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。

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”。

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——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

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。