Somebody said that when your PHP code and application use global variables then it must be spaghetti code (I assume this). I use WordPress a lot. As far as I know, it's the best thing near great PHP software. And it uses many global variables to interact between its components.
But forget about that, because frankly, that's the only thing I know. So it's completely biased ;D
So, I am just curious, What is the characteristic of spaghetti code?
PS: the only thing I know is WordPress. So, hopefully, maybe this will help somebody give a great answer for somebody who has little experience in developing a full web application on PHP (for example, the Stack Overflow website).
No modularity (everything in one file, class, module, namespace, package, or whatever your language uses to provide modularity),
Plenty of goto's,
Poor organization,
No clear separation of functionality and purpose. (That is, all-encompassing classes or functions)
Long functions.
Poor naming.
No consistent coding style throughout.
No clear interface contract between implementation and clients of code. (That is, no specification of what the inputs, outputs, pre- and post-conditions of functions are)
Over-reliance on internals of data structures with little abstraction.
Functions randomly permute/modify global state without any mention of it in documentation.
Lack of comments or documentation of non-trivial code.
Code that is more complicated than it needs to be.
Lack of reuse. (plenty of duplicated code, a.k.a. copypasta)
No verification or unit testing (it works on faith).
Magic numbers.
In essence, a lack of design and
forethought, and just a mishmash of
hacks slapped together. This applies to any language, not just PHP.
for somebody who has little experience in developing a full web application on PHP (for example, the Stack Overflow website)
Just FYI, but Stack Overflow was not developed with PHP.
Well, talking of comment you posted, the explanation is very simple.
Using global operator makes source of a variable is unknown, like other end of spaghetti noodle. It can be defined everywhere. So, when you call your function, you have no idea what value this variable has. Instead of it, direct passing a variable makes it plain and clear:
function hello_testing($conditional_random) {
if ($conditional_random)) {
echo "foo is inside";
}
}
P.S. http://en.wikipedia.org/wiki/Spaghetti_code
Spaghetti code has specific characteristics which distinguish it from plain poor code. Spaghetti is extremely complicated and unstructured, so it is hard to follow the flow of a process through the program. It is like trying to untangle the noodles in a bowl of bolognese.
This is why GOTO statements (dread word!) are often cited in this context: a GOTO statement transfers control to another arbitrarily defined location in the code base. Most programming languages have commands which can be abused to simulate goto style behaviour; for instance, using exceptions to implement regular business logic rather than handling errors.
Global variables contribute to spaghetti code because the values are assigned outside of the scope of the current program unit. This can make it difficult to determine where in the code base a variable is set to a given value (or indeed whether it is set to any value at all).
Spaghetti code can be functionally correct and performative. It's a problem because it's hard to understand, so we can't be sure it is bug free and the lack of structure makes it difficult to troubleshoot. For similar reasons spaghetti code is brittle and difficult to change; the risk of introducing a bug is high.
Incidentally, the use of goto statements does not mean a program is spaghetti. It is perfectly possible to write clear, well-structured code using goto, it is just requires a lot of self-discipline not to abuse its flexibility. Modern programming languages have made its use unnecessary, and undesirable.
WordPress is the biggest piece of spaghetti code PHP I have seen around. There is a shocking mix of PHP, HTML, JavaScript and all things in between all lumped in the same files. If you want another example of spaghetti code look at osCommerce or Zen Cart.
In fact I dare say a large majority of open source PHP applications are pretty shocking examples of how to program in PHP. If you want to look at a good structured example (that is, non-spaghetti) then look at Yii framework or Zend Framework. Frameworks like CodeIgniter and Kohana, although not spaghetti, are not very good examples of how to structure things in PHP 5 as they use many of the features used in PHP 4 simply because there was no better way of doing them until PHP 5 (for example, using path based inheritance instead of true object inheritance).
If you want a reasonbly good example of procedural programming done right look at Drupal. It might not be the best performing PHP application, because of the complexity, but it sure beats WordPress and you can do many of the same things with it.
Related
I know there are countless questions about the difference between OOP and procedural, when to use either and whether the benefits outweigh the extra overhead, learning the syntax, inheritance confusion, etc. Most of what I've found tends to just discuss differences and benefits and not whether its necessary.
I generally mix OOP and procedural within the same sites scripts depending on what I'm doing. I'm still fairly new to OOP and actually quite like the modular nature of OOP and the benefits it gives, even if there's a minor overhead. Inheritance can get a little confusing at times though!
To me the major benefits only seem to be in better organisation and protection of the code. Of which, the developer or team of developers are the only people to appreciate it. I guess there's a case for deployment speed but wouldn't say there's a lot in it for most sites unless you've inherited someone else's birdsnest :)
Is OOP necessary in most PHP apps though, especially when execution speed is the holy grail for most sites? ok, so the milliseconds overhead won't really notice unless a heavy use site but as a fan of electronic music speed is king!
