I am porting a library from C++ to Java. This is my first time and I am not sure
about what "porting" really means? Specifically, what if the author named a variable
as 'A' and I think that a better name would be 'B'. Same for methods, classes and namespaces.
Also, what if I think something can be done better? Does porting mean that I should try
to keep as much of the original code spirit as possible, but still allow myself freedom
to improve stuff?
Thanks
It doesn't necessarily have to be a one-to-one translation (and in many cases, it can't be done). Porting is just rewriting a piece of software in a different language/environment/etc. Sometimes porting will require you to tweak things and implement them in different ways altogether, so I think the last sentence of your post pretty much captures the gist of things.
I view it as comparable to translating a book from English to another language. There will be instances where judgment calls need to be made in terms of how to express the intent/function of the source material.
When porting from System A to System B, the world is your oyster. You can pretty much change anything if you believe it's an improvement. The only caveats to that are when dealing with interfaces. Say, you are porting an API, for example, it wouldn't be a good idea to name externally-available methods, as that would break something down the road. Tracing naming issues across multiple classes is a major pain.
As someone who's done a fair bit of porting from language to language, I would recommend sticking to implementation details first and foremost. A good engineering principle is to change one thing at any time. That way, when things don't run as expected, you'll know that it's your implementation that is to blame, and not some silly naming issue. And when you do come to renaming, I suppose it goes without saying, be very careful and backup often. This is one case where software versioning may save you hours of time.
When "porting" a library from one platform to another, you are porting functionality. You are not porting style of code. It isn't like in literature, where one must maintain the style of the piece, keeping in mind metaphors and iambic pentameter or what have you.
I have decided to write a small interpreter as my next project, in Ruby. What knowledge/skills will I need to have to be successful?
I haven't decided on the language to interpret yet, but I am looking for something that is not a toy language, but would be relatively easy to write an interpreter for.
Thanks in advance.
You will have to learn at least:
lexical analysis (grouping characters into tokens)
parsing (grouping tokens together into structure)
abstract syntax trees (representing program structure in a data structure)
data representation (assuming your language will have variables)
an evaluation loop that "runs" your program
An excellent introduction to some of these topics can be found in the introductory text Structure and Interpretation of Computer Programs. The language used in that book is Scheme, which is a robust, well-specified language that is ideally suited for your first interpreter implementation. Highly recommended.
I haven't decided on the language to interpret yet, but I am looking for
something that is not a toy language, but would be relatively easy to write an
interpreter for. Thanks in advance.
Try some dialect of Lisp like Scheme or Clojure. (Now there's an idea: Clojure-in-Ruby, which integrates with Ruby as well as Clojure does with Java.)
With Lisp, there is no need to bother with idiosyncracies of syntax, as Lisp's syntax is much closer to the abstract syntax tree.
This SICP chapter shows how to write a Lisp interpreter in Lisp (a metacircular evaluator). In my opinion this is the best place to start. Then you can move on to Lisp in Small Pieces to learn how to write advanced interpreters and compilers for Lisp. The advantage of implementing a language like Lisp (in Lisp itself!) is that you get the lexical analyzer, parser, AST, data/program representation and REPL for free. You can concentrate on the task of getting your great language working!
There is Tree top project wich can be helpful for you http://treetop.rubyforge.org/
You can checkout Ruby Draft Specification http://ruby-std.netlab.jp/
I had a similar idea a couple of days ago. LISP is by far the easiest to implement because the syntax is so simple, and the data structures that the language manipulates are the same structures that the code is written in. Hence you need only a minimal implementation, and can define the rest in terms of itself.
However, if you are trying to learn about parsing, you may want to do a more complex language with Abstract Syntax Trees, etc.
If you want to check out my (literally two days old) Java implementation of lisp, check out mylisp.googlecode.com. I'm still working on it but it is incredible how short a time it took to get the existing stuff working.
