6 ms·

Don't Use ISO/IEC 14977:1996 Extended Backus-Naur Form (EBNF) (2023)

51 points by gslin 1y ago

dwheeler 1y ago
I'm the author. Ask me anything!
- teddyh 1y ago
  Is there any documented version of EBNF suitable for general use?
  - ggm 1y ago
    IETF uses ABNF https://datatracker.ietf.org/doc/html/rfc5234 https://datatracker.ietf.org/doc/html/rfc5234 Commentaries say its different, but the differences might not matter?
- ucarion 1y ago
  Is the main objection to RFC5234 its weird syntax for repetition (*THING, [THING], 5THING)?
  - dwheeler 1y ago
    More or less. It's a perfectly workable approach, but it's the reverse of what practically everyone else does, as well as the reverse of regex notation. Common notations should be common.
- 90s_dev 1y ago
  What's your favorite programming language and why is it C?
  - dwheeler 1y ago
    I like lots of programming languages, I don't really have 1 favorite. I like the relative simplicity of C, and I have long familiarity with it, but its lack of memory safety and endless undefined behavior make it more difficult to safely use than necessary.
- arh68 1y ago
  Great article. Informative, qualified. I had not realized how splintered the [E]BNF syntax is, like in the way I already knew timestamps are (3339 vs 8601 vs mm/dd/yyyy &c &c). Q: what's your ideal way to write Unicode characters clearly? In the W3C/XML spec they'll have stuff like [#x200C-#x200D] but I have no idea what those are, without like a dictionary on hand. Points for specificity, but it doesn't scream "readable". Your point about standards-not-publicly-available is unfortunately similar to, well, laws. In some areas, "the laws" themselves are not public (!) though perhaps it's a digression better to not get into pedantically, s/unabiguously/unambiguously/g
  - dwheeler 1y ago
    In many cases I think the character itself is clearest. Thankfully most tools can handle Unicode today. That's not always unambiguous, so sometimes an annotation may be helpful, and if the spec is freely available you can copy amd paste from it.
- alfiedotwtf 1y ago
  So if not EBNF for defining grammars, what should people be using ie what would be your preferred that would be powerfully enough and general purpose? I’ve never seen PCRE used for grammars and not sure if it would be powerfully enough for all cases, but personally I think it would be pretty cool since I find it easier to read than EBNF and it’s so widely used people can slot a specification right into code and it should just work
  - dwheeler 1y ago
    One of those alternative specifications is in the W3C Extensible Markup Language (XML) 1.0 (Fifth Edition). As I note, "The W3C specification is much more similar to typical regex syntax making it much easier for today's software developers to understand), avoids the key problems of 14977:1996, and is already clearly described." It's freely available from w3c. You don't need to use XML to use it.
    - alfiedotwtf 1y ago
      Oops, sorry. When you said that I thought there was going to be an actual specification of the grammar specification itself rather than the application in use. Ok cool, got it thanks :)
HarHarVeryFunny 1y ago
> One of the most common operations in a grammar, by far, is concatenation (aka sequencing). ISO/IEC 14977:1996 requires that a comma be used for every concatenation, so any sequence of N symbols will have N-1 commas. It seems they were trying really hard to make EBNF NOT look like the grammar it is representing. > ISO/IEC 14977:1996 represents “one or more” as { symbol }- which means “0 or more symbols, and then subtract the empty set It's almost as if they were looking for a special syntax to express "1 or more". Bizarre ... just bizarre.
mannyv 1y ago
(4) -> I would say that the lack of regex use is a plus. Yes I use them all the time. For anything complicated I'll just ask ChatGPT to make it. And of course, are regexs all the same across everything? (5) -> Once you get it you get it. Every language specification is odd in its own special way. In any case I haven't really seen BNF/EBNF used in the last few years, so this is probably a moot discussion.
- alfiedotwtf 1y ago
  How are the grammars you see on a regular basis specified if not by BNF?
  - mdaniel 1y ago
    I'm not the person you asked but I believe there are two broad classes of answers: hand rolled parsers and PEG grammars e.g. https://github.com/llvm/llvm-project/blob/llvmorg-20.1.5/clang/lib/Parse/Parser.cpp#L54 https://github.com/llvm/llvm-project/blob/llvmorg-20.1.5/cla... and https://github.com/python/cpython/blob/v3.13.3/Grammar/python.gram https://github.com/python/cpython/blob/v3.13.3/Grammar/pytho...
    - alfiedotwtf 1y ago
      Hand rolled and PEG are nice implementation wise, but to think about the grammar itself and needing to debug it not so much. Maybe it's a personal thing, but I'd rather something textual that generated a parser than have to hand roll rec decent etc.
Mofpofjis 1y ago
The complaints about ISO Prolog, and about ISO standards generally not being available publicly / at no cost, resonate a lot with me. If you compare eg. the POSIX and the ISO C development workflows, and their respective end results, there's a world of difference. "ISO" (International Standards Organization), where you have "national member bodies", should absolutely be a thing of the past for programming languages (or for anything related to computing). My country's national body's own homepage has a huge tirade, aiming to "dispel myths", such as the "myth" that "standards should be available free of charge". Meanwhile, lots of std orgs have published computing standards with various degrees of openness already. ISO is a relic when it comes to computing. (So are other standards bodies that choose to remain proprietary; like those behind PCI, SCSI, ... Computing is ubiquitous and these bodies should be forced open by law, for the public interest.)
- convolvatron 1y ago