I get using OOP in complex things like gaming and real-time cloud software, but static websites? even database heavy ones?
Does anyone have real world examples of typical sites that benefit from OOP and why?
Assuming both cases are well structured, would heavy use sites like ebay or monster.co.uk benefit more from OOP or the speed improvement of procedural ()? and why?
At least with procedural you can debug from the top down without having to bounce around the script to check classes and extensions and interfaces.
Can't I just apply OOP modular thinking with clear MVC and well commented code?
For example, I keep re-usable functions in include files and group related functions together. All I have to do is include the file like I would a class file and call up the functions. If the function needs to change, it gets changed in just one place, similar to a class.
And a kind of inheritance already exists in procedural without having to jump through hoops to declare it. You don't have the same level of control but it gets the job done nice and quick.
You could even simulate a class by grouping functions within a parent function and use a selector function to access them. That's taking it a bit far though!
Also, as far as I'm aware when a function is called it stays in memory making subsequent uses quicker. Whereas with OOP you would have to create two objects of the various methods to use the same function for two different variables. Correct me if I'm wrong.
Why create an object and use a method to 'get' a value when I could just reference the value directly with procedural?
well done for getting this far, hadn't realised I'd typed so much. Anyway, before I digress any further I'm going to end it here.
So if you've got any good examples of actual sites or parts of sites that benefit from either OOP or procedural I would really appreciate the clarity.
People managed to write good, clear, well organized code long before OO languages became popular. I see no reason why it can't still be done now.
Generally OO principles make it easier (which is one reason why OO is so popular) but they are by no means a necessity.
There are lots of questions here. I recall writing a long essay addressing some of these points while at university, but I don't want to reproduce something similar here, so instead, let me share a few thoughts:
Is OOP necessary in most PHP apps though, especially when execution speed is the holy grail for most sites? ok, so the miliseconds overhead won't really notice unless a heavy use site but as a fan of electronic music speed is king!
I think that if execution speed is a really big deal for you, php is not the right language for your website. Compared to the large performance cost of using an interpreted language, a little overhead is negligiable compared to the advantages of the OOP programming style for creating large systems. Do not discount the importance of making something easy for programmers to do, as this means faster releases, less bugs, and more maintainable code.
I get using OOP in complex things like gaming and real-time cloud software, but static websites? even database heavy ones?
I agree with you here. A nice thing about websites is that they are naturally modular because they are separated into pages. It hard to write code so bad that future programmers can't maintain it (for a fairly simple, static website).
For example, I keep re-usable functions in include files and group related functions together. All I have to do is include the file like I would a class file and call up the functions. If the function needs to change, it gets changed in just one place, similar to a class.
You can write good procedural code, but its harder than writing good OOP code. Weaker programmers are less likely to write insane spagetti code when given an OOP system to work with.
Why create an object and use a method to 'get' a value when I could just reference the value directly with procedural?
This is your only real implemenation question. The idea with Getters/Setters is so that you can change the internal workings of the class without breaking other code that depends on it.
I get using OOP in complex things like gaming and real-time cloud software, but static websites? even database heavy ones?
Implying that you don't want speed in games but want speed in websites.
PHP is never the bottleneck, if it is, write it in C.
Don't write procedural code because it is "faster". That's silly.
Does anyone have real world examples of typical sites that benefit from OOP and why?
Websites benefit from modular code that is maintainable and well organized.
You don't need OO for this, you can do it with functional or imperative styles. However PHP is known for it's ease to write bad code in a procedural style.
I would personally say that it's more likely your code is modular and maintainable if it was OO.
It's not necessary though.
And a kind of inheritance already exists in procedural without having to jump through hoops to declare it. You don't have the same level of control but it gets the job done nice and quick.
In OO programming it's all about encapsulation which means binding a lump of data to some functions that manipulate it.
You can do this just as well with a set of functions which take a data object as the first argument or classes.
Classes and OO just gives you sugar and utility.
It's a tool to write modular code, if it's helps you use it.
Don't pre maturely optimize OO away because it's "slow". If you care about that kind of micro optimization then start writing C or ASM.
I think a lot of people who promote OO are younger and have only ever written/been taught OO and so have a very negative view of procedural as old fashioned and 'legacy'.
IMO it is easy to write modular procedural PHP that is DRY and can be easily maintained. The trick is to use functions and 'include' to reuse standard php and html respectively. At the end of the day PHP is just reading DBs and generating html - there is no specific need to add the extra complexity of OO if you don't want to.
I'm about to start coding a new website. My problem is that I'm still stuck in using old school coding methods.
I recently downloaded some open source code from Question2Answer.org and was really intrigued in how it was set out.
