What kinds of patterns could I enforce on the code to make it easier to translate to another programming language?
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.
Answer
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.