It's not sooo hard. here's a LISP interpreter in ruby and the source is so small you are supposed to copy/paste it. but are you gonna learn LISP now? hehe.
If you're just doing this for fun, make up your own, simple language and just try it. My recommendation would be something like a really simple classic BASIC (no visual basic or object oriented stuff). With line numbers, GOTO, INPUT and PRINT and that's it. You get to do the basics, and you get a better understanding of how things work.
The knowledge you'll need?
Tokenizing (turning that huge chunk of characters into something more efficiently readable, effectively splitting it up into 'words')
Parsing (going over the tokens and building a data structure from it)
Interpreting (looping over the data structure and executing each command)
And for that last one you'll also need a way to keep around variables. Usually you'd just implement a "stack", one huge block of data where you can mark off an area at the end.
It's not implemented in Lisp, but I found Write Yourself A Scheme in 48 Hours to be a very useful document while I was starting out with Haskell (though I didn't get anywhere near finishing it after 48 hours; YMMV). It also gives you a lot of insight into interpreters in general.
I can recommend this book. It discusses patterns for writing parsers and interpreters and more:
http://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=language+implementation+patterns&x=0&y=0
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I am a hobbyist programmer (started with VBA to make excel quicker) and have been working with VB.NET / C#.NET and am trying to learn ADO.NET.
A facet of programming that has always frustrated me is what does 'good' look like? I am not a professional so have little to compare against. What makes a better programmer?
Is it:
They have a better understanding of
all the objects / classes / methods
in a given language?
Their programs are more efficient?
The design of their programs are much
better in terms of better
documentation, good choice of names
for functions etc.?
Put another way, if I were to look at the code of a professional programmer, what is the first thing that I would notice about their code relative to mine? For example, I read books like 'Professional ASP.NET' by Wrox press. Are the code examples in that book 'world class'? Is that the pinnacle? Would any top-gun programmer look at that code and think it was good code?
The list below is not comprehensive, but these are the things that I thought of in considering your question.
Good code is well-organized. Data and operations in classes fit together. There aren't extraneous dependencies between classes. It does not look like "spaghetti."
Good code comments explain why things are done not what is done. The code itself explains what is done. The need for comments should be minimal.
Good code uses meaningful naming conventions for all but the most transient of objects. the name of something is informative about when and how to use the object.
Good code is well-tested. Tests serve as an executable specification of the code and examples of its use.
Good code is not "clever". It does things in straightforward, obvious ways.
Good code is developed in small, easy to read units of computation. These units are reused throughout the code.
I haven't read it yet, but the book I'm planning to read on this topic is Clean Code by Robert C. Martin.
The first thing you'd notice is that their code follows a consistent coding-style. They always write their structure blocks the same, indent religiously and comment where appropriate.
The second things you'd notice is that their code is segmented into small methods / functions spanning no more than a couple dozen lines at the most. They also use self describing method names and generally their code is very readable.
The third thing you'd notice, after you messed around with the code a little is that the logic is easy to follow, easy to modify - and therefore easily maintainable.
After that, you'll need some knowledge and experience in software design techniques to understand the specific choices they took constructing their code architecture.
Regarding books, I haven't seen many books where the code could be considered "world-class". In books they try mostly to present simple examples, which might be relevant to solving very simple problems but aren't reflective of more complex situations.
Quoting Fowler, summizing readability:
Any fool can write code that a computer can understand.
Good programmers write code that humans can understand.
'nough said.
Personally, I'll have to quote "The Zen of Python" by Tim Peters. It tells Python programmers what their code should look like, but I find that it applies to basically all code.
Beautiful is better than ugly. Explicit is better than
implicit. Simple is better than complex. Complex is better
than complicated. Flat is better than nested. Sparse is
better than dense. Readability counts. Special cases
aren't special enough to break the rules. Although practicality
beats purity. Errors should never pass silently. Unless
explicitly silenced. In the face of ambiguity, refuse the
temptation to guess. There should be one-- and preferably only
one --obvious way to do it. Although that way may not be obvious
at first unless you're Dutch. Now is better than never.