Does anyone know of any sources? or something that I could possibly download, a template or example to help me get started with a new site?
The site won't be anything fancy but I want to start moving into Web 2.0 and OO programming.
In short I want to do it right. Any advice would be appreciated.
It seems like this question isn't getting many answers, so I'll try my hand at it (even though I'd recommend a different language, just because PHP is such a terrible language ). PHP was the first language I ever wrote anything big in, and the one thing that I wish I had known at the time was the MVC design pattern. It has some advantages like:
Separation of logic and UI means less ad-hoc code, more functions (try to follow the "each function does one thing" rule -- It makes things much easier when you go back and look at code)
Functions are easier to verify correctness than huge function-less pages
Functions can be unit tested (do this!)
It's easier to figure out where things are (database logic is in one file, HTML in another, and "controller" logic in another)
Here's a pretty good high-level intro to MVC.
Key points:
The model talks to the database (or whatever other storage you're using).
The view displays things (HTML)
The controller does everything else
I found two good-looking PHP MVC tutorials here and here. Hopefully they're not too complicated, and feel free to ask more questions if anything doesn't make sense.
Good luck!
PS - Don't forget the point about unit tests! If you can't find a way to unit test a function, it's probably too complicated.
There is a reason why people chose PHP as a server-side scripting language. It's extremely easy to pick up and offers many different coding options. Many functions are included without having to know prior importing, and you do NOT have to write OO code if you don't want to.
This all leads to a huge downfall as well, because there are less restrictions on the structure of the code, it's much easier to write bad code.
I suggest using a framework. It saves you time, energy, and the opportunity to write bad code:
CakePHP is a rapid development framework for PHP that provides an extensible architecture for developing, maintaining, and deploying applications. Using commonly known design patterns like MVC and ORM within the convention over configuration paradigm, CakePHP reduces development costs and helps developers write less code.
http://cakephp.org/
http://www.phpframeworks.com/
IMO MVC ( and this OOP ) is nothing really fancy. It's basically a function to register a pointer to a function in an array for example like a hook. This array is looked by another function to do some stuff. A good MVC should have a callback function. IMO this OOP thing is more a bussines logic to help you to monetize your application. It's not really something difficult to understand.
I have been writing a lot of code for work in PHP/MySQL. So far it has all been procedural making use of functions for functionality occuring multiple times/places. Starting to find some of the site hard to manage - time to go OO.
I want to learn about MVC with object oriented PHP & MySQL. I have some experience in Java and MVC but never anything to do with web technologies, i.e. HTML/CSS/JS etc. I don't really understand how the dynamically generated HTML fits in with the classes etc.
I am after some recommndations about where I can start. Ideally some sites with great examples from the ground up. I don't really want to use a framework at this point because I find that it does too much for you. Once I understand the OO approach with MVC I'll probably use a framework to managei easier.
Cheers,
Evan
You have a lot of code, and despite being hard to change, it probably works. Making an overhaul is going to set you back a while, so the best approach is incremental. Find some ONE thing that would seriously benefit from using objects, and use objects there. Refactor as you are able to run tests. You can work this in with the ordinary flow of events, and things work out OK.
Frankly, if you're serious about removing redundancy and duplication, you'll often find places where just writing a small function can make a dent. If you do this often enough, you'll find groups of functions that work on the same data. That will suggest where to look for objects.
In other words, if you listen with the right kind of ears, the code will tell you.
If you just want a good online course, maybe you should have a look at
http://www.lynda.com/home/DisplayCourse.aspx?lpk2=653&srchtrk=index%3A1%0Alinktypeid%3A2%0Aq%3APHP%20Object%20Oriented%0Apage%3A1%0As%3Arelevance%0Asa%3Atrue%0Aproducttypeid%3A2%0Acategory_facet%3APHP
This course was really useful for a friend of me. The progress he booked when he finished this tutorial was really great. It takes you through the basics of building a CMS in PHP and object oriented.
I think you should start with small pieces to get into OOP step by step. I mean that you should write simple classes for things you use often like handling forms, image uploads, site messages, session handling.
When you get used to programming classes and working on objects it will be time to jump for something bigger like refactor whole "engine" to OOP using mvc and other stuff.
Not really an answer but too big to be a comment.
Actually HTML has nothing to so with classes and JS and CSS has nothing to do with PHP, MVC and OOP at all.
Your code should produce some data which is going to be displayed one or another way. You can use some class to render this data, but it's not that important class.
While JS and CSS are totally separate files usually, called by browser directly from the server, avoiding your application code (unless being generated dynamically). Anyway you should have not too much concern in it.
Although it's still hard and non-trivial task to tie classes hierarchy with such a discrete way of execution of a typical PHP application. Once wise man have said once, "If Windows were have to restart after each user's click, it were written completely different".
welcome to SO.
