Douglas Crockford makes reference to Simplified JavaScript in this article about Top Down Operator Precedence. Unfortunately he only makes references to it in this article. The best explanation I found here. But I still don't have a clue what it actually is.
What is Simplified JavaScript?
In the context of the Top Down Operator Precedence article, "simplified" means "easier to write a parser for". That is, he wanted to demonstrate how to write a Javascript parser in Javascript, without overcomplicating things by trying to support all the full specfications loveable quirks.
Yes, the features supported in "Simplified Javascript" are very close to those listed as "Good Parts", but the point of that article was not to promote Crockford's world view or book, but to show how to write a language parser (no easy task).
From the article:
We don't have time in this short
chapter to deal with the whole
JavaScript language, and perhaps we
wouldn't want to because the language
is a mess. But it has some brilliant
stuff in it that is well worth
consideration. We will build a parser
that can process Simplified
JavaScript.
This is probably a reference to Crockford's book Javascript: The Good Parts. In this book, he describes which features of Javascript he feels are "good", as well as those that are "bad" and shouldn't be used.
Simplified JavaScript is not quite a language (yet), but it could be. There is Pratt's/Crockford's Simplified Parser TDOP, and that is a first step, but you would need either a code generator or an interpretor to process the parser's output "tree" into working JavaScript. IMHO that will be a good thing. Then an IDE that would help anyone learn and write this proposed language "Simplified JavaScript", will follow.
Related
I read Kyle's book, and I found it really informative.
But I am a little confused on the discussion in "You Don't Know JS: this & Object Prototypes".
That series say that Object Linking to Other Object design pattern is cleaner and simpler then object oriented design pattern. I do agree with this.
But I notice that many react code snippet prefer using ES6 class keyword which represents object oriented code style.
First and foremost you don't have to teach programmers that you introduce to your project to OOP pattern as it is by far the most popular one and people are used to it. Teaching someone consumes time and money - two things that business value the most.
Second thing would be that it is hard to prove that one programming paradigm is better than the other - especially if you can count on one hand implementations of OLOO when OOP has hundreds of thousands.
Just because something is better (I also agree with Kyle) doesn't mean it will be the most popular choice.
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Edit: I recently learned about a project called CommonMark, which
correctly identifies and deals with the ambiguities in the original
Markdown specification. http://commonmark.org/ It has great C# library
support.
You can find the syntax here.
The source that follows with the download is written in Perl, which I have no intentions of honoring. It is riddled with regular expressions, and it relies on MD5 hashes to escape certain characters. Something is just wrong about that!
I'm about to hard code a parser for Markdown. What is experience with this?
If you don't have anything meaningful to say about the actual parsing of Markdown, spare me the time. (This might sound harsh, but yes, I'm looking for insight, not a solution, that is, a third-party library).
To help a bit with the answers, regular expressions are meant to identify patterns! NOT to parse an entire grammar. That people consider doing so is foobar.
If you think about Markdown, it's fundamentally based around the concept of paragraphs.
As such, a reasonable approach might be to split the input into paragraphs.
There are many kinds of paragraphs, for example, heading, text, list, blockquote, and code.
The challenge is thus to identify these paragraphs and in what context they occur.
I'll be back with a solution, once I find it's worthy to be shared.
The only markdown implementation I know of, that uses an actual parser, is Jon MacFarleane’s peg-markdown. Its parser is based on a Parsing Expression Grammar parser generator called peg.
EDIT: Mauricio Fernandez recently released his Simple Markup Markdown parser, which he wrote as part of his OcsiBlog Weblog Engine. Because the parser is written in OCaml, it is extremely simple and short (268 SLOC for the parser, 43 SLOC for the HTML emitter), yet blazingly fast (20% faster than discount (written in hand-optimized C) and sixhundred times faster than BlueCloth (Ruby)), despite the fact that it isn't even optimized for performance yet. Because it is only intended for internal use by Mauricio himself for his weblog, there are a few deviations from the official Markdown specification, but Mauricio has created a branch which reverts most of those changes.