  doesn’t have to be a legal enforcement if vendors just don’t work with bodies that insist on this counterproductive notion of standards as a profit center. unfortunately i think there us some degree of collusion here. it’s easier to get your existing proprietary standards ratified if there are fewer players in the room and the palms that need to be greased are clearly marked
  - OneDeuxTriSeiGo 1y ago
    It's not exactly clear cut. Standards are unfortunately in general quite expensive to produce and maintain. Software oriented standards are certainly cheaper than metallurgy, machining, manufacturing, building construction, environmental, health & safety, and the other big classes of standards however they still have quite a cost. Historically the ISO standards development process for software standards (like the C or C++ standards) happened only in small part asynchronously and historically required large, extended-duration committee meetings where all the details were hashed out in person. This process only really started to change during COVID but even then it's still a very in-person synch-heavy process and that's not exactly cheap to run. And with most standards, the FDIS (final draft international standard) revisions are made public. They can be found online even if they can be annoying to dig up. For 99% of cases the FDIS revision is more than sufficient and is identical to the published standard minus a typo or grammar mistake here and there. As the average SW dev or engineer of course you don't need to fork over the cost for the published standard but any large company will probably purchase a catalog of standards rather than deal with the overhead of dealing with FDIS (and any legal risk from not following the "true" standard). It is also worth noting that pretty much every university library (and many public non-university libraries) has some contract or service that provides access to copies of the standard to members free of cost.
userbinator 1y ago
IMHO ABNF is just as annoying to read, especially its insistence on '/' instead of '|' for alternation (when | has universally become the "OR" symbol in other languages) and "%x" for hex prefixes. My preference for syntax descriptions is [ ] for "optional", ",,," or "+" for "1 or more", "*" for "0 or more", and "{min,max}" or "{n}" for repetition ranges, which closely mirrors regex syntax; I'm not sure if this has been standardised or even has a name, but I've seen it far more often than ABNF/EBNF.
- Pet_Ant 1y ago
  First thing I noticed was > rule = definition; comment CR LF ...so it uses CR/LF as a line terminator as opposed to just LF like POSIX does so you are gonna have text editors and other tools just breaking the format on save.
kstenerud 1y ago
These are some of the many reasons why I developed the Dogma metalanguage. https://dogma-lang.org/ https://dogma-lang.org/ https://github.com/kstenerud/dogma/blob/master/v1/dogma_v1.0.md https://github.com/kstenerud/dogma/blob/master/v1/dogma_v1.0... That, and also I needed a language that could describe binary data.
- mdaniel 1y ago
  Do I correctly understand that you don't currently have a parser generator for .dogma files? I tried clicking around in a few of the adjacent repos without much luck Anyway, my go-to tire kicking for any such binary file description format is parsing .pdf files, since they are ferociously hard, and include backreferences
  - kstenerud 1y ago
    I haven't gotten around to writing a parser generator yet. I only wanted this for documentation purposes, and haven't had the spare time to take it further yet. There are definitely going to be horribly convoluted formats that it can't describe without help (which is where custom functions come in). But it's been able to describe most formats I've thrown at it, so that's good enough for Dogma v1. I was able to get it to describe minidump without function help...
PJBaker 1y ago
> every single language document uses its own notation, which is more often than not, a dialect of the (Extended) Backus-Naur Form I came across this recently while writing an article that references Lua, Go and Python (3.8) syntax. Each of them uses a slightly different form of EBNF. To make them more easily comparable, I wanted to convert all three to the same format. Looking for something fairly standard (not entirely ad-hoc but also not as formal as ISO/IEC EBNF or RFC 5234 ABNF), I came across Wirth Syntax Notation [0] [1]: syntax = { production } . production = identifier "=" expression "." . expression = term { "|" term } . term = factor { factor } . factor = identifier | literal | "(" expression ")" | "[" expression "]" | "{" expression "}" . literal = "\"" character {character} "\"" . It turns out that the Go specification already uses WSN [2]. I converted Lua and Python, and then could work with all three language grammars in a consistent, machine-readable notation. [0] https://en.wikipedia.org/wiki/Wirth_syntax_notation https://en.wikipedia.org/wiki/Wirth_syntax_notation [1] https://dl.acm.org/doi/10.1145/359863.359883 https://dl.acm.org/doi/10.1145/359863.359883 [2] https://go.dev/ref/spec#Notation https://go.dev/ref/spec#Notation
- egberts1 1y ago
  Here is a list of all variants of EBNFs: https://github.com/egberts/vim-syntax-ebnf https://github.com/egberts/vim-syntax-ebnf And a EBNF format detector: https://github.com/egberts/filetype-ebnf-grammars https://github.com/egberts/filetype-ebnf-grammars And a master list of all variants of EBNF: http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html
egberts1 1y ago
Still use EBNF, so some of my tools, citations, and sources are: Here is a list of all variants of EBNFs as well as Vim syntax highlighter for EBNF variants: https://github.com/egberts/vim-syntax-ebnf https://github.com/egberts/vim-syntax-ebnf And a EBNF format detector: https://github.com/egberts/filetype-ebnf-grammars https://github.com/egberts/filetype-ebnf-grammars And a master list of all variants of EBNF: http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html
- deleted 1y ago
  [deleted]