I am by no means a PHP expert, here is my thought comes on top of my head:
OO programming with PHP does help, but not very likely as the rescue to removing all the redundancies or making your code "neater". The traditional way of coding in PHP can produce nice code. OO helps to encapsulate your code to hide the implementation and reuse others' code by direct composition and/or via inheritance.
I would say the real concern here is how to decouple the "static" html from the "dynamic" PHP.
For instance, there shouldn't be many :
echo "<h1>A title</h1";
echo "<p>blah blah blah...";
Even in conditional printing.
I would say separate the whole business logic on one page/script into several functions, and at least hide the nitty-gritty inside a function.
For your questions on popular OO frameworks of PHP, CakePHP is a good one, got its inspiration from Ruby on Rails.
Read MVC Tutorial in PHP to understand what MVC is then start using a MVC Framework like Zend, CakePHP, Symphony or CodeIgniter...
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I am setting out to do a side project that has the goal of translating code from one programming language to another. The languages I am starting with are PHP and Python (Python to PHP should be easier to start with), but ideally I would be able to add other languages with (relative) ease. The plan is:
This is geared towards web development. The original and target code will be be sitting on top of frameworks (which I will also have to write). These frameworks will embrace an MVC design pattern and follow strict coding conventions. This should make translation somewhat easier.
I am also looking at IOC and dependency injection, as they might make the translation process easier and less error prone.
I'll make use of Python's parser module, which lets me fiddle with the Abstract Syntax Tree. Apparently the closest I can get with PHP is token_get_all(), which is a start.
From then on I can build the AST, symbol tables and control flow.
Then I believe I can start outputting code. I don't need a perfect translation. I'll still have to review the generated code and fix problems. Ideally the translator should flag problematic translations.
Before you ask "What the hell is the point of this?" The answer is... It'll be an interesting learning experience. If you have any insights on how to make this less daunting, please let me know.
EDIT:
I am more interested in knowing what kinds of patterns I could enforce on the code to make it easier to translate (ie: IoC, SOA ?) the code than how to do the translation.
I've been building tools (DMS Software Reengineering Toolkit) to do general purpose program manipulation (with language translation being a special case) since 1995, supported by a strong team of computer scientists. DMS provides generic parsing, AST building, symbol tables, control and data flow analysis, application of translation rules, regeneration of source text with comments, etc., all parameterized by explicit definitions of computer languages.
The amount of machinery you need to do this well is vast (especially if you want to be able to do this for multiple languages in a general way), and then you need reliable parsers for languages with unreliable definitions (PHP is perfect example of this).
There's nothing wrong with you thinking about building a language-to-language translator or attempting it, but I think you'll find this a much bigger task for real languages than you expect. We have some 100 man-years invested in just DMS, and another 6-12 months in each "reliable" language definition (including the one we painfully built for PHP), much more for nasty languages such as C++. It will be a "hell of a learning experience"; it has been for us. (You might find the technical Papers section at the above website interesting to jump start that learning).
People often attempt to build some kind of generalized machinery by starting with some piece of technology with which they are familiar, that does a part of the job. (Python ASTs are great example). The good news, is that part of the job is done. The bad news is that machinery has a zillion assumptions built into it, most of which you won't discover until you try to wrestle it into doing something else. At that point you find out the machinery is wired to do what it originally does, and will really, really resist your attempt to make it do something else. (I suspect trying to get the Python AST to model PHP is going to be a lot of fun).
The reason I started to build DMS originally was to build foundations that had very few such assumptions built in. It has some that give us headaches. So far, no black holes. (The hardest part of my job over the last 15 years is to try to prevent such assumptions from creeping in).
Lots of folks also make the mistake of assuming that if they can parse (and perhaps get an AST), they are well on the way to doing something complicated. One of the hard lessons is that you need symbol tables and flow analysis to do good program analysis or transformation. ASTs are necessary but not sufficient. This is the reason that Aho&Ullman's compiler book doesn't stop at chapter 2. (The OP has this right in that he is planning to build additional machinery beyond the AST). For more on this topic, see Life After Parsing.
The remark about "I don't need a perfect translation" is troublesome. What weak translators do is convert the "easy" 80% of the code, leaving the hard 20% to do by hand. If the application you intend to convert are pretty small, and you only intend to convert it once well, then that 20% is OK. If you want to convert many applications (or even the same one with minor changes over time), this is not nice. If you attempt to convert 100K SLOC then 20% is 20,000 original lines of code that are hard to translate, understand and modify in the context of another 80,000 lines of translated program you already don't understand. That takes a huge amount of effort. At the million line level, this is simply impossible in practice. (Amazingly there are people that distrust automated tools and insist on translating million line systems by hand; that's even harder and they normally find out painfully with long time delays, high costs and often outright failure.)