I released a new parser-based Markdown Java implementation last week, called pegdown.
pegdown uses a PEG parser to first build an abstract syntax tree, which is subsequently written out to HTML. As such it is quite clean and much easier to read, maintain and extend than a regex based approach.
The PEG grammar is based on John MacFarlanes C implementation "peg-markdown".
Maybe something of interest to you...
If I was to try to parse markdown (and its extension Markdown extra) I think I would try to use a state machine and parse it one char at a time, linking together some internal structures representing bits of text as I go along then, once all is parsed, generating the output from the objects all stringed together.
Basically, I'd build a mini-DOM-like tree as I read the input file.
To generate an output, I would just traverse the tree and output HTML or anything else (PS, LaTex, RTF,...)
Things that can increase complexity:
The fact that you can mix HTML and markdown, although the rule could be easy to implement: just ignore anything that's between two balanced tags and output it verbatim.
URLs and notes can have their reference at the bottom of the text. Using data structures for hyperlinks could simply record something like:
[my text to a link][linkkey]
results in a structure like:
URLStructure:
| InnerText : "my text to a link"
| Key : "linkkey"
| URL : <null>
Headers can be defined with an underline, that could force us to use a simple data structure for a generic paragraph and modify its properties as we read the file:
ParagraphStructure:
| InnerText : the current paragraph text
| (beginning of line until end of line).
| HeadingLevel : <null> or 1-4 when we can assess
| that paragraph heading level, if any.
Anyway, just some thoughts.
I'm sure that there are many small details to take care of and I'm pretty sure that Regexes could become handy during the process.
After all, they were meant to process text.
I'd probably read the syntax specification enough times to know it, and get a feel for how to parse it.
Reading the existing parser code is of course brilliant, both to see what seems to be the main source of complexity, and if any special clever tricks are being used. The use of MD5 checksumming seems a bit weird, but I haven't studied the code enough to understand why it's being done. A comment in a routine called _EscapeSpecialChars() states:
We're replacing each such character with its corresponding MD5 checksum value;
this is likely overkill, but it should prevent us from colliding with the escape
values by accident.
Replacing a single character by a full MD5 does seem extravagant, but perhaps it really makes sense.
Of course, it'd be clever to consider creating a "true" syntax, for a tool such as Flex to get out of the regex bog.
If Perl isn't your thing, there are Markdown implementations in at least 10 other languages. They probably don't all have 100% compatibility, but tend to be pretty close.
MarkdownPapers is another Java implementation whose parser is defined in a JavaCC grammar.
If you are using a programming language that has more than three other
users, you should be able to find a library to parse it for you. A
quick Google-ing reveals libraries for CL, Haskell, Python,
JavaScript, Ruby, and so on. It is highly unlikely that you will need
to reinvent this wheel.
If you really have to write it from scratch, I recommend writing a
proper parser. With this technique, you won't have to escape things
with MD5 hashes. (I agree that if you have to do something like this,
it's time to reconsider your design.)
There are libraries available in a number of languages, including php, ruby, java, c#, javascript. I'd suggest looking at some of these for ideas.
It depends on which language you wish to use, for the best way to implement it, there will be idiomatic and non idiomatic ways to do it.
Regexes work in perl, because perl and regex are best friends.
Markdown is a JAWL (just another wiki language)
There are plenty of open source wiki's out there that you can examine the code of the parser. Most use REGEX
Check out the screwturn wiki, is has an interesting multi pass formatter pipeline, a very nice technique - see /core/Formatter.cs and /core/FormatterPipeline.cs
Best is to use/join an existing project, these sorts of things are always much harder than they appear
Here you can find a JavaScript-implementation of Markdown. It also relies heavily on regular expressions, as this is just the fastest and easiest way to parse the text.
But it spares the MD5 part.
I cannot help directly with the coding of the parsing, but maybe this link can help you one way or another.
