命令式编程:告诉“机器”如何做某事，结果你想发生的事情就会发生。

声明式编程:告诉“机器”你想要发生什么，然后让计算机想出如何去做。

命令式的例子

function makeWidget(options) {
    const element = document.createElement('div');
    element.style.backgroundColor = options.bgColor;
    element.style.width = options.width;
    element.style.height = options.height;
    element.textContent = options.txt;

    return element;
}

声明性的例子

function makeWidget(type, txt) {
    return new Element(type, txt);
}

注:区别不在于简洁、复杂或抽象。如前所述，区别在于如何做和做什么。

当务之急:如何实现我们的目标

   Take the next customer from a list.
   If the customer lives in Spain, show their details.
   If there are more customers in the list, go to the beginning

声明性:我们想要达到的目标

   Show customer details of every customer living in Spain

命令式编程:告诉“机器”如何做某事，结果你想发生的事情就会发生。

声明式编程:告诉“机器”你想要发生什么，然后让计算机想出如何去做。

命令式的例子

function makeWidget(options) {
    const element = document.createElement('div');
    element.style.backgroundColor = options.bgColor;
    element.style.width = options.width;
    element.style.height = options.height;
    element.textContent = options.txt;

    return element;
}

声明性的例子

function makeWidget(type, txt) {
    return new Element(type, txt);
}

注:区别不在于简洁、复杂或抽象。如前所述，区别在于如何做和做什么。

现在，新的焦点是:我们需要旧的分类吗?

命令式/声明式/函数式在过去很好地分类了泛型语言，但是现在所有的“大语言”(如Java、Python、Javascript等)都有一些选项(通常是框架)来表达其主要关注点(通常是命令式)之外的“其他关注点”，并表达并行进程、声明式函数、lambdas等。

所以这个问题的一个很好的变体是“今天对框架进行分类的优点是什么?” ．.． 一个重要的方面是我们可以标记为“编程风格”……

重点研究数据与算法的融合

这是一个很好的例子。你可以在维基百科上读到jQuery，

jQuery的一系列核心特性——DOM元素选择、遍历和操作——由它的选择器引擎(…)支持，创建了一种新的“编程风格”，融合了算法和DOM数据结构

因此jQuery是关注“新编程风格”的最佳(流行)例子，它不仅是面向对象的，而且是“融合算法和数据结构”。jQuery有点像电子表格，但不是“面向单元格”，是“面向dom节点”…比较本文中的主要风格:

No fusion: in all "big languages", in any Functional/Declarative/Imperative expression, the usual is "no fusion" of data and algorithm, except by some object-orientation, that is a fusion in strict algebric structure point of view. Some fusion: all classic strategies of fusion, in nowadays have some framework using it as paradigm... dataflow, Event-driven programming (or old domain specific languages as awk and XSLT)... Like programming with modern spreadsheets, they are also examples of reactive programming style. Big fusion: is "the jQuery style"... jQuery is a domain specific language focusing on "fusing algorithms and DOM-data-structures". PS: other "query languages", as XQuery, SQL (with PL as imperative expression option) are also data-algorith-fusion examples, but they are islands, with no fusion with other system modules... Spring, when using find()-variants and Specification clauses, is another good fusion example.

命令式——表达式描述要执行的动作序列(关联的)

声明性——表达式是对程序行为做出贡献的声明(关联、交换、幂等、单调)

函数式表达式的值仅为效果;语义支持等式推理

声明性编程是通过在输入和输出之间表达一些永恒的逻辑来编程，例如，在伪代码中，下面的例子将是声明性的:

def factorial(n):
  if n < 2:
    return 1
  else:
    return factorial(n-1)

output = factorial(argvec[0])

We just define a relationship called the 'factorial' here, and defined the relationship between the output and the input as the that relationship. As should be evident here, about any structured language allows declarative programming to some extend. A central idea of declarative programming is immutable data, if you assign to a variable, you only do so once, and then never again. Other, stricter definitions entail that there may be no side-effects at all, these languages are some times called 'purely declarative'.

在命令式风格中，同样的结果将是:

a = 1
b = argvec[0]
while(b < 2):
  a * b--

output = a

在这个例子中，我们没有在输入和输出之间表达永恒的静态逻辑关系，我们手动更改内存地址，直到其中一个保存所需的结果。很明显，所有语言在某种程度上都允许声明性语义，但并非所有语言都允许命令式语义，一些“纯”声明性语言允许副作用和突变。

Declarative languages are often said to specify 'what must be done', as opposed to 'how to do it', I think that is a misnomer, declarative programs still specify how one must get from input to output, but in another way, the relationship you specify must be effectively computable (important term, look it up if you don't know it). Another approach is nondeterministic programming, that really just specifies what conditions a result much meet, before your implementation just goes to exhaust all paths on trial and error until it succeeds.

纯声明性语言包括Haskell和Pure Prolog。从一种到另一种的比例可以是:Pure Prolog, Haskell, OCaml, Scheme/Lisp, Python, Javascript, C——，Perl, PHP, c++， Pascall, C, Fortran, Assembly