What you have to shoot for to translate large-scale systems is high nineties percentage conversion rates, or it is likely that you can't complete the manual part of the translation activity.
Another key consideration is size of code to be translated. It takes a lot of energy to build a working, robust translator, even with good tools. While it seems sexy and cool to build a translator instead of simply doing a manual conversion, for small code bases (e.g., up to about 100K SLOC in our experience) the economics simply don't justify it. Nobody likes this answer, but if you really have to translate just 10K SLOC of code, you are probably better off just biting the bullet and doing it. And yes, that's painful.
I consider our tools to be extremely good (but then, I'm pretty biased). And it is still very hard to build a good translator; it takes us about 1.5-2 man-years and we know how to use our tools. The difference is that with this much machinery, we succeed considerably more often than we fail.
My answer will address the specific task of parsing Python in order to translate it to another language, and not the higher-level aspects which Ira addressed well in his answer.
In short: do not use the parser module, there's an easier way.
The ast module, available since Python 2.6 is much more suitable for your needs, since it gives you a ready-made AST to work with. I've written an article on this last year, but in short, use the parse method of ast to parse Python source code into an AST. The parser module will give you a parse tree, not an AST. Be wary of the difference.
Now, since Python's ASTs are quite detailed, given an AST the front-end job isn't terribly hard. I suppose you can have a simple prototype for some parts of the functionality ready quite quickly. However, getting to a complete solution will take more time, mainly because the semantics of the languages are different. A simple subset of the language (functions, basic types and so on) can be readily translated, but once you get into the more complex layers, you'll need heavy machinery to emulate one language's core in another. For example consider Python's generators and list comprehensions which don't exist in PHP (to my best knowledge, which is admittedly poor when PHP is involved).
To give you one final tip, consider the 2to3 tool created by the Python devs to translate Python 2 code to Python 3 code. Front-end-wise, it has most of the elements you need to translate Python to something. However, since the cores of Python 2 and 3 are similar, no emulation machinery is required there.
Writing a translator isn't impossible, especially considering that Joel's Intern did it over a summer.
If you want to do one language, it's easy. If you want to do more, it's a little more difficult, but not too much. The hardest part is that, while any turing complete language can do what another turing complete language does, built-in data types can change what a language does phenomenally.
For instance:
word = 'This is not a word'
print word[::-2]
takes a lot of C++ code to duplicate (ok, well you can do it fairly short with some looping constructs, but still).
That's a bit of an aside, I guess.
Have you ever written a tokenizer/parser based on a language grammar? You'll probably want to learn how to do that if you haven't, because that's the main part of this project. What I would do is come up with a basic Turing complete syntax - something fairly similar to Python bytecode. Then you create a lexer/parser that takes a language grammar (perhaps using BNF), and based on the grammar, compiles the language into your intermediate language. Then what you'll want to do is do the reverse - create a parser from your language into target languages based on the grammar.
The most obvious problem I see is that at first you'll probably create horribly inefficient code, especially in more powerful* languages like Python.
But if you do it this way then you'll probably be able to figure out ways to optimize the output as you go along. To summarize:
read provided grammar
compile program into intermediate (but also Turing complete) syntax
compile intermediate program into final language (based on provided grammar)
...?
Profit!(?)
*by powerful I mean that this takes 4 lines:
myinput = raw_input("Enter something: ")
print myinput.replace('a', 'A')
print sum(ord(c) for c in myinput)
print myinput[::-1]
Show me another language that can do something like that in 4 lines, and I'll show you a language that's as powerful as Python.
There are a couple answers telling you not to bother. Well, how helpful is that? You want to learn? You can learn. This is compilation. It just so happens that your target language isn't machine code, but another high-level language. This is done all the time.
There's a relatively easy way to get started. First, go get http://sourceforge.net/projects/lime-php/ (if you want to work in PHP) or some such and go through the example code. Next, you can write a lexical analyzer using a sequence of regular expressions and feed tokens to the parser you generate. Your semantic actions can either output code directly in another language or build up some data structure (think objects, man) that you can massage and traverse to generate output code.
You're lucky with PHP and Python because in many respects they are the same language as each other, but with different syntax. The hard part is getting over the semantic differences between the grammar forms and data structures. For example, Python has lists and dictionaries, while PHP only has assoc arrays.
The "learner" approach is to build something that works OK for a restricted subset of the language (such as only print statements, simple math, and variable assignment), and then progressively remove limitations. That's basically what the "big" guys in the field all did.
Oh, and since you don't have static types in Python, it might be best to write and rely on PHP functions like "python_add" which adds numbers, strings, or objects according to the way Python does it.
Obviously, this can get much bigger if you let it.
