Is it possible to write code to write code?

Question

I've heard that there are some things one cannot do as a computer programmer, but I don't know what they are. One thing that occurred to me recently was: wouldn't it be nice to have a class that could make a copy of the source of the program it runs, modify that program and add a method to the class that it is, and then run the copy of the program and terminate itself. Is it possible for code to write code?

Answer 1

Sure it is. That's how a lot of viruses work!

Answer 2

Yes it is possible to create code generators. Most of the time they take user input and produce valid code. But there are other possibilities.

Self modifying programes are also possible. But they were more common in the dos era.

Answer 3

There is a whole class of such things called "Code Generators". (Although, a compiler also fits the description as you set it). And those describe the two areas of these beasts.

Most code generates, take some form of user input (most take a Database schema) and product source code which is then compiled.

More advanced ones can output executable code. With .NET, there's a whole namespace (System.CodeDom) dedicated to the create of executable code. The these objects, you can take C# (or another language) code, compile it, and link it into your currently running program.

Answer 4

Reflection gives you the ability to create code on the fly

Answer 5

Yes it certainly is, though maybe not in the context you are referring to check out this post on t4.

Answer 6

Of course you can! In fact, if you use a dynamic language, the class can change itself (or another class) while the program is still running. It can even create new classes that didn't exist before. This is called metaprogramming, and it lets your code become very flexible.

Answer 7

If you want to learn about the limits of computability, read about the halting problem

In computability theory, the halting problem is a decision problem which can be stated as follows: given a description of a program and a finite input, decide whether the program finishes running or will run forever, given that input.

Alan Turing proved in 1936 that a general algorithm to solve the halting problem for all possible program-input pairs cannot exist

Answer 8

Get your head around this: computability theory.

Answer 9

Start by looking at quines, then at Macro-Assemblers and then lex & yacc, and flex & bison. Then consider self-modifying code.

Here's a quine (formatted, use the output as the new input):

#include<stdio.h>

main()
{
  char *a = "main(){char *a = %c%s%c; int b = '%c'; printf(a,b,a,b,b);}";
  int b = '"';
  printf(a,b,a,b,b);
}

Now if you're just looking for things programmers can't do look for the opposite of np-complete.

Answer 10

I do this in PHP.

To persist settings for a class, I keep a local variable called $data. $data is just a dictionary/hashtable/assoc-array (depending on where you come from).

When you load the class, it includes a php file which basically defines data. When I save the class, it writes the PHP out for each value of data. It's a slow write process (and there are currently some concurrency issues) but it's faster than light to read. So much faster (and lighter) than using a database.

Something like this wouldn't work for all languages. It works for me in PHP because PHP is very much on-the-fly.

Answer 11

You are confusing/conflating two meanings of the word "write". One meaning is the physical writing of bytes to a medium, and the other is designing software. Of course you can have the program do the former, if it was designed to do so.

The only way for a program to do something that the programmer did not explicitly intend it to do, is to behave like a living creature: mutate (incorporate in itself bits of environment), and replicate different mutants at different rates (to avoid complete extinction, if a mutation is terminal).

Answer 12

If you look at Functional Programming that has many opportunities to write code that generates further code, the way that a language like Lisp doesn't differentiate between code and data is a significant part of it's power.

Rails generates the various default model and controller classes from the database schema when it's creating a new application. It's quite standard to do this kind of thing with dynamic languages- I have a few bits of PHP around that generate php files, just because it was the simplest solution to the problem I was dealing with at the time.

So it is possible. As for the question you are asking, though- that is perhaps a little vague- what environment and language are you using? What do you expect the code to do and why does it need to be added to? A concrete example may bring more directly relevant responses.

Answer 13

Sure it is. I wrote an effect for Paint.NET* that gives you an editor and allows you to write a graphical effect "on the fly". When you pause typing it compiles it to a dll, loads it and executes it. Now, in the editor, you only need to write the actual render function, everything else necessary to create a dll is written by the editor and sent to the C# compiler.

You can download it free here: http://www.boltbait.com/pdn/codelab/

In fact, there is even an option to see all the code that was written for you before it is sent to the compiler. The help file (linked above) talks all about it.

The source code is available to download from that page as well.

*Paint.NET is a free image editor that you can download here: http://getpaint.net

Answer 14

This is one of the fundamental questions of Artificial Intelligence. Personally I hope it is not possible - otherwise soon I'll be out of a job!!! :)

Answer 15

In relation to artificial intelligence, take a look at Evolutionary algorithms.

Answer 16

It has always been possible to write code generators. With XML technology, the use of code generators can be an essential tool. Suppose you work for a company that has to deal with XML files from other companies. It is relatively straightforward to write a program that uses the XML parser to parse the new XML file and write another program that has all the callback functions set up to read XML files of that format. You would still have to edit the new program to make it specific to your needs, but the development time when a new XML file (new structure, new names) is cut down a lot by using this type of code generator. In my opinion, this is part of the strength of XML technology.

Answer 17

make a copy of the source of the program it runs, modify that program and add a method to the class that it is, and then run the copy of the program and terminate itself

You can also generate code, build it into a library instead of an executable, and then dynamically load the library without even exiting the program that is currently running.

Answer 18

Dynamic languages usually don't work quite as you suggest, in that they don't have a completely separate compilation step. It isn't necessary for a program to modify its own source code, recompile, and start from scratch. Typically the new functionality is compiled and linked in on the fly.

Common Lisp is a very good language to practice this in, but there are others where you can created code and run it then and there. Typically, this will be through a function called "eval" or something similar. Perl has an "eval" function, and it's generally common for scripting languages to have the ability.

There are a lot of programs that write other programs, such as yacc or bison, but they don't have the same dynamic quality you seem to be looking for.

Answer 19

Yes, that's what most Lisp macros do (for just one example).

Answer 20

Lisp lisp lisp lisp :p

Joking, if you want code that generates code to run and you got time to loose learning it and breaking your mind with recursive stuff generating more code, try to learn lisp :)

(eval '(or true false))

Answer 21

Take a look at Langtom's loop. This is the simplest example of self-reproducing "program".

Answer 22

wouldn't it be nice to have a class that could make a copy of the source of the program it runs, modify that program and add a method to the class that it is, and then run the copy of the program and terminate itself

There are almost no cases where that would solve a problem that cannot be solved "better" using non-self-modifying code..

That said, there are some very common (useful) cases of code writing other code.. The most obvious being any server-side web-application, which generates HTML/Javascript (well, HTML is markup, but it's identical in theory). Also any script that alters a terminals environment usually outputs a shell script that is eval'd by the parent shell. wxGlade generates code to that creates bare-bone wx-based GUIs.