable state everywhere this generator does
a lot of copying and allocation.)
"""
def merge_sets(self, config_set_a, config_set_b):
"""Merge the two config sets, by keeping the item cores but merging
the lookahead sets for each item.
"""
assert len(config_set_a) == len(config_set_b)
merged = []
for index, a in enumerate(config_set_a):
b = config_set_b[index]
assert a.replace(lookahead=()) == b.replace(lookahead=())
new_lookahead = a.lookahead + b.lookahead
new_lookahead = tuple(sorted(set(new_lookahead)))
merged.append(a.replace(lookahead=new_lookahead))
return tuple(merged)
def sets_equal(self, a, b):
a_no_la = tuple(s.replace(lookahead=()) for s in a)
b_no_la = tuple(s.replace(lookahead=()) for s in b)
return a_no_la == b_no_la
def gen_sets(self, config_set, F):
"""Recursively generate all configuration sets starting from the
provided set, and merge them with the provided set 'F'.
The difference between this method and the one in GenerateLR0, where
this comes from, is in the part that stops recursion. In LALR we
compare for set equality *ignoring lookahead*. If we find a match,
then instead of returning F unchanged, we merge the two equal sets
and replace the set in F, returning the modified set.
"""
config_set_no_la = tuple(s.replace(lookahead=()) for s in config_set)
for index, existing in enumerate(F):
existing_no_la = tuple(s.replace(lookahead=()) for s in existing)
if config_set_no_la == existing_no_la:
merged_set = self.merge_sets(config_set, existing)
return F[:index] + (merged_set,) + F[index + 1 :]
# No merge candidate found, proceed.
new_F = F + (config_set,)
for successor in self.gen_all_successors(config_set):
new_F = self.gen_sets(successor, new_F)
return new_F
def find_set_index(self, sets, set):
"""Find the specified set in the set of sets, and return the
index, or None if it is not found.
"""
for i, s in enumerate(sets):
if self.sets_equal(s, set):
return i
return None
###############################################################################
# Formatting
###############################################################################
def format_node(node):
"""Print out an indented concrete syntax tree, from parse()."""
lines = ["{name}".format(name=node[0])] + [
" " + line for child in node[1] for line in format_node(child).split("\n")
]
return "\n".join(lines)
def format_table(generator, table):
"""Format a parser table so pretty."""
def format_action(state, terminal):
action = state.get(terminal, ("error",))
if action[0] == "accept":
return "accept"
elif action[0] == "shift":
return "s" + str(action[1])
elif action[0] == "error":
return ""
elif action[0] == "reduce":
return "r" + str(action[1])
header = " | {terms} | {nts}".format(
terms=" ".join("{0: <6}".format(terminal) for terminal in sorted(generator.terminals)),
nts=" ".join("{0: <5}".format(nt) for nt in sorted(generator.nonterminals)),
)
lines = [
header,
"-" * len(header),
] + [
"{index: <3} | {actions} | {gotos}".format(
index=i,
actions=" ".join(
"{0: <6}".format(format_action(row, terminal))
for terminal in sorted(generator.terminals)
),
gotos=" ".join(
"{0: <5}".format(row.get(nt, ("error", ""))[1])
for nt in sorted(generator.nonterminals)
),
)
for i, row in enumerate(table)
]
return "\n".join(lines)
###############################################################################
# Examples
###############################################################################
# OK, this is a very simple LR0 grammar.
grammar_simple = [
("E", ["E", "+", "T"]),
("E", ["T"]),
("T", ["(", "E", ")"]),
("T", ["id"]),
]
gen = GenerateLR0("E", grammar_simple)
table = gen.gen_table()
tree = parse(table, ["id", "+", "(", "id", ")"])
print(format_node(tree) + "\n")
print()
# This one doesn't work with LR0, though, it has a shift/reduce conflict.
grammar_lr0_shift_reduce = grammar_simple + [
("T", ["id", "[", "E", "]"]),
]
try:
gen = GenerateLR0("E", grammar_lr0_shift_reduce)
table = gen.gen_table()
assert False
except ValueError as e:
print(e)
print()
# Nor does this: it has a reduce/reduce conflict.
grammar_lr0_reduce_reduce = grammar_simple + [
("E", ["V", "=", "E"]),
("V", ["id"]),
]
try:
gen = GenerateLR0("E", grammar_lr0_reduce_reduce)
table = gen.gen_table()
assert False
except ValueError as e:
print(e)
print()
# Nullable symbols just don't work with constructs like this, because you can't
# look ahead to figure out if you should reduce an empty 'F' or not.
grammar_nullable = [
("E", ["F", "boop"]),
("F", ["beep"]),
("F", []),
]
try:
gen = GenerateLR0("E", grammar_nullable)
table = gen.gen_table()
assert False
except ValueError as e:
print(e)
gen = GenerateSLR1("E", grammar_lr0_shift_reduce)
print("First: {first}".format(first=str(gen.gen_first(["E"]))))
print("Follow: {follow}".format(follow=str(gen.gen_follow("E"))))
table = gen.gen_table()
print(format_table(gen, table))
tree = parse(table, ["id", "+", "(", "id", "[", "id", "]", ")"])
print(format_node(tree) + "\n")
print()
# SLR1 can't handle this.
grammar_aho_ullman_1 = [
("S", ["L", "=", "R"]),
("S", ["R"]),
("L", ["*", "R"]),
("L", ["id"]),
("R", ["L"]),
]
try:
gen = GenerateSLR1("S", grammar_aho_ullman_1)
table = gen.gen_table()
assert False
except ValueError as e:
print(e)
print()
# Here's an example with a full LR1 grammar, though.
grammar_aho_ullman_2 = [
("S", ["X", "X"]),
("X", ["a", "X"]),
("X", ["b"]),
]
gen = GenerateLR1("S", grammar_aho_ullman_2)
table = gen.gen_table()
print(format_table(gen, table))
parse(table, ["b", "a", "a", "b"], trace=True)
print()
# What happens if we do LALR to it?
gen = GenerateLALR("S", grammar_aho_ullman_2)
table = gen.gen_table()
print(format_table(gen, table))
print()
# A fun LALAR grammar.
grammar_lalr = [
("S", ["V", "E"]),
("E", ["F"]),
("E", ["E", "+", "F"]),
("F", ["V"]),
("F", ["int"]),
("F", ["(", "E", ")"]),
("V", ["id"]),
]
gen = GenerateLALR("S", grammar_lalr)
table = gen.gen_table()
print(format_table(gen, table))
print()

1950
parser.py
View file

File diff suppressed because it is too large Load diff

1295
parser_faster.py
View file

File diff suppressed because it is too large Load diff

13
pyproject.toml Normal file
View file

@ -0,0 +1,13 @@
[project]
name = "lrparsers"
descrption = "a small LR parser generator library"
authors = [
{name = "John Doty", email = "john@d0ty.me"},
]
classifiers = [
"Private :: Do Not Upload", # Probably.
"License :: OSI Approved :: MIT License",
]
[tool.black]
line-length=100