I will second #EliBendersky point of view regarding using ast.parse instead of parser (which I did not know about before). I also warmly recommend you to review his blog. I used ast.parse to do Python->JavaScript translator (#https://bitbucket.org/amirouche/pythonium). I've come up with Pythonium design by somewhat reviewing other implementations and trying them on my own. I forked Pythonium from https://github.com/PythonJS/PythonJS which I also started, It's actually a complete rewrite . The overall design is inspired from PyPy and http://www.hpl.hp.com/techreports/Compaq-DEC/WRL-89-1.pdf paper.
Everything I tried, from beginning to the best solution, even if it looks like Pythonium marketing it really isn't (don't hesitate to tell me if something doesn't seem correct to the netiquette):
Implement Python semantic in Plain Old JavaScript using prototype inheritance: AFAIK it's impossible to implement Python multiple inheritance using JS prototype object system. I did try to do it using other tricks later (cf. getattribute). As far as I know there is no implementation of Python multiple inheritance in JavaScript, the best that exists is Single inhertance + mixins and I'm not sure they handle diamond inheritance. Kind of similar to Skulpt but without google clojure.
I tried with Google clojure, just like Skulpt (compiler) instead of actually reading Skulpt code #fail. Anyway because of JS prototype based object system still impossible. Creating binding was very very difficult, you need to write JavaScript and a lot of boilerplate code (cf. https://github.com/skulpt/skulpt/issues/50 where I am the ghost). At that time there was no clear way to integrate the binding in the build system. I think that Skulpt is a library and you just have to include your .py files in the html to be executed, no compilation phase required to be done by the developer.
Tried pyjaco (compiler) but creating bindings (calling Javascript code from Python code) was very difficult, there was too much boilerplate code to create every time. Now I think pyjaco is the one that more near Pythonium. pyjaco is written in Python (ast.parse too) but a lot is written in JavaScript and it use prototype inheritance.
I never actually succeed at running Pyjamas #fail and never tried to read the code #fail again. But in my mind PyJamas was doing API->API tranlation (or framework to framework) and not Python to JavaScript translation. The JavaScript framework consume data that is already in the page or data from the server. Python code is only "plumbing". After that I discovered that pyjamas was actually a real python->js translator.
Still I think it's possible to do API->API (or framework->framework) translation and that's basicly what I do in Pythonium but at lower level. Probably Pyjamas use the same algorithm as Pythonium...
Then I discovered brython fully written in Javascript like Skulpt, no need for compilation and lot of fluff... but written in JavaScript.
Since the initial line written in the course of this project, I knew about PyPy, even the JavaScript backend for PyPy. Yep, you can, if you find it, directly generate a Python interpreter in JavaScript from PyPy. People say, it was a disaster. I read no where why. But I think the reason is that the intermediate language they use to implement the interpreter, RPython, is a subset of Python tailored to be translated to C (and maybe asm). Ira Baxter says you always make assumptions when you build something and probably you fine tune it to be the best at what it's meant to do in the case of PyPy: Python->C translation. Those assumptions might not be relevant in another context worse they can infere overhead otherwise said direct translation will most likely always be better.
Having the interpreter written in Python sounded like a (very) good idea. But I was more interested in a compiler for performance reasons also it's actually more easy to compile Python to JavaScript than interpret it.
I started PythonJS with the idea of putting together a subset of Python that I could easily translate to JavaScript. At first I didn't even bother to implement OO system because of past experience. The subset of Python that I achieved to translate to JavaScript are:
function with full parameters semantic both in definition and calling. This is the part I am most proud of.
while/if/elif/else
Python types were converted to JavaScript types (there is no python types of any kind)
for could iterate over Javascript arrays only (for a in array)
Transparent access to JavaScript: if you write Array in the Python code it will be translated to Array in javascript. This is the biggest achievement in terms of usability over its competitors.
You can pass function defined in Python source to javascript functions. Default arguments will be taken into account.
It add has special function called new which is translated to JavaScript new e.g: new(Python)(1, 2, spam, "egg") is translated to "new Python(1, 2, spam, "egg").
"var" are automatically handled by the translator. (very nice finding from Brett (PythonJS contributor).
global keyword
closures
lambdas
list comprehensions
imports are supported via requirejs
single class inheritance + mixin via classyjs
This seems like a lot but actually very narrow compared to full blown semantic of Python. It's really JavaScript with a Python syntax.
The generated JS is perfect ie. there is no overhead, it can not be improved in terms of performance by further editing it. If you can improve the generated code, you can do it from the Python source file too. Also, the compiler did not rely on any JS tricks that you can find in .js written by http://superherojs.com/, so it's very readable.
