I would like to do the following . I have declared a structure in my program and in run time when the program is being executed, if there is a user input, I should be able to create another new structure/edit that structure in my code. How can we do that? Is this what "A self modifying code"? Please clarify .Please give some examples .Thanks
Let me give an idea of what I wish to do , I have a "Structure/Class " called "student", which contains variables like "int roll_no" and "int reg_no". If the user wishes to add a new variable like "char name" in run time how can it be done?
Have a look on this:
http://bracha.org/Site/Talks.html
There is talk about reflection, which is probably what you want - reflection is not only about introspection (which most of developers already knows) but also about changing meaning of language constructs and runtime code manipulation.
Best languages for this kind of stuff are probably ruby and smalltalk.
If your language does not support these capabilities, you have still option to leverage code-generation - which is possible in almost every programming language but it is much easier in dynamic ones with "eval" support. For example this kind of stuff is possible even in C/C++ but your app has to embed compiler.
Java is good choice too (thx to classloaders and a lot of libraries for bytecode manipulation)
Oh, I've almost forgot, have a look on lisp and metacircular evaluation.
Sounds like you don't need to modify the existing code, but rather emit some new code in runtime. It is easy to do with any environment where your compiler is present in runtime. It is true for .NET, for JVM-based environments, various Lisps, etc.
Related
In the autoconf manual, it is noted that
AC_INIT (package, version, [bug-report], [tarname], [url])
defines multiple macro names such as AC_PACKAGE_NAME and PACKAGE_NAME.
Running configure also generates a config file with definition like the following:
define HAVE_LIBGMP 1
As I am writing C++ code, I find these macros annoying yet useful. In fact, it happened many times that I needed to link with a library that uses the autotools and thus has these macros in its headers. So the situation is that there is conflict on headers macros such as:
define PACKAGE_NAME "library"
define PACKAGE_NAME "mine"
So, I was wondering if there was a way to tell the autotools to define at least some of these macros inside some kind of structure as follows:
`struct header_information{
static string package_name;
static bug_report;
....
}`
and then initialize it with the right macro names.
This solution would keep these informations encapsulated and does not pollute the global namespace ?
It seems to me like you want to abuse a package-private, build-system-ony configuration header file (config.h) that just so happens to define a convenient macro name that you'd like to use. I think the pretty obvious answer is "don't do that", or else you're on your own.
Unless I'm misunderstanding you?
Those defines are there so that the particular library can use them. It's not meant for other things to include. In fact, the majority of the things in config.h are completely useless outside of the particular package.
That doesn't mean that the library that config.h file belongs to couldn't provide what you're looking for, by defining a public struct in a header that uses those variables. Or perhaps a library that uses pkg-config (if you're just looking for package names) can provide some of information for you. But I don't think that autotools would or should provide that information to you.
I'm wrapping a C++ library in PHP using SWIG and there have been some occasions where I want to modify the generated code (both generated C++ and PHP):
Fix code-generation errors
Add code that makes sense in PHP, but not in C++ (e.g. type checking)
Add documentation tags (e.g. phpDoc)
I'm currently automating these modifications with patch. This approach works, but it seems high-maintenance and fragile. Is there a better way of doing this?
The best bet is to have your code generator generate correct code for your needs. Hand-tweaking generated output is unsustainable. You'll have to tweak it again any time the input changes.
If a tool is producing flatly erroneous output, it's ideal to repair it and submit patches back to the maintainer. If the output is correct for some circumstances but wrong for yours, I'd suggest to add an option that changes the behavior to what you need.
Sometimes, you can use a short program that automatically does an intelligent job of patching your generated code, so that you don't need a manual process to make patches.
Alternatively, you could write your own code generator, but I suspect that's much more work than you want. It also depends on what you're doing. Sometimes code-generation is really just macro-expansion, and there are plenty of good tools for that in the wild.
Good luck!
You may end up having a maintenance nightmare later on. Instead of SWIG you might consider using another generative approach that:
Let you add your custom code directly on the model (so that you won't need to add it post-generation)
Let you define your own generator. This feature alone could take out the need to add custom code all along.
The problem of using third-party generators is that they never really generate what you want. The problem of writing your own code generators is that it's much more work. You choose.
But correcting an automation with another automation...
Code generation is quite a wide topic and there are definitely many other approaches, which might be more interresting to you as mentioned above.
But if you do not want to use other tool, depending on what code is generated and on the PHP OO capabilities, you might use the Generation Gap pattern.