I'm reading the book Structure and Interpretation
of Computer Programs, and I'd like to code a scheme interpreter gradually.
Do you knows the implementation of the scheme most easy to read (and short)?
I will make a JavaScript in C.
SICP itself has several sections detailing how to build a meta-circular interpreter, but I would suggest that you take a look at the following two books for better resources on Scheme interpreters: Programming Languages: Application and Interpretation and Essentials of Programming Languages. They're both easy to read and gradually guide you through building interpreters.
I would recommend the blog series Scheme from scratch which incrementally builds up a scheme interpreter in C.
Christian Queinnec's book Lisp In Small Pieces is superb. More modern that EoPL. Covers both Lisp and Scheme, and goes into detail about the gory low-level stuff that most books omit.
I would recommend reading Kent Dybvig's dissertation "Three Implementation Models for Scheme". Not the whole dissertation, but the first part (up to chapter 3) where he discusses the Heap-Based Model is very suitable for a naive implementation of Scheme.
Another great resource (if I understood it correctly and you want to implement it in C) is Nils Holm's "Scheme 9 from Empty Space". This link is to Nils's page, and there's a link at the bottom to the old, public domain, edition of the book and to the newer, easier to read, commercially available edition. Read both and loved 'em.
I can give you an overview on how my interpreter works, maybe it can give you an idea of the general thing.
Although the answer is pretty late, I hope this can help someone else, who has come to this thread and wants a general idea.
For each line of scheme entered , a Command object is created. If the command is partial then its nest level is stored(number of remaining right brackets to complete the expression). If the command is complete an Expression Object is created and the evaluators are fired on this object.
There are 4 types of evaluator classes defined , each derived from the base class Evaluator
a) Define_Evaluator :for define statements
b) Funcall_Evaluator :for processing other user defined functions
c) Read_Evaluator :for reading an expression and converting it to a scheme object
d) Print_Evaluator :prints the object depending on the type of the object.
e) Eval_Evaluator :does the actual processing of the expression.
3.-> First each expression is read using the Read Evaluator which will create a scheme object out of the expression. Nested expressions are calculated recursively until the expression is complete.
->Next, the Eval_Evaluator is fired which processes the Scheme Expression Object formed in the first step.
this happens as so
a) if the expression to be evaluated is a symbol. Return its value. Therefore the variable blk will return the object for that block.
b) if the expression to be evaluated is a list. Print the list.
c) if the expression to be evaluated is a function. Look for the definition of the function which will return the evaluation using the Funcall_Evaluator.
->Lastly the print evaluator is fired to print the outcome , this print will depend on what type the output expression is.
Disclaimer:
This is how my interpreter works , doesnt have to be that way.
I've been on a similar mission but several years later, recommendations:
Peter Michaux's scheme from scratch: http://michaux.ca/articles/scheme-from-scratch-introduction, and his github repo: https://github.com/petermichaux/bootstrap-scheme/blob/v0.21/scheme.c. Sadly his royal scheme effort seems to have stalled. There were promises of a VM, which with his clarity of explanations would have been great.
Peter Norvigs lis.py: http://norvig.com/lispy.html, although written in python, is very understandable and exploits all the advantages of using a dynamic, weakly typed language to create another. He has a follow up article that adds more advanced features.
Anthony C. Hay used lis.py as an inspiration to create an implementation in C++:
http://howtowriteaprogram.blogspot.co.uk/2010/11/lisp-interpreter-in-90-lines-of-c.html
A more complete implementation is chibi scheme: http://synthcode.com/scheme/chibi/ which does include a VM, but the code base is still not too large to comprehend.
I'm still searching for good blog posts on creating a lisp/scheme VM, which could be coupled with JIT (important for any competitive JS implementation :).
Apart from Queinnec's book, which probably is the most comprehensive one in scheme
to C conversion, you can read also literature from the old platform library.readscheme.org.
I've noticed many languages like Ruby and CofeeScript (well a transcompiler) support everything being an expression.
