What is declarative programming? [closed]

Question

I keep hearing this term tossed around in several different contexts. What is it?

Answer 1

imagine an excel page. With columns populated with formulas to calculate you tax return.

All the logic is done declared in the cells, the order of the calculation is by determine by formula itself rather than procedurally.

That is sort of what declarative programming is all about. You declare the problem space and the solution rather than the flow of the program.

Prolog is the only declarative language I've use. It requires a different kind of thinking but it's good to learn if just to expose you to something other than the typical procedural programming language.

Answer 2

A couple other examples of declarative programming:

• ASP.Net markup for databinding. It just says "fill this grid with this source", for example, and leaves it to the system for how that happens.
• Linq expressions

Declarative programming is nice because it can help simplify your mental model* of code, and because it might eventually be more scalable.

For example, let's say you have a function that does something to each element in an array or list. Traditional code would look like this:

foreach (object item in MyList)
{
   DoSomething(item);
}

No big deal there. But what if you use the more-declarative syntax and instead define DoSomething() as an Action? Then you can say it this way:

MyList.ForEach(DoSometing);

This is, of course, more concise. But I'm sure you have more concerns than just saving two lines of code here and there. Performance, for example. The old way, processing had to be done in sequence. What if the .ForEach() method had a way for you to signal that it could handle the processing in parallel, automatically? Now all of a sudden you've made your code multi-threaded in a very safe way and only changed one line of code. And, in fact, there's a an extension for .Net that lets you do just that.

• If you follow that link, it takes you to a blog post by a friend of mine. The whole post is a little long, but you can scroll down to the heading titled "The Problem" _and pick it up there no problem.*

Answer 3

Declarative programming is the picture, where imperative programming is instructions for painting that picture.

You're writing in a declarative style if you're "Telling it what it is", rather than describing the steps the computer should take to get to where you want it.

When you use XML to mark-up data, you're using declarative programming because you're saying "This is a person, that is a birthday, and over there is a street address".

Some examples of where declarative and imperative programming get combined for greater effect:

• Windows Presentation Foundation uses declarative XML syntax to describe what a user interface looks like, and what the relationships (bindings) are between controls and underlying data structures.

• Structured configuration files use declarative syntax (as simple as "key=value" pairs) to identify what a string or value of data means.

• HTML marks up text with tags that describe what role each piece of text has in relation to the whole document.

Answer 4

The other answers already do a fantastic job explaining what declarative programming is, so I'm just going to provide some examples of why that might be useful.

Context Independence

Declarative Programs are context-independent. Because they only declare what the ultimate goal is, but not the intermediary steps to reach that goal, the same program can be used in different contexts. This is hard to do with imperative programs, because they often depend on the context (e.g. hidden state).

Take yacc as an example. It's a parser generator aka. compiler compiler, an external declarative DSL for describing the grammar of a language, so that a parser for that language can automatically be generated from the description. Because of its context independence, you can do many different things with such a grammar:

• Generate a C parser for that grammar (the original use case for yacc)
• Generate a C++ parser for that grammar
• Generate a Java parser for that grammar (using Jay)
• Generate a C# parser for that grammar (using GPPG)
• Generate a Ruby parser for that grammar (using Racc)
• Generate a tree visualization for that grammar (using GraphViz)
• simply do some pretty-printing, fancy-formatting and syntax highlighting of the yacc source file itself and include it in your Reference Manual as a syntactic specification of your language

And many more …

Optimization

Because you don't prescribe the computer which steps to take and in what order, it can rearrange your program much more freely, maybe even execute some tasks in parallel. A good example is a query planner and query optimizer for a SQL database. Most SQL databases allow you to display the query that they are actually executing vs. the query that you asked them to execute. Often, those queries look nothing like each other. The query planner takes things into account that you wouldn't even have dreamed of: rotational latency of the disk platter, for example or the fact that some completely different application for a completely different user just executed a similar query and the table that you are joining with and that you worked so hard to avoid loading is already in memory anyway.

There is an interesting trade-off here: the machine has to work harder to figure out how to do something than it would in an imperative language, but when it does figure it out, it has much more freedom and much more information for the optimization stage.

Answer 5

I'd explain it as DP is a way to express

• A goal expression, the conditions for - what we are searching for. Is there one, maybe or many?
• Some known facts
• Rules that extend the know facts

...and where there is a deduct engine usually working with a unification algorithm to find the goals.

Answer 6

It's a method of programming based around describing what something should do or be instead of describing how it should work.

In other words, you don't write algorithms made of expressions, you just layout how you want things to be. Two good examples are HTML and WPF.

This Wikipedia article is a good overview: http://en.wikipedia.org/wiki/Declarative_programming