I'm creating a game in XNA and was thinking of creating my own scripting language (extremely simple mind you). I know there's better ways to go about this (and that I'm reinventing the wheel), but I want the learning experience more than to be productive and fast.
When confronted with code at run time, from what I understand, the usual approach is to parse into a machine code or byte code or something else that is actually executable and then execute that, right? But, for instance, when Chrome first came out they said their JavaScript engine was fast because it compiles the JavaScript into machine code. This implies other engines weren't compiling into machine code.
I'd prefer not compiling to a lower language, so are there any known modern techniques for parsing and executing code without compiling to low level? Perhaps something like parsing the code into some sort of tree, branching through the tree, and comparing each symbol and calling some function that handles that symbol? (Wild guessing and stabbing in the dark)
I personally wouldn't roll your own parser ( turning the input into tokens ) or lexer ( checking the input tokens for your language grammar ). Take a look at ANTLR for parsing/lexing - it's a great framework and has full source code if you want to dig into the guts of it.
For executing code that you've parsed, I'd look at running a simple virtual machine or even better look at llvm which is an open-source(ish) attempt to standardise the virtual machine byte code format and provide nice features like JITing ( turning your script compiled byte code into assembly ).
I wouldn't discourage you from the more advanced options that you machine such as native machine code execution but bear in mind that this is a very specialist area and gets real complex, real fast!
Earlz pointed out that my reply might seem to imply 'don't bother doing this yourself. Re-reading my post it does sound a bit that way. The reason I mentioned ANTLR and LLVM is they both have heaps of source code and tutorials so I feel this is a good reference source. Take it as a base and play
You can try this framework for building languages (it works well with XNA):
http://www.meta-alternative.net/mbase.html
There are some tutorials:
http://www.meta-alternative.net/calc.pdf
http://www.meta-alternative.net/pfront.pdf
Python is great as a scripting language. I would recommend you make a C# binding for its C API and use that. Embedding Python is easy. Your application can define functions, types/classes and variables inside modules which the Python interpreter can access. The application can also call functions in Python scripts and get a result back. These two features combined gives you a two-way communication scheme.
Basically, you get the Python syntax and semantics for free. What you would need to implement is the API your application exposes to Python. An example could be access to game logic functions and render functions. Python scripts would then define functions which calls these, and the host application would invoke the Python functions (with parameters) to get work done.
EDIT: Seems like IronPython can save you even more work. It's a C# implementation of CPython, and has its own embedding API: http://www.ironpython.net/
Interpreted languages are usually more high-level and therefore have features as dynamic typing (including creating new variables dynamically without declaration), the infamous eval and many many other features that make a programmer's life easier - but why can't compiled languages have these as well?
I don't mean languages like Java that run on a VM, but those that compile to binary like C(++).
I'm not going to make a list now but if you are going to ask which features I mean, please look into what PHP, Python, Ruby etc. have to offer.
Which common features of interpreted languages can't/don't/do exist in compiled languages? Why?
Whether source code is compiled - to native binaries, some kind of intermediate language (Java Bytecode/IL) - or interpreted is absolutely no trait of the language. It's just a question of the implementation.
You can actually have both compilers and interpreters for the same language like
Haskell: GHC <-> GHCI
C: gcc <-> ch
VB6: VS IDE <-> VB6 compiler
Certain language features like eval or dynamic typing may suggest a distinction between so called "dynamic languages" and static ones, but how this is run can never be the primary question.
Initially, one of the largest benefits of interpreted languages was debugging. That way you can get incredibly accurate and detailed information when looking for the reason a program isn't working. However, most compilers have become advanced enough that that is not too big of a deal any more.
The other main benefit (in my opinion anyway), is that with interpreted languages, you don't have to wait for eternity for your project to compile to test it out.
You couldn't plausibly do eval, for example, for reasons I'd have thought were pretty obvious: exactly how would you implement it? Make the runtime contain a full copy of the compiler? Every time you wanted to evaluate a string (keeping in mind that each time it could be different!) you'd save the string to a file, run the compiler on it to make a DLL/shared-lib, then load that DLL/shared-lib and call your code? You can't see why this might be a wee bit impractical? ;)
You can find this kind of thing in dynamic languages all over the place that you can't do with static code short of basically running an interpreter, in effect, behind the scenes.