Now it makes the language somewhat simple to understand and definitely seems neat at the surface, but I was looking maybe for some scholarly publications about the positives and negatives of the two approaches.
It would be beneficial if the publications had clear examples that compared the benefits of having everything be an expression vs., well, not.
Examples in CoffeeScript vs Javascript would be nice, but not required.
The concept is definitely cool, but I'm still slightly unsure how revolutionary the whole idea really is (obviously something being revolutionary is somewhat an opinion).
Thanks!
There is nothing revolutionary about this per se. The expression-oriented approach is a functional programming technique.
Expression-oriented code is simpler and less cluttered than statement-oriented code, because of fewer assignments and no explicit return statements. The lack of distinction between expressions and commands enables conceptual uniformity (see Referential transparency) and bottom-up structure.
Some modern languages have adopted functional programming concepts (e.g. C#, Python, Ruby).
Some scholarly insight on the benefits of functional practices:
Can Programming Be Liberated from the von Neumann Style? A Functional Style and Its Algebra of Programs - John Backus
Interesting articles:
Why Functional Programming is Important in a Mixed Environment
Is C# becoming a functional language?
As to the comment about performance concerns, the possible overhead related to choice of paradigm is probably negligible. Even in C, most statements evaluate as an expression - however, a comparison between a compiled language (C) and an interpreted language (CoffeeScript) is rather useless.
On a theoretical note, an imperative language represents the control flow in more of a machine-oriented way, which may allow for easier hand-optimization than a functional language.
Language performance and its significance depend heavily on the use case. Concerning JavaScript and whatever code transformation on top of it, this performance discussion is completely irrelevant. The gains in productivity outweigh any slight performance hit.
By "everything is an expression," I assume you mean what is described at http://jashkenas.github.com/coffee-script/
It kinda sounds like what you're asking about are functional languages. Consider, for example, Lisp, which did this sort of thing back in the '50s. This ultimately comes out of the Lambda Calculus, in which code and data are really the same thing, and you can pass code around as though it were data (because it is).
I don't know of any scholarly articles discussing this specifically, but now you at least have some more keywords to search for.
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Closed 10 years ago.
So I have been exploring different methods to clean up and test my JavaScript. I figured just like any other language one way to get better is to read good code. jQuery is very popular so it must have a certain degree of good coding.
So why when I run jQuery through JSLint's validation it gives me this message:
Error:
Problem at line 18 character 5:
Expected an identifier and instead saw
'undefined' (a reserved word).
undefined,
Problem at line 24 character 27:
Missing semicolon.
jQuery = window.jQuery = window.$ =
function( selector, context ) {
Problem at line 24 character 28:
Expected an identifier and instead saw
'='.
jQuery = window.jQuery = window.$ =
function( selector, context ) {
Problem at line 24 character 28:
Stopping, unable to continue. (0%
scanned).
This was done using JSLint and jquery-1.3.1.js
JSLint tests one particular person's (Douglas Crockford) opinions regarding what makes good JavaScript code. Crockford is very good, but some of his opinions are anal retentive at best, like the underscore rule, or the use of the increment/decrement operators.
Many of the issues being tagged by JSLint in the above output are issues that Crockford feels leads to difficult to maintain code, or they are things that he feels has led him to doing 'clever' things in the past that can be hard to maintain.
There are some things Crockford identifies as errors that I agree with though, like the missing semicolons thing. Dropping semicolons forces the browser to guess where to insert the end-of-statement token, and that can sometimes be dangerous (it's always slower). And several of those errors are related to JSLint not expecting or supporting multiple assignments like jQuery does on line 24.
If you've got a question about a JSLint error, e-mail Crockford, he's really good about replying, and with his reply, you'll at least know why JSLint was implemented that way.
Oh, and just because a library is popular doesn't mean it's code is any good. JQuery is popular because it's a relatively fast, easy to use library. That it's well implemented is rather inconsequential to it's popularity among many. However, you should certainly be reading more code, we all should.