51 points by gslin 1y ago

dwheeler 1y ago

I'm the author. Ask me anything!

teddyh 1y ago
Is there any documented version of EBNF suitable for general use?
- ggm 1y ago
  IETF uses ABNF https://datatracker.ietf.org/doc/html/rfc5234 https://datatracker.ietf.org/doc/html/rfc5234 Commentaries say its different, but the differences might not matter?
ucarion 1y ago
Is the main objection to RFC5234 its weird syntax for repetition (*THING, [THING], 5THING)?
- dwheeler 1y ago
  More or less. It's a perfectly workable approach, but it's the reverse of what practically everyone else does, as well as the reverse of regex notation. Common notations should be common.
90s_dev 1y ago
What's your favorite programming language and why is it C?
- dwheeler 1y ago
  I like lots of programming languages, I don't really have 1 favorite. I like the relative simplicity of C, and I have long familiarity with it, but its lack of memory safety and endless undefined behavior make it more difficult to safely use than necessary.
arh68 1y ago
Great article. Informative, qualified. I had not realized how splintered the [E]BNF syntax is, like in the way I already knew timestamps are (3339 vs 8601 vs mm/dd/yyyy &c &c). Q: what's your ideal way to write Unicode characters clearly? In the W3C/XML spec they'll have stuff like [#x200C-#x200D] but I have no idea what those are, without like a dictionary on hand. Points for specificity, but it doesn't scream "readable". Your point about standards-not-publicly-available is unfortunately similar to, well, laws. In some areas, "the laws" themselves are not public (!) though perhaps it's a digression better to not get into pedantically, s/unabiguously/unambiguously/g
- dwheeler 1y ago
  In many cases I think the character itself is clearest. Thankfully most tools can handle Unicode today. That's not always unambiguous, so sometimes an annotation may be helpful, and if the spec is freely available you can copy amd paste from it.
alfiedotwtf 1y ago
So if not EBNF for defining grammars, what should people be using ie what would be your preferred that would be powerfully enough and general purpose? I’ve never seen PCRE used for grammars and not sure if it would be powerfully enough for all cases, but personally I think it would be pretty cool since I find it easier to read than EBNF and it’s so widely used people can slot a specification right into code and it should just work
- dwheeler 1y ago
  One of those alternative specifications is in the W3C Extensible Markup Language (XML) 1.0 (Fifth Edition). As I note, "The W3C specification is much more similar to typical regex syntax making it much easier for today's software developers to understand), avoids the key problems of 14977:1996, and is already clearly described." It's freely available from w3c. You don't need to use XML to use it.
  - alfiedotwtf 1y ago
    Oops, sorry. When you said that I thought there was going to be an actual specification of the grammar specification itself rather than the application in use. Ok cool, got it thanks :)

