What:
A functional language is a programming language built over and around logical functions or procedures within its programming structure. It is based on and is similar to mathematical functions in its program flow.
函数式语言是在其编程结构中的逻辑函数或过程周围构建的编程语言。它基于并且类似于其程序流程中的数学函数。
include：
Functional languages derive their basic structure from the mathematical framework of Lambda calculus and combinatory logic. Erlang, LISP, Haskell and Scala are the most well-known functional languages.
函数式语言是在其编程结构中的逻辑函数或过程周围构建的编程语言。它基于并且类似于其程序流程中的数学函数。
函数式语言从Lambda演算和组合逻辑的数学框架中得出它们的基本结构。Erlang，LISP，Haskell和Scala是最着名的函数式语言。
Why popular?
Techopedia explains Functional Language
Comprised primarily of functions, functional language emphasizes the semantics, rather than compilation, of a program. Functional language does not have the side effects of traditional imperative styles of programming in that functional language does not change the state of a program and will return the same results until functions are passed with the same arguments.
功能语言主要由函数组成，强调程序的语义而不是编译。功能语言没有传统命令式编程风格的副作用，因为函数式语言不会改变程序的状态，并且在使用相同的参数传递函数之前将返回相同的结果。