Although never is often better than right now. If the
implementation is hard to explain, it's a bad idea. If the
implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!
Code is poetry.
Start from this point of logic and you can derive many of the desirable qualities of code. Most importantly, observe that code is read far more than it is written, hence write code for the reader. Rewrite, rename, edit, and refactor for the reader.
A follow on corollary:
The reader will be you at time n from the code creation date. The payoff of writing code for the reader is a monotonically increasing function of n. A reader looking at your code for the first time is indicated by n == infinity.
In other words, the larger the gap of time from when you wrote the code to when you revisit the code, the more you will appreciate your efforts to write for the reader. Also, anyone you hand your code off to will gain great benefit from code written with the reader as the foremost consideration.
A second corollary:
Code written without consideration for the reader can be unnecessarily difficult to understand or use. When the consideration for the reader drops below a certain threshold, the reader derives less value from the code than the value gained by rewriting the code. When this occurs the previous code is thrown away and, tragically, much work is repeated during the rewrite.
A third corollary:
Corollary two has been known to repeat itself multiple times in a vicious cycle of poorly documented code followed by forced rewrites.
I've been programming for 28 years and I find this a tough question to answer. To me good code is a complete package. The code is cleanly written, with meaningful variable and method names. It has well placed comments that comment the intent of the code and doesn't just regurgitate the code you can already read. The code does what it is supposed to in an efficient manner, without wasting resources. It also has to be written with an eye towards maintainability.
The bottom line though is that it means different things to different people. What I might label as good code someone else might hate. Good code will have some common traits which I think I've identified above.
The best thing you can do is expose yourself to code. Look at other people's code. Open Source projects are a good source for that. You will find good code and bad code. The more you look at it, the better you will recognize what you determine to be good code and bad code.
Ultimately you will be your own judge. When you find styles and techniques you like adopt them, over time you will come up with your own style and that will change over time. There is no person on here that can wave a wand and say what is good and that anything else is bad.
Read the book Code Complete. This explains a lot of ideas about how to structure code and the the reasons for doing so. Reading it should short-circuit your time to aquiring the experience necessary to tell good from bad.
http://www.amazon.com/Code-Complete-Practical-Handbook-Construction/dp/0735619670/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1229267173&sr=8-1
Having been programming for nearly 10 years now myself and having worked with others I can say without bias that there is no difference between a good programmer and an average programmers code
All programmers at a competent level:
Comment Correctly
Structure Efficiently
Document Cleanly
I once overheard a co-worker say "I've always been very logical and rational minded. I think that's why I enjoy developing"
That in my opinion, is the mind of an average programmer. One who sees the world in terms of rules and logic and ultimately obeys those rules when designing and writing a program.
The expert programmer, understands the rules, but also their context. This ultimately leads to them coming up with new ideas and implementations, the mark of an expert programmer. Programming is ultimately an art form.
Succinctly put, a good programmer's code can be read and understood.
In my opinion, a good programmer's code is language-agnostic; well-written code can be read and understood in a short amount of time with minimal thinking, regardless of the programming language used. Whether the code is in Java, Python, C++ or Haskell, well-written code is understandable by people who don't even program in that particular language.
Some characteristics of code that is easy to read are, methods that are well-named, absence of "tricks" and convoluted "optimization", classes are well-designed, to name a few. As others have mentioned, coding style is consistent, succinct and straight-forward.
For example, the other day, I was taking a look at the code for TinyMCE to answer one of the questions on Stack Overflow. It is written in JavaScript, a language that I've hardly used. Yet, because of the coding style and the comments that are included, along with the structuring of the code itself, it was fairly understandable, and I was able to navigate through the code in a few minutes.
One book that was quite an eye-opener for me in the regard of reading good programmer's code is Beautiful Code. It has many articles written by authors of various programming projects in various programming languages. Yet, when I read it, I could understand what the author was writing in his code despite the fact that I've never even programmed in that particular language.