The direct descendant of this part of PythonJS is the Pythonium Veloce mode. The full implementation can be found # https://bitbucket.org/amirouche/pythonium/src/33898da731ee2d768ced392f1c369afd746c25d7/pythonium/veloce/veloce.py?at=master 793 SLOC + around 100 SLOC of shared code with the other translator.
An adapted version of pystones.py can be translated in Veloce mode cf. https://bitbucket.org/amirouche/pythonium/src/33898da731ee2d768ced392f1c369afd746c25d7/pystone/?at=master
After having setup basic Python->JavaScript translation I choosed another path to translate full Python to JavaScript. The way of glib doing object oriented class based code except the target language is JS so you have access to arrays, map-like objects and many other tricks and all that part was written in Python. IIRC there is no javascript code written by in Pythonium translator. Getting single inheritance is not difficult here are the difficult parts making Pythonium fully compliant with Python:
spam.egg in Python is always translated to getattribute(spam, "egg") I did not profile this in particular but I think that where it loose a lot of time and I'm not sure I can improve upon it with asm.js or anything else.
method resolution order: even with the algorithm written in Python, translating it to Python Veloce compatible code was a big endeavour.
getattributre: the actual getattribute resolution algorithm is kind of tricky and it still doesn't support data descriptors
metaclass class based: I know where to plug the code, but still...
last bu not least: some_callable(...) is always transalted to "call(some_callable)". AFAIK the translator doesn't use inference at all, so every time you do a call you need to check which kind of object it is to call it they way it's meant to be called.
This part is factored in https://bitbucket.org/amirouche/pythonium/src/33898da731ee2d768ced392f1c369afd746c25d7/pythonium/compliant/runtime.py?at=master It's written in Python compatible with Python Veloce.
The actual compliant translator https://bitbucket.org/amirouche/pythonium/src/33898da731ee2d768ced392f1c369afd746c25d7/pythonium/compliant/compliant.py?at=master doesn't generate JavaScript code directly and most importantly doesn't do ast->ast transformation. I tried the ast->ast thing and ast even if nicer than cst is not nice to work with even with ast.NodeTransformer and more importantly I don't need to do ast->ast.
Doing python ast to python ast in my case at least would maybe be a performance improvement since I sometime inspect the content of a block before generating the code associated with it, for instance:
var/global: to be able to var something I must know what I need to and not to var. Instead of generating a block tracking which variable are created in a given block and inserting it on top of the generated function block I just look for revelant variable assignation when I enter the block before actually visiting the child node to generate the associated code.
yield, generators have, as of yet, a special syntax in JS, so I need to know which Python function is a generator when I want to write the "var my_generator = function"
So I don't really visit each node once for each phase of the translation.
The overall process can be described as:
Python source code -> Python ast -> Python source code compatible with Veloce mode -> Python ast -> JavaScript source code
Python builtins are written in Python code (!), IIRC there is a few restrictions related to bootstraping types, but you have access to everything that can translate Pythonium in compliant mode. Have a look at https://bitbucket.org/amirouche/pythonium/src/33898da731ee2d768ced392f1c369afd746c25d7/pythonium/compliant/builtins/?at=master
Reading JS code generated from pythonium compliant can be understood but source maps will greatly help.
The valuable advice I can give you in the light of this experience are kind old farts:
extensively review the subject both in literature and existing projects closed source or free. When I reviewed the different existing projects I should have given it way more time and motivation.
ask questions! If I knew beforehand that PyPy backend was useless because of the overhead due to C/Javascript semantic mismatch. I would maybe had Pythonium idea way before 6 month ago maybe 3 years ago.
know what you want to do, have a target. For this project I had different objectives: pratice a bit a javascript, learn more of Python and be able to write Python code that would run in the browser (more and that below).
failure is experience
a small step is a step
start small
dream big
do demos
iterate
With Python Veloce mode only, I'm very happy! But along the way I discovered that what I was really looking for was liberating me and others from Javascript but more importantly being able to create in a comfortable way. This lead me to Scheme, DSL, Models and eventually domain specific models (cf. http://dsmforum.org/).
About what Ira Baxter response:
The estimations are not helpful at all. I took me more or less 6 month of free time for both PythonJS and Pythonium. So I can expect more from full time 6 month. I think we all know what 100 man-year in an enterprise context can mean and not mean at all...
When someone says something is hard or more often impossible, I answer that "it only takes time to find a solution for a problem that is impossible" otherwise said nothing is impossible except if it's proven impossible in this case a math proof...
If it's not proven impossible then it leaves room for imagination:
finding a proof proving it's impossible
and
If it is impossible there may be an "inferior" problem that can have a solution.
or
if it's not impossible, finding a solution
It's not just optimistic thinking. When I started Python->Javascript everybody was saying it was impossible. PyPy impossible. Metaclasses too hard. etc... I think that the only revolution that brings PyPy over Scheme->C paper (which is 25 years old) is some automatic JIT generation (based hints written in the RPython interpreter I think).