JSLint can be very helpful in identifying problems with the code, even if JQuery doesn't pass the standards it desires.
JSLint helps you catch problems, it isn't a test of validity or a replacement for thinking. jQuery is pretty advanced as js goes, which makes such a result understandable. I mean the first couple of lines are speed hacks, no wonder the most rigid js parser is going have a couple of errors.
In any case, the assumption that popular code is perfectly correct code or even 'good' is flawed. JQuery code is good, and you can learn a lot of from reading it. You should still run your stuff through JSLint, if only because it's good to hear another opinion on what you've written.
From JSLint's description:
JSLint takes a JavaScript source and scans it. If it finds a problem, it returns a message describing the problem and an approximate location within the source. The problem is not necessarily a syntax error, although it often is. JSLint looks at some style conventions as well as structural problems. It does not prove that your program is correct. It just provides another set of eyes to help spot problems.
JSLint defines a professional subset of JavaScript, a stricter language than that defined by Edition 3 of the ECMAScript Language Specification. The subset is related to recommendations found in Code Conventions for the JavaScript Programming Language.
"jQuery is very popular so it must have a certain degree of good coding."
One would like to hope this is the case with jQuery, but unfortunately it's not really true. jQuery is useful and popular, but it is not a well written JavaScript library. David Mark recently posted a scathing critique of jQuery in comp.lang.javascript that examines a large number of examples of the poor code found in jQuery:
http://groups.google.com/group/comp.lang.javascript/msg/37cb11852d7ca75c?hl=en&
If you're not actively developing jQuery itself, why even run JSLint over it at all? If it works, it works, and you don't have to worry about it.
The jQuery developers' goals are not the same as your goals. jQuery is built for speed and compactness and achieving those goals trumps readability and maintainability.
Crockford's tests in JSLint have more to do with achieving something that he would feel comfortable taking home to meet his mother, which is a valid concern if you will be married to your code for some time.
The purpose of JsLint is clearly stated in the FAQ [1]:
"JSLint defines a professional subset of JavaScript, a stricter language than that defined by Edition 3 of the ECMAScript Language Specification. The subset is related to recommendations found in Code Conventions for the JavaScript Programming Language."
Now if you are confused: ECMA3 is already a subset of the JS capabilities provided by any of todays JS interpreters (see [2] for an overview of the relation between JavasScript and ECMAScript versions)
To answer the quesition "what good is JSlint":
* use JsLint to verify you are using a "safe" subset of Javascript that is unlikly to break accross JS implementations.
* use Jslint to verify you followed the crockford code conventions [4]
[1] http://www.jslint.com/lint.html
[2] https://developer.mozilla.org/en/Core_JavaScript_1.5_Guide/JavaScript_Overview#Relationship_between_JavaScript_Versions_and_ECMAScript_Editions
[3] https://developer.mozilla.org/en/New_in_JavaScript_1.7#Avoiding_temporary_variables
[4] http://javascript.crockford.com/code.html
I've found one case where JSLint is very, very useful: when you grab one of those big-arse libraries that float around the 'Net, then another, then again one other, you soon find yourself loading 50k of Javascript on every new page load (caching may help, but it's not a cure-all solution).
What would you do then? Compress those libraries. But your host doesn't do compression for non-html file! So what? You use a Javascript compressor.
The best I've found is Dean Edward's; I used it to compress John Fraser's Showdown (a Markdown for Javascript library), but unfortunately, the compression broke the code. Since Showdown isn't supported anymore, I had to correct it myself - and JSlint was invaluable for that.
In short, JSlint is useful to prepare your JS code for heavy duty compression.
If you like to daisy-chain methods like jQuery allows you, you might appreciate YUI3.
JQuery is of course not the best thing in the world. That's already clear when you look at the notation. The dollar parentheses combination is really bad for your eyes. Programming should be clear and simple. JQuery is far from that. That reason is enough for me not to use it. That it's not properly written doesn't surprise me and only underscores my thoughts on this JavaScript library.