Perhaps what we should keep in mind is that programming is also about communication, not only to the computer but to people, so good programmer's code is almost like a well-written book, which can communicate to the reader about the ideas it wants to convey.
Easy to read
easy to write
easy to maintain
everything else is filigree
Good code should be easily understood.
It should be well commented.
Difficult parts should be even better commented.
Good code is readable. You'd have no trouble understanding what the code is doing on the first read through of code written by a good professional programmer.
Rather then repeat everyone else's great suggestions, I will instead suggest that you read the book Code Complete by Steve McConnell
Essentially it is a book packed full of programming best practices for both functionality and style.
[Purely subjective answer]
For me, good code is a form of art, just like a painting. I might go further and say that it's actually a drawing that includes characters, colors, "form" or "structure" of code, and with all this being so readable/performant. The combination of readability, structure (i.e. columns, indentation, even variable names of the same length!), color (class names, variable names, comments, etc.) all make what I like to see as a "beautiful" picture that can make me either very proud or very detestful of my own code.
(As said before, very subjective answer. Sorry for my English.)
I second the recommendation of Bob Martin's "Clean Code".
"Beautiful Code" was highly acclaimed a couple of years ago.
Any of McConnell's books are worth reading.
Perhaps "The Pragmatic Programmer" would be helpful, too.
%
Just wanted to add my 2 cents... comments in your code -- and your code itself, generally -- should say what your code does, now how it does it. Once you have the concept of 'client' code, which is code that calls other code (simplest example is code that calls a method), you should always be most worried about making your code comprehensible from the "client's" perspective. As your code grows, you'll see that this is... uh, good.
A lot of the other stuff about good code is about the mental leaps that you'll make (definitely, if you pay attention)... 99% of them have to do with doing a bit more work now to spare you a ton of work later, and reusability. And also with doing things right: I almost always want to run the other way rather than using regular expressions, but every time I get into them, I see why everybody uses them in every single language I work in (they're abstruse, but work and probably couldn't be better).
Regarding whether to look at books, I would say definitely not in my experience. Look at APIs and frameworks and code conventions and other people's code and use your own instincts, and try to understand why stuff is the way it is and what the implications of things are. The thing that code in books almost never does is plan for the unplanned, which is what error checking is all about. This only pays off when somebody sends you an email and says, "I got error 321" instead of "hey, the app is broke, yo."
Good code is written with the future in mind, both from the programmer's perspective and the user's perspective.
This is answered pretty well in Fowler's book, "Refactoring", It's the absence of all the "smells" he describes throughout the book.
I haven't seen 'Professional ASP.NET', but I'd be surprised if it's better than OK. See this question for some books with really good code. (It varies, of course, but the accepted answer there is hard to beat.)
This seems to be (should be) a FAQ. There is an ACM article about beautiful code recently. There seems to be a lot of emphasis on easy to read/understand. I'd qualifier this with "easy to read/understand by domain experts". Really good programmers tend to use the best algorithms (instead of naive easy to understand O(n^2) algorithms) for any given problems, which could be hard to follow, if you're not familiar with the algorithm, even if the good programmer gives a reference to the algorithm.
Nobody is perfect including good programmers but their code tend to strive for:
Correctness and efficiency with proven algorithms (instead of naive and adhoc hacks)
Clarity (comment for intent with reference to non-trivial algorithms)
Completeness to cover the basics (coding convention, versioning, documentation, unit tests etc.)
Succinctness (DRY)
Robustness (resilient to arbitrary input and disruption of change requests)
i second the recommendation for uncle bob's "clean code". but you may wish to take a look at http://www.amazon.com/Implementation-Patterns-Addison-Wesley-Signature-Kent/dp/0321413091 as i think this deals with your specific question a bit better. good code should leap off the page and tell you what it does/how it works.
