What's the difference between data and code?

问题:

To take an example, consider a set of discounts available to a supermarket shopper.

We could define these rules as data in some standard fashion (lists of qualifying items, applicable dates, coupon codes) and write generic code to handle these. Or, we could write each as a chunk of code, which checks for the appropriate things given the customer's shopping list and returns any applicable discounts.

You could reasonably store the rules as objects, serialised into Blobs or stored in code files, so that each rule could choose its own division between data and code, to allow for future rules that wouldn't fit the type of generic processor considered above.

It's often easy to criticise code that mixes data in, via if statements that check for 6 different things that should be in a file or a database, but is there a rule that helps in the edge cases?

Or is this the point of Object Oriented design, to stop us worrying about the line between data and code?

To clarify, the underlying question is this: How would you code the above example? Is there a rule of thumb that made you decide what is data and what is code?

(Note: I know, code can be compiled, but in a world of dynamic languages and JIT compilation, even that is a blurry concept.)

回答1:

Fundamentally, there is of course no difference between data and code, but for real software infrastructures, there can be a big difference. Apart from obvious things like, as you mentioned, compilation, the biggest issue is this:

Most sufficiently large projects are designed to produce "releases" that are one big bundle, produced in 3-month (or longer) cycles, tested extensively and cannot be changed afterwards except in tightly controlled ways. "Code" most definitely cannot be changed, so anything that does need to be changed has to be factored out and made "configuration data" so that changing it becomes palatable those whose job it is to ensure that a release works.

Of course, in most cases bad configuration data can break a release just as thoroughly as bad code, so the whole thing is largely an illusion - in reality it doesn't matter whether it's code or "configuration data" that changes, what matters is that the interface between the main system and the parts that change is narrow and well-defined enough to give you a good chance that the person who does the change understands all consequences of what he's doing.

This is already harder than most people think when it's really just a few strings and numbers that are configured (I've personally witnessed a production mainframe system crash because it had one boolean value set differently than another system it was talking to). When your "configuration data" contains complex logic, it's almost impossible to achieve. But the situation isn't going to be any better ust because you use a badly-designed ad hoc "rules configuration" language instead of "real" code.

回答2:

This is a rather philosophical question (which I like) so I'll answer it in a philosophical way: with nothing much to back it up. ;)

Data is the part of a system that can change. Code defines behavior; the way in which data can change into new data.

To put it more accurately: Data can be described by two components: a description of what the datum is supposed to represent (for instance, a variable with a name and a type) and a value. The value of the variable can change according to rules defined in code. The description does not change, of course, because if it does, we have a whole new piece of information. The code itself does not change, unless requirements (what we expect of the system) change.

To a compiler (or a VM), code is actually the data on which it performs its operations. However, the to-be-compiled code does not specify behavior for the compiler, the compiler's own code does that.

回答3:

It all depends on the requirement. If the data is like lookup data and changes frequently you dont really want to do it in code, but things like Day of the Week, should not chnage for the next 200 years or so, so code that.

You might consider changing your topic, as the first thing I thought of when I saw it, was the age old LISP discussion of code vs data. Lucky in Scheme code and data looks the same, but thats about it, you can never accidentally mix code with data as is very possible in LISP with unhygienic macros.

回答4:

Data are information that are processed by instructions called Code. I'm not sure I feel there's a blurring in OOD, there are still properties (Data) and methods (Code). The OO theory encapsulates both into a gestalt entity called a Class but they are still discrete within the Class.

How flexible you want to make your code in a matter of choice. Including constant values (what you are doing by using if statements as described above) is inflexible without re-processing your source, whereas using dynamically sourced data is more flexible. Is either approach wrong? I would say it really depends on the circumstances. As Leppie said, there are certain 'data' points that are invariate, like the days of the week that can be hard coded but even there it may be advantageous to do it dynamically in certain circumstances.

回答5:

In Lisp, your code is data, and your data is code

In Prolog clauses are terms, and terms are clauses.

回答6:

The important note is that you want to separate out the part of your code that will execute the same every time, (i.e. applying a discount) from the part of your code which could change (i.e. the products to be discounted, or the % of the discount, etc.)

