简而言之，一种编程风格越是强调“做什么”(What (to do))而忽略了“如何做”(How (to do))的细节，这种风格就越被认为是声明性的。而imperative则相反。函数式编程与声明式风格相关联。

简而言之，一种编程风格越是强调“做什么”(What (to do))而忽略了“如何做”(How (to do))的细节，这种风格就越被认为是声明性的。而imperative则相反。函数式编程与声明式风格相关联。

命令式编程是指任何一种编程风格，其中你的程序是由描述计算机执行的操作如何发生的指令构成的。

声明式编程是指任何一种编程风格，其中你的程序是对问题或解决方案的描述，但没有显式地说明如何完成工作。

函数式编程是通过计算函数和函数的函数来编程。函数式编程(严格定义)是指通过定义无副作用的数学函数来编程，因此它是一种声明式编程，但它不是唯一的声明式编程。

逻辑编程(例如在Prolog中)是另一种形式的声明式编程。它包括通过决定一个逻辑语句是否为真(或是否可以满足它)来计算。程序通常是一系列事实和规则——即描述，而不是一系列指令。

术语重写(例如CASL)是声明式编程的另一种形式。它涉及代数项的符号变换。它完全不同于逻辑编程和函数式编程。

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

这里有一些关于所提到的“类型”的好答案。

我还提供了一些额外的、更“奇特”的概念，通常与函数式编程人群有关:

领域特定语言或DSL编程:创建一种新的语言来处理手头的问题。 元编程:当你的程序编写其他程序时。 进化编程:你构建一个系统，它可以不断地自我改进，或者不断地生成更好的子程序。

命令式和声明式描述了两种相反的编程风格。命令式是传统的“循序渐进”方法，而声明式则更多地是“这就是我想要的，现在你来研究如何去做”。

这两种方法贯穿于整个编程过程——即使使用相同的语言和相同的程序。一般来说，声明式方法被认为是更可取的，因为它使程序员不必指定如此多的细节，同时也减少了出现错误的机会(如果您描述了您想要的结果，并且一些经过良好测试的自动流程可以从该结果向后工作以定义步骤，那么您可能希望事情比手工指定每个步骤更可靠)。

另一方面，命令式方法为您提供了更多的低级控制——这是编程的“微观管理方法”。这可以让程序员利用有关问题的知识来给出更有效的答案。因此，程序的某些部分以声明式的风格编写并不罕见，但对速度至关重要的部分则更加必要。

as you might imagine, the language you use to write a program affects how declarative you can be - a language that has built-in "smarts" for working out what to do given a description of the result is going to allow a much more declarative approach than one where the programmer needs to first add that kind of intelligence with imperative code before being able to build a more declarative layer on top. so, for example, a language like prolog is considered very declarative because it has, built-in, a process that searches for answers.

so far, you'll notice that i haven't mentioned functional programming. that's because it's a term whose meaning isn't immediately related to the other two. at its most simple, functional programming means that you use functions. in particular, that you use a language that supports functions as "first class values" - that means that not only can you write functions, but you can write functions that write functions (that write functions that...), and pass functions to functions. in short - that functions are as flexible and common as things like strings and numbers.

it might seem odd, then, that functional, imperative and declarative are often mentioned together. the reason for this is a consequence of taking the idea of functional programming "to the extreme". a function, in it's purest sense, is something from maths - a kind of "black box" that takes some input and always gives the same output. and that kind of behaviour doesn't require storing changing variables. so if you design a programming language whose aim is to implement a very pure, mathematically influenced idea of functional programming, you end up rejecting, largely, the idea of values that can change (in a certain, limited, technical sense).

如果你这样做——如果你限制变量的改变方式——那么你几乎会意外地迫使程序员编写更具声明性的程序，因为命令式编程的很大一部分是描述变量如何改变，而你再也不能这样做了!因此，函数式编程——特别是用函数式语言编程——倾向于给出更多的声明性代码。

总结一下:

命令式和声明式是两种相反的编程风格(鼓励这些风格的编程语言使用相同的名称) 函数式编程是一种编程风格，其中函数变得非常重要，因此，更改值变得不那么重要。在值中指定更改的有限能力迫使使用更声明性的风格。

因此，“函数式编程”通常被描述为“声明式的”。