Jeff Atwood wrote a nice article about how coders are Typists first reference:
http://www.codinghorror.com/blog/archives/001188.html
When being a typist you always need to be elegant in your work, having strucutre and proper "grammar" is highly important. Now converting this to "programming"-typing would catch the same outcome.
Structure
Comments
Regions
I'm a software engineere which means during my education i've come across many different languages but my programming always "feel" the same, as my writing does on fekberg.wordpress.com, i have a "special" way for typing.
Now programming different applications and in different languages, such as Java, C#, Assembler, C++,C i've come to the "standard" of writing that i like.
I see everything as "boxes" or regions and each region has it's explaining commenting. A region might be "class Person" and inside this Region i have a couple of methods for properties, which i may call "Access Methods" or such and each property and region has it's own explaining commenting.
This is highly important, i always see my code that i do, as "being a part of an api", when creating an API structure and elegance is VERY important.
Think about this. Also read my paper on Communication issues when adapting outsourcing which explains in rough, how bad code can conflict, Enterpret as you like: http://fekberg.wordpress.com/2008/12/14/communication-issues-when-adapting-outsourcing/
Good code is easy to understand, easy to maintain, and easy to add to. Ideally, it is also as efficient as possible without sacrificing other indicators.
Great code to me is something that is simple to grasp yet sophisticated. The things that make you go, "wow, of course, why didn't I think of it that way?". Really good code is not hard to understand, it simply solves the problem at hand in a straight-forward way (or a recursive way, if that is even simpler).
Good code is where you know what the method does from the name. Bad code is where you have to work out what the code does, to make sense of the name.
Good code is where if you read it, you can understand what it's doing in not much more time than it takes to read it. Bad code is where you end up looking at it for ages trying to work out wtf it does.
Good code has things named in such a way as to make trivial comments unnecessary.
Good code tends to be short.
Good code can be reused to do what it does anywhere else, since it doesn't rely on stuff that is really unrelated to its purpose.
Good code is usually a set of simple tools to do simple jobs (put together in well organised ways to do more sophisticated jobs). Bad code tends to be huge multi-purpose tools that are easy to break and difficult to use.
Code is a reflection of a programmer's skills and mindset. Good programmers always have an eye on the future - how the code will function when requirements or circumstances are not exactly what they are today. How scalabale it will be? How convenient it will be when I am not the one maintaining this code? How reusable the code will be, so that someone else doing similar stuff can reuse the code and not write it again. What when someone else is trying to understand the code that I have written.
When a programmer has that mindset, all the other stuff falls in place nicely.
Note: A code base is worked on by many programmers over time and typically there is not a specific designation of code base to a programmer. Hence good code is a reflection of all the company's standards and quality of their workforce.
(I use "he" below because this is the person that I aspire to be, sometimes with success).
I believe that the core of a good programmer's philosophy is that he is always thinking "I am coding for myself in the future when I will have forgotten all about this task, why I was working on it, what were the risks and even how this code was supposed to work."
As such, his code has to:
Work (it doesn't matter how fast code gets to the wrong answer. There's no partial credit in the real world).
Explain how he knows that this code works. This is a combination of documentation (javadoc is my tool of choice), exception handling and test code. In a very real sense, I believe that, line for line, test code is more valuable than functional code if for no other reason than it explains "this code works, this is how it should be used, and this is why I should get paid."
Be maintained. Dead code is a nightmare. Legacy code maintenance is a chore but it has to be done (and remember, it's "legacy" the moment that it leaves your desk).
On the other hand, I believe that the good programmer should never do these things:
Obsess over formatting. There are plenty of IDEs, editors and pretty-printers that can format code to exactly the standard or personal preference that you feel is appropriate. I use Netbeans, I set up the format options once and hit alt-shift-F every now and then. Decide how you want the code to look, set up your environment and let the tool do the grunt work.
Obsess over naming conventions at the expense of human communication. If a naming convention is leading you down the road of naming your classes "IElephantProviderSupportAbstractManagerSupport" rather than "Zookeeper", change the standard before you make it harder for the next person.