HarHarVeryFunny 1y ago

> One of the most common operations in a grammar, by far, is concatenation (aka sequencing). ISO/IEC 14977:1996 requires that a comma be used for every concatenation, so any sequence of N symbols will have N-1 commas. It seems they were trying really hard to make EBNF NOT look like the grammar it is representing. > ISO/IEC 14977:1996 represents “one or more” as { symbol }- which means “0 or more symbols, and then subtract the empty set It's almost as if they were looking for a special syntax to express "1 or more". Bizarre ... just bizarre.

mannyv 1y ago

(4) -> I would say that the lack of regex use is a plus. Yes I use them all the time. For anything complicated I'll just ask ChatGPT to make it. And of course, are regexs all the same across everything? (5) -> Once you get it you get it. Every language specification is odd in its own special way. In any case I haven't really seen BNF/EBNF used in the last few years, so this is probably a moot discussion.

alfiedotwtf 1y ago
How are the grammars you see on a regular basis specified if not by BNF?
- mdaniel 1y ago
  I'm not the person you asked but I believe there are two broad classes of answers: hand rolled parsers and PEG grammars e.g. https://github.com/llvm/llvm-project/blob/llvmorg-20.1.5/clang/lib/Parse/Parser.cpp#L54 https://github.com/llvm/llvm-project/blob/llvmorg-20.1.5/cla... and https://github.com/python/cpython/blob/v3.13.3/Grammar/python.gram https://github.com/python/cpython/blob/v3.13.3/Grammar/pytho...
  - alfiedotwtf 1y ago
    Hand rolled and PEG are nice implementation wise, but to think about the grammar itself and needing to debug it not so much. Maybe it's a personal thing, but I'd rather something textual that generated a parser than have to hand roll rec decent etc.

Mofpofjis 1y ago

The complaints about ISO Prolog, and about ISO standards generally not being available publicly / at no cost, resonate a lot with me. If you compare eg. the POSIX and the ISO C development workflows, and their respective end results, there's a world of difference. "ISO" (International Standards Organization), where you have "national member bodies", should absolutely be a thing of the past for programming languages (or for anything related to computing). My country's national body's own homepage has a huge tirade, aiming to "dispel myths", such as the "myth" that "standards should be available free of charge". Meanwhile, lots of std orgs have published computing standards with various degrees of openness already. ISO is a relic when it comes to computing. (So are other standards bodies that choose to remain proprietary; like those behind PCI, SCSI, ... Computing is ubiquitous and these bodies should be forced open by law, for the public interest.)

convolvatron 1y ago
doesn’t have to be a legal enforcement if vendors just don’t work with bodies that insist on this counterproductive notion of standards as a profit center. unfortunately i think there us some degree of collusion here. it’s easier to get your existing proprietary standards ratified if there are fewer players in the room and the palms that need to be greased are clearly marked
- OneDeuxTriSeiGo 1y ago
  It's not exactly clear cut. Standards are unfortunately in general quite expensive to produce and maintain. Software oriented standards are certainly cheaper than metallurgy, machining, manufacturing, building construction, environmental, health & safety, and the other big classes of standards however they still have quite a cost. Historically the ISO standards development process for software standards (like the C or C++ standards) happened only in small part asynchronously and historically required large, extended-duration committee meetings where all the details were hashed out in person. This process only really started to change during COVID but even then it's still a very in-person synch-heavy process and that's not exactly cheap to run. And with most standards, the FDIS (final draft international standard) revisions are made public. They can be found online even if they can be annoying to dig up. For 99% of cases the FDIS revision is more than sufficient and is identical to the published standard minus a typo or grammar mistake here and there. As the average SW dev or engineer of course you don't need to fork over the cost for the published standard but any large company will probably purchase a catalog of standards rather than deal with the overhead of dealing with FDIS (and any legal risk from not following the "true" standard). It is also worth noting that pretty much every university library (and many public non-university libraries) has some contract or service that provides access to copies of the standard to members free of cost.