Continuing on from Dario - I think you are really asking why a compiled program can't evaluate statements at runtime (e.g. eval). Here's some reasons I can think of:
The full compiler would have to be distributed with the program (or be part of the program)
For an eval function to have access to type information and symbols (such as variable names and function names) in the environment it was used the original program would have to be compiled with those symbols accessible (compiled languages usually remove these symbols at compile time).
Edit: As noted neither of these reasons make it impossible for a language/compiler to be able to evaluate code at runtime, but they are definitely things that need to be taken into consideration when developing a compiler or when designing a language.
Maybe the question is not about interpreted/compiled languages (compile is ambiguous anyway) but about languages that do/don't carry their own compiler around with them? For instance we've said C++ could do eval with a handy compiler floating around in the app, and reflection presumably is similar in some ways.
I am attempting to determine prior art for the following idea:
1) user types in some code in a language called (insert_name_here);
2) user chooses a destination language from a list of well-known output candidates (javascript, ruby, perl, python);
3) the processor translates insert_name_here into runnable code in destination language;
4) the processor then runs the code using the relevant system call based on the chosen language
The reason this works is because there is a pre-established 1 to 1 mapping between all language constructs from insert_name_here to all supported destination languages.
(Disclaimer: This obviously does not produce "elegant" code that is well-tailored to the destination language. It simply does a rudimentary translation that is runnable. The purpose is to allow developers to get a quick-and-dirty implementation of algorithms in several different languages for those cases where they do not feel like re-inventing the wheel, but are required for whatever reason to work with a specific language on a specific project.)
Does this already exist?
The .NET CLR is designed such that C++.Net, C#.Net, and VB.Net all compile to the same machine language, and you can "decompile" that CLI back in to any one of those languages.
So yes, I would say it already exists though not exactly as you describe.
There are converters available for different languages. The problem you are going to have is dealing with libraries. While mapping between language statements might be easy, finding mappings between library functions will be very difficult.
I'm not really sure how useful that type of code generator would be. Why would you want to write something in one language and then immediately convert it to something else? I can see the rationale for 4th Gen languages that convert diagrams or models into code but I don't really see the point of your effort.
Yes, a program that transform a program from one representation to another does exist. It's called a "compiler".
And as to your question whether that is always possible: as long as your target language is at least as powerful as the source language, then it is possible. So, if your target language is Turing-complete, then it is always possible, because there can be no language that is more powerful than a Turing-complete language.
However, there does not need to be a dumb 1:1 mapping.
For example: the Microsoft Volta compiler which compiles CIL bytecode to JavaScript sourcecode has a problem: .NET has threads, JavaScript doesn't. But you can implement threads with continuations. Well, JavaScript doesn't have continuations either, but you can implement continuations with exceptions. So, Volta transforms the CIL to CPS and then implements CPS with exceptions. (Newer versions of JavaScript have semi-coroutines in the form of generators; those could also be used, but Volta is intended to work across a wide range of JavaScript versions, including obviously JScript in Internet Explorer.)
This seems a little bizarre. If you're using the term "prior art" in its most common form, you're discussing a potentially patentable idea. If that is the case, you have:
1/ Published the idea, starting the clock running on patent filing - I'm assuming, perhaps incorrectly, that you're based in the U.S. Other jurisdictions may have other rules.
2/ Told the entire planet your idea, which means it's pretty much useless to try and patent it, unless you act very fast.
If you're not thinking about patenting this and were just using the term "prior art" in a laypersons sense, I apologize. I work for a company that takes patents very seriously and it's drilled into us, in great detail, what we're allowed to do with information before filing.
Having said that, patentable ideas must be novel, useful and non-obvious. I would think that your idea would not pass on the third of these since you're describing a language translator which would have the prior art of the many pascal-to-c and fortran-to-c converters out there.
The one glimmer of hope would be the ability of your idea to generate one of multiple output languages (which p2c and f2c don't do) but I think even that would be covered by the likes of cross compilers (such as gcc) which turn source into one of many different object languages.
IBM has a product called Visual Age Generator in which you code in one (proprietary) language and it's converted into COBOL/C/Java/others to run on different target platforms from PCs to the big honkin' System z mainframes, so there's your first problem (thinking about patenting an idea that IBM, the biggest patenter in the world, is already using).
Tons of them. p2c, f2c, and the original implementation s of C++ and Objective C strike me immediately. Beyond that, it's kind of hard to distinguish what you're describing from any compiler, especially for us old guys whose compilers generated ASM code for an intermediate represetation anyway.