Most people that say that a thing is "hard" or "impossible" don't provide the reasons. C++ is hard to parse? I know that, still they are (free) C++ parser. Evil is in the detail? I know that. Saying it's impossible alone is not helpful, It's even worse than "not helpful" it's discouraging, and some people mean to discourage others. I heard about this question via https://stackoverflow.com/questions/22621164/how-to-automatically-generate-a-parser-code-to-code-translator-from-a-corpus.
What would be perfection for you? That's how you define next goal and maybe reach the overall goal.
I am more interested in knowing what kinds of patterns I could enforce
on the code to make it easier to translate (ie: IoC, SOA ?) the code
than how to do the translation.
I see no patterns that can not be translated from one language to another language at least in a less than perfect way. Since language to language translation is possible, you'd better aim for this first. Since, I think according to http://en.wikipedia.org/wiki/Graph_isomorphism_problem, translation between two computer languages is a tree or DAG isomorphism. Even if we already know that they are both turing complete, so...
Framework->Framework which I better visualize as API->API translation might still be something that you might keep in mind as a way to improve the generated code. E.g: Prolog as very specific syntax but still you can do Prolog like computation by describing the same graph in Python... If I was to implement a Prolog to Python translator I wouldn't implement unification in Python but in a C library and come up with a "Python syntax" that is very readable for a Pythonist. In the end, syntax is only "painting" for which we give a meaning (that's why I started scheme). Evil is in the detail of the language and I'm not talking about the syntax. The concepts that are used in the language getattribute hook (you can live without it) but required VM features like tail-recursion optimisation can be difficult to deal with. You don't care if the initial program doesn't use tail recursion and even if there is no tail recursion in the target language you can emulate it using greenlets/event loop.
For target and source languages, look for:
Big and specific ideas
Tiny and common shared ideas
From this will emerge:
Things that are easy to translate
Things that are difficult to translate
You will also probably be able to know what will be translated to fast and slow code.
There is also the question of the stdlib or any library but there is no clear answer, it depends of your goals.
Idiomatic code or readable generated code have also solutions...
Targeting a platform like PHP is much more easy than targeting browsers since you can provide C-implementation of slow and/or critical path.
Given you first project is translating Python to PHP, at least for the PHP3 subset I know of, customising veloce.py is your best bet. If you can implement veloce.py for PHP then probably you will be able to run the compliant mode... Also if you can translate PHP to the subset of PHP you can generate with php_veloce.py it means that you can translate PHP to the subset of Python that veloce.py can consume which would mean that you can translate PHP to Javascript. Just saying...
You can also have a look at those libraries:
https://bitbucket.org/logilab/astroid
https://bitbucket.org/logilab/pylint-brain
Also you might be interested by this blog post (and comments): https://www.rfk.id.au/blog/entry/pypy-js-poc-jit/
This Google Tech Talk from Ira Baxter is interesting https://www.youtube.com/watch?v=C-_dw9iEzhA
You could take a look at the Vala compiler, which translates Vala (a C#-like language) into C.
As far as I know, and have gathered from other SO posts, there are no proper tools for refactoring PHP code yet, so when it comes to refactoring, it's probably good old search-and-replace for most of us, with a good amount of praying that we didn't overlook something.
I would like to know whether there are any coding recommendations on how to write code friendly for manual refactoring. Never to construct variable names from strings, would be one thing that comes to mind because a construct like that is impossible to grep:
$object->{"field_".$fieldname}
I could imagine there are several such do's and don'ts. Maybe somebody knows good resources / articles on the issue. It wouldn't have to be PHP specific, either.
Unit tests always help me identify places where I've broken code due to a refactor. Unit tests in dynamic languages (PHP, Ruby, Python, etc.) provide assistance where static typing in other languages (Java, C#) would typically allow you to more safely refactor.
Avoid magic as much as possible: variable variables, eval, masking errors with # and storing code in the database will come back to bite you.
Well, The best way to write refactoring friendly code is to write loose coupled ,highly cohesive code and object oriented code.
You should try as much abstraction as you can, after all abstraction is the keyword while programming.
Moroever, you should be layering your code into presentation layer, business layer, data layer etc.. and Using design patterns is a pretty good solution.
I d recommend you to read Martin Fowler.
Your question makes a certain amount of sense. But—at the same time—it implies that the implementation is known to be inadequate, and written to be replaced. Why not just architect it properly the first time?
First, make sure your variable names make sense. If possible, go as OOP as you possibly can, or at least keep everything organized (image function file, database file, etc)
Second, and this is handy, check your IDE. Netbeans has options for refactoring. You can search in a file, in a folder, in a project, etc.