userbinator 1y ago

IMHO ABNF is just as annoying to read, especially its insistence on '/' instead of '|' for alternation (when | has universally become the "OR" symbol in other languages) and "%x" for hex prefixes. My preference for syntax descriptions is [ ] for "optional", ",,," or "+" for "1 or more", "*" for "0 or more", and "{min,max}" or "{n}" for repetition ranges, which closely mirrors regex syntax; I'm not sure if this has been standardised or even has a name, but I've seen it far more often than ABNF/EBNF.

Pet_Ant 1y ago
First thing I noticed was > rule = definition; comment CR LF ...so it uses CR/LF as a line terminator as opposed to just LF like POSIX does so you are gonna have text editors and other tools just breaking the format on save.

kstenerud 1y ago

These are some of the many reasons why I developed the Dogma metalanguage. https://dogma-lang.org/ https://dogma-lang.org/ https://github.com/kstenerud/dogma/blob/master/v1/dogma_v1.0.md https://github.com/kstenerud/dogma/blob/master/v1/dogma_v1.0... That, and also I needed a language that could describe binary data.

mdaniel 1y ago
Do I correctly understand that you don't currently have a parser generator for .dogma files? I tried clicking around in a few of the adjacent repos without much luck Anyway, my go-to tire kicking for any such binary file description format is parsing .pdf files, since they are ferociously hard, and include backreferences
- kstenerud 1y ago
  I haven't gotten around to writing a parser generator yet. I only wanted this for documentation purposes, and haven't had the spare time to take it further yet. There are definitely going to be horribly convoluted formats that it can't describe without help (which is where custom functions come in). But it's been able to describe most formats I've thrown at it, so that's good enough for Dogma v1. I was able to get it to describe minidump without function help...

PJBaker 1y ago

> every single language document uses its own notation, which is more often than not, a dialect of the (Extended) Backus-Naur Form I came across this recently while writing an article that references Lua, Go and Python (3.8) syntax. Each of them uses a slightly different form of EBNF. To make them more easily comparable, I wanted to convert all three to the same format. Looking for something fairly standard (not entirely ad-hoc but also not as formal as ISO/IEC EBNF or RFC 5234 ABNF), I came across Wirth Syntax Notation [0] [1]: syntax = { production } . production = identifier "=" expression "." . expression = term { "|" term } . term = factor { factor } . factor = identifier | literal | "(" expression ")" | "[" expression "]" | "{" expression "}" . literal = "\"" character {character} "\"" . It turns out that the Go specification already uses WSN [2]. I converted Lua and Python, and then could work with all three language grammars in a consistent, machine-readable notation. [0] https://en.wikipedia.org/wiki/Wirth_syntax_notation https://en.wikipedia.org/wiki/Wirth_syntax_notation [1] https://dl.acm.org/doi/10.1145/359863.359883 https://dl.acm.org/doi/10.1145/359863.359883 [2] https://go.dev/ref/spec#Notation https://go.dev/ref/spec#Notation

egberts1 1y ago
Here is a list of all variants of EBNFs: https://github.com/egberts/vim-syntax-ebnf https://github.com/egberts/vim-syntax-ebnf And a EBNF format detector: https://github.com/egberts/filetype-ebnf-grammars https://github.com/egberts/filetype-ebnf-grammars And a master list of all variants of EBNF: http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html

egberts1 1y ago

Still use EBNF, so some of my tools, citations, and sources are: Here is a list of all variants of EBNFs as well as Vim syntax highlighter for EBNF variants: https://github.com/egberts/vim-syntax-ebnf https://github.com/egberts/vim-syntax-ebnf And a EBNF format detector: https://github.com/egberts/filetype-ebnf-grammars https://github.com/egberts/filetype-ebnf-grammars And a master list of all variants of EBNF: http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html http://www.cs.man.ac.uk/~pjj/bnf/ebnf.html

deleted 1y ago
[deleted]