what is Procedual, what is declartive?

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happycow 101 May 07, 2005 at 03:19

I’m a non-programmer and I’m curious about the different types of programming, for example procedural and declarative. In my understanding procedural is where you tell the computer what to do step by step. That’s easy enough to understand, but declarative is a “what with no how” language? Where you set up a data base of true statements and procedures then ask the computer a question or something and let it figure out the problem. What i don’t get is how the computer is able to this? I mean how does it know how to reason with what it knows? My second question is, in theory could you make a declarative program with a procedural language ? Is a declarative language just a higher from of a procedural language? I mean the way i see a procedural language at it basics (reminder that i have no programming experience or knowledge) is just operations of what the computer can do, witch would be true false operations, which make up logical operations such as If, or, and, which are used to make add subtraction and divide multiply. So how should I see a declarative language in relations with the possessor? My last question is, is there any other forms of languages besides procedural and declarative?

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Reedbeta 167 May 07, 2005 at 03:37

Yes, you can build a logic-based (declarative) language out of an imperative (procedural) language - and vice versa. All the basic models of computation are essentially equivalent as regards what can and cannot be computed (Church-Turing thesis). However, different programming paradigms can be better suited to different kinds of things. Imperative languages are popular because they most closely model the way the computer actually works at the hardware level, which is basically to execute a sequence of instructions that tell it to do things like store and load data to/from memory, and perform basic math operations on data.

Another programming paradigm besides imperative and logic-based is functional programming, where you define functions (like in math) that take one or more parameters and return an output value. The tricky thing is that functional programming doesn’t allow you to create loops like in imperative programming, so you have to use recursion a lot to get stuff done. This sounds like it would be really cumbersome, but functional programming can actually be a really elegant way to express some algorithms.

I don’t know much about the internal construction of logic-based languages, but I suspect that they are based on some kind of path-finding algorithm. Basically, you can imagine it using an analogy of a road map: all the possible statements (true and false) the system can generate are like intersections, and there are rules that enable you to deduce one statement from another, like roads connecting the intersections. Then, determining whether a statement is true or false is equivalent to determining whether there is a path from some given starting point (the database of true statements) to the statement you are interested in. If there is, the “path” you followed is a proof of the statement. This sort of path-finding problem has been studied a lot by computer scientists and there are a whole load of algorithms to deal with it.

If you are interested in knowing more about formal logical systems and how computers can be programmed to “reason” in them, a couple of good books (oriented for non-programmers) are: The Advent of the Algorithm by David Berlinski and Godel, Escher, Bach by Douglas R. Hofstadter.