This is simply for safety. If a discount changes, you won't have to re-write your discount code, you'll only need to go into your discounts repository (DB, or app file, or xml file, or however you choose to implement it) and make a small change to a number.

Also, if the discount code is separated into an XML file, then you can give the entire application to a manager, and with sufficient instructions, they won't need to pester you whenever they want to change the discount rates.

When you mix in data and code, you are exponentially increasing the odds of breaking when anything changes. So, as leppie said, you need to extract the constantly changing parts, and put them in a separate place.

回答7:

Huge difference. Data is a given to system while code is a part of system.

Wrong data is senseless: our code===handler is good and what you put that you take, it is not a trouble of system that you meant something else. But if code is bad - system is bad.

In example, let's consider some JSON, some bad code parser.js by me and let's say good V8. For my system bad parser.js is a code and my system works wrong. But for Google system my bad parser is data that no how says about quality of V8.

The question is very practical, no sophistic. https://en.wikipedia.org/wiki/Systems_engineering tries to make good answer and money.

回答8:

Data is information. It's not about where you decide to put it, be it a db, config file, config through code or inside the classes.

The same happens for behaviors / code. It's not about where you decide to put it or how you choose to represent it.

回答9:

The line between data and code (program) is blurry. It's ultimately just a question of terminology - for example, you could say that data is everything that is not code. But, as you wrote, they can be happily mixed together (although usually it's better to keep them separate).

回答10:

Code is any data which can be executed. Now since all data is used as input to some program at some point of time, it can be said that this data is executed by a program! Thus your program acts as a virtual machine for your data. Hence in theory there is no difference between data and code!

In the end what matters is software engineering/development considerations like performance, efficiency etc. For example data driven programs may not be as efficient as programs which have hard coded (and hence fragile) conditional statements. Hence I choose to define code as any data which can be efficiently executed and all else being plain data.

It's a tradeoff between flexibility and efficiency. Executable data (like XML rules) offers more flexibility (sometimes) while the same data/rules when coded as part of the application will run more efficiently but changing it frequently becomes cumbersome. In other words executable data is easy to deploy but is inefficient and vice-versa. So ultimately the decision rests with you - the software designer.

Please correct me if I wrong.

回答11:

Relationship between code and data is as follows:

code after compiled to a program processes the data while execution

program can extract data, transform data, load data, generate data ...

Also program can extract code, transform code, load code, generate code tooooooo...

Hence code without compiled or interperator is useless, data is always worth..., but code after compiled can do all the above activities....

For eg)

Sourcecontrolsystem process Sourcecodes

here source code itself is a code

Backupscripts process files

here files is a data and so on...

回答12:

I would say that the distinction between data, code and configuration is something to be made within the context of a particular component. Sometimes it's obvious, sometimes less so.

For example, to a compiler, the source code it consumes and the object code it creates are both data - and should be separated from the compiler's own code.

In your case you seem to be describing the option of a particularly powerful configuration file, which can contain code. Much as, for example, the GIMP lets you 'configure' plugins using Scheme. As the developer of the component that reads this configuration, you would think of it as data. When working at a different level -- writing the configuration -- you would think of it as code.

This is a very powerful way of designing.

Applying this to the underlying question ("How would you code the above example?"), one option might be to adopt or design a high level Domain Specific Language (DSL) for specifying rules. At startup, or when first required, the server reads the rule and executes it.

Provide an admin interface allowing the administrator to

• test a new rule file
• replace the current configuration with that from a new rule file

... all of which would happen at runtime.

A DSL might be something as simple as a table parser or an XML parser, or it could be something as sophisticated as a scripting language. From C, it's easy to embed Python or Lua. From Java it's easy to embed Groovy or Clojure.

You could switch in compiled code at runtime, with clever linking or classloader tricks. This seems more difficult and less valuable than the embedded DSL option, in my opinion.

回答13:

The best practical answer to this question I found is this: Any class that needs to be serialized, now or in any foreseeable future, is data. Everything else is code. That's why, for example, Java's HashMap is data - although it has a lot of code, API methods and specific implementation (i.e., it might look as code at first glance).