Forget that he works as a team with actual human beings.
Forget that the primary source of coding errors is sitting at his keyboard right now. If there's a mistake or an error, he should look to himself first.
Forget that what goes around comes around. Any work that he does now to make his code more accessible to future readers will almost certainly benefit him directly (because who's going to be the first person asked to look at his code? He is).
It works
It has unit tests that prove that it works
The rest is icing...
The best code has a certain elegance that you recognise as soon as you see it.
It looks crafted, with care and attention to detail. It's obviously produced with someone with skill and has an art about it - you could say it looks sculpted and polished, rather than rough and ready.
It's consistent and reads easily.
It's split into small, highly cohesive functions each of which do one thing and do it well.
It's minimally coupled, meaning that dependencies are few and strictly controlled,
usually by...
Functions and classes have dependencies on abstractions rather than implementations.
Ironically the better the programmer the less indispensable he/she becomes because the code produced is better maintainable by anyone (as stated by general consent by Eran Galperin).
My experience tells the opposite is also true. The worse the programmer the more difficult to maintain his/her code is, so more indispensable he/she becomes, since no other soul can understand the riddles produced.
I have a good example :
Read GWT (google web tookit) Source code, you will see that every fool understand it (some english books are harder to read than this code).
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In my day job I, and others on my team write a lot of hardware models in Verilog-AMS, a language supported primarily by commercial vendors and a few opensource simulator projects.
One thing that would make supporting each others code more helpful would be a LINTER that would check our code for common problems and assist with enforcing a shared code formatting style.
I of course want to be able to add my own rules and, after I prove their utility to myself, promote them to the rest of the team..
I don't mind doing the work that has to be done, but of course also want to leverage the work of other existing projects.
Does having the allowed language syntax in a yacc or bison format give me a leg up?
or should I just suck each language statement into a perl string, and use pattern matching to find the things I don't like?
(most syntax and compilation errors are easily caught by the commercial tools.. but we have some of our own extentions.)
lex/flex and yacc/bison provide easy-to-use, well-understood lexer- and parser-generators, and I'd really recommend doing something like that as opposed to doing it procedurally in e.g. Perl. Regular expressions are powerful stuff for ripping apart strings with relatively-, but not totally-fixed structure. With any real programming language, the size of your state machine gets to be simply unmanageable with anything short of a Real Lexer/Parser (tm). Imagine dealing with all possible interleavings of keywords, identifiers, operators, extraneous parentheses, extraneous semicolons, and comments that are allowed in something like Verilog AMS, with regular expressions and procedural code alone.
There's no denying that there's a substantial learning curve there, but writing a grammar that you can use for flex and bison, and doing something useful on the syntax tree that comes out of bison, will be a much better use of your time than writing a ton of special-case string-processing code that's more naturally dealt with using a syntax-tree in the first place. Also, what you learn writing it this way will truly broaden your skillset in ways that writing a bunch of hacky Perl code just won't, so if you have the means, I highly recommend it ;-)
Also, if you're lazy, check out the Eclipse plugins that do syntax highlighting and basic refactoring for Verilog and VHDL. They're in an incredibly primitive state, last I checked, but they may have some of the code you're looking for, or at least a baseline piece of code to look at to better inform your approach in rolling your own.
I've written a couple verilog parsers and I would suggest PCCTS/ANTLR if your favorite programming language is C/C++/Java. There is a PCCTS/ANTLR Verilog grammar that you can start with. My favorite parser generator is Zebu which is based on Common Lisp.
Of course the big job is to specify all the linting rules. It makes sense to make some kind of language to specify the linting rules as well.
Don't underestimate the amount of work that goes into a linter. Parsing is the easy part because you have tools (bison, flex, ANTLR/PCCTS) to automate much of it.
