What is the difference between declarative and imp

Question

I have been searching the web looking for a definition for declarative and imperative programming that would shed some light for me. However, the language used at some of the resources that I have found is daunting - for instance at Wikipedia. Does anyone have a real-world example that they could show me that might bring some perspective to this subject (perhaps in C#)?

Answer 1

I liked an explanation from a Cambridge course + their examples:

• Declarative - specify what to do, not how to do it
  • E.g.: HTML describes what should appear on a web page, not how it should be drawn on the screen
• Imperative - specify both what and how
  • int x; - what (declarative)
  • x=x+1; - how

Answer 2

I'll add another example that rarely pops up in declarative/imperative programming discussion: the User Interface!

In C#, you can build an UI using various technologies.

On the imperative end, you could use DirectX or OpenGL to very imperatively draw your buttons, checkboxes, etc... line-by-line (or really, triangle by triangle). It is up to you to say how to draw the user interface.

At the declarative end, you have WPF. You basically write some XML (yeah, yeah, "XAML" technically) and the framework does the work for you. You say what the user interface looks like. It is up to the system to figure out how to do it.

Anyway, just another thing to think about. Just because one language is declarative or imperative does not mean that it doesn't have certain features of the other.

Also, one benefit of declarative programming is that purpose is usually more easily understood from reading the code whereas imperative gives you finer control over execution.

The gist of it all:

Declarative -> what you want done

Imperative -> how you want it done

Answer 3

In computer science, declarative programming is a programming paradigm that expresses the logic of a computation without describing its control flow.

From http://en.wikipedia.org/wiki/Declarative_programming

in a nutshell the declarative language is simpler because it lacks the complexity of control flow ( loops, if statements, etc. )

A good comparison is the ASP.Net 'code-behind' model. You have declarative '.ASPX' files and then the imperative 'ASPX.CS' code files. I often find that if I can do all I need in the declarative half of the script a lot more people can follow what's being done.

Answer 4

declarative program is just a data for its some more-or-less "universal" imperative implementation/vm.

pluses: specifying just a data, in some hardcoded (and checked) format, is simpler and less error-prone than specifying variant of some imperative algorithm directly. some complex specifications just cant be written directly, only in some DSL form. best and freq used in DSLs data structures is sets and tables. because you not have dependencies between elements/rows. and when you havent dependencies you have freedom to modify and ease of support. (compare for example modules with classes - with modules you happy and with classes you have fragile base class problem) all goods of declarativeness and DSL follows immediately from benefits of that data structures (tables and sets). another plus - you can change implementation of declarative language vm, if DSL is more-or-less abstract (well designed). make parallel implementation, for example. or port it to other os etc. all good specifed modular isolating interfaces or protocols gives you such freedom and easyness of support.

minuses: you guess right. generic (and parameterized by DSL) imperative algorithm/vm implementation may be slower and/or memory hungry than specific one. in some cases. if that cases is rare - just forget about it, let it be slow. if it's frequient - you always can extend your DSL/vm for that case. somewhere slowing down all other cases, sure...

P.S. Frameworks is half-way between DSL and imperative. and as all halfway solutions ... they combines deficiences, not benefits. they not so safe AND not so fast :) look at jack-of-all-trades haskell - it's halfway between strong simple ML and flexible metaprog Prolog and... what a monster it is. you can look at Prolog as a Haskell with boolean-only functions/predicates. and how simple its flexibility is against Haskell...