1. What functional programming is and what Haskell is
What is imperative language?
--Give a computer sets of tasks and excute it
What is Functional Programming?
--Don't tell a computer what to do but tell it what a stuff you want to excute is
What is Haskell?
Haskell is a purely functional programming.
2. Properties of Functional programming.
(1). Function has not side effects.
The only thing a function can do is calculate something and return it as a result. One consequence of absence of side effect is Single assignment: A defined variable can not be redefined.
(2). Lazy execution
Functions will not be executed and calculated until its really need to be shown.
来源：https://www.cnblogs.com/Joshua1989/p/3670748.html