But once you have a parse, then what? You must build a semantic tree for the design. Depending on how complicated your inputs are, you must elaborate the Verilog-AMS design (i.e. resolving parameters, unrolling generates, etc. If you use those features). And only then can you try to implement rules.
I'd seriously consider other possible solutions before writing a linter, unless the number of users and potential time savings thereby justify the development time.
In trying to find my answer, I found this on ANTLR - might be of use
If you use Java at all (and thus IDEA), the IDE's extensions for custom languages might be of use
yacc/bison definitely gives you a leg up, since good linting would require parsing the program. Regex (true regex, at least) might cover trivial cases, but it is easy to write code that the regexes don't match but are still bad style.
ANTLR looks to be an alternative path to the more common (OK I heard about them before) YACC/BISON approach, which it turns out also commonly use LEX/FLEX as a front end.
a Quick read of the FLEX man page kind of make me think It could be the framework for that regex type of idea..
Ok.. I'll let this stew a little longer, then see how quickly I can build a prototype parser in one or the other.
and a little bit longer
I often need to implement some sort of rule system that is user-editable -- the requirements are generally different enough that the same system isn't directly applicable, so I frequently run into the same problem--how do I design a rule system that
is maintainable
properly balances expressiveness with ease of use
is easily extended (if/when I get (2) wrong).
I think Rule systems / DSLs are extremely valuable, but I don't feel comfortable with my ability to design them properly.
What references / tips do you have to offer that may help make this easier?
Because of the nature of the problems I run into, existing languages are generally not applicable. (For example, you would not require that general computer users learn python in order to write an email filter.) Similarly, rule languages, such as JESS, are only a partial solution, since some (simpler) user interface needs to be built on-top of the rule language so non-programmers can make use of it. This interface invariably involves removing some features, or making those features more difficult to use, and that process poses the same problems described above.
Edit: To clarify, the question is about designing a rule engine, I'm not looking for a pre-built rule engine. If you suggest a rule engine, please explain how it addresses the question about making good design decisions.
We had an in-house demo of this tool by it's vendor:
http://www.rulearts.com/rulexpress.php
As a company, we have a lot of experience with rule engines (e.g. Cleverpath Aion), but mostly developer-oriented tools. This tool (rulexpress) is very business-people oriented. It's not a rule engine. But it can output all the data in xml (so basically any format you like), and this is something we would then consider as input for a real rule engine, e.g. Windows Workflow Foundation (not one of the bigger/better rule engines, but still).
The tool in itself looked pretty good, some stuff I had never seen in any developer-oriented tool.
There are also some tools for rule management built around WF, if that's your rule engine of choice, check out InRule.
Edited after original question was clarified:
Although I have dabbled in this a long time ago (writing a little language in javacc), I would consider this a bad time investment now. My comment above is in the same spirit: take a simple rule engine, a simple (commercial) UI that makes it easy for business users to maintain, and only invest time in tying the two together.
We have had luck with this: http://msdn.microsoft.com/en-us/library/bb472424.aspx
A Ruby implementation to consider is Ruleby (http://ruleby.org/wiki/Ruleby)
One thing I've found is that being able to define rules as expression trees makes implementation so much simpler. As you correctly mentioned, the requirements from project to project are so different that you just about have to reimplement every time. Expression trees coupled with something like the visitor pattern make for a very (no pun intended) expressive framework that is easily extensible. And you can easily put a very dynamic GUI on top of expression trees which meets that aspect of your requirement.
Hopefully this doesn't sound like I'm saying that everything looks like a nail with my hammer because that's not the case ... it's just that in my experience, this has come in handy more than once :-)
First of all, normally it is not advised to let end-users define the rules. That's because they do not have development background and could simply write "code" that goes into infinite loop or does other weird things.
So either the system has to protect against that kind of behavior (thus, making it more complex), accept such possibility, or disallow end-users to do this.
If you are working with .NET then it is hideously easy to create your own DSL by extending the Boo compiler (i.e. with Rhino.DSL you can have simple DSL with one class).