# Why Functional Programming Matters

## 1. Introduction

传统的解释fp编程的特点, 没有说服力

- no assignment
- no side-effect
- no control flow

## 2.  An Analogy with Structured Programming

结构化编程的优点?  `no goto`?

核心还是结构化编程的模块化设计.

- 小模块能够快速实现
- 模块容易复用
- 模块容易独立测试

fp为组合程序提供了两种更新, 更好的胶水(glue)

## 3. Glueing Functions Together

列表处理为例

listof的类型签名如下:
```haskell
listof X ::= nil | cons X (listof X)  
```

定义一个列表求和函数`sum`

```haskell
sum nil = 0
sum (cons num list) = num + sum list
```

`sum`的这种递归模式, 就是`reduce`.

可以改写为

```haskell
add x y = x + y
sum = reduce add 0
```

`reduce`的实现如下

```haskell
(reduce f x) nil = x
(reduce f x) (cons a l) = f a ((reduce f x) l)
```

同理

```haskell
product = reduce multiply 1
anytrue = reduce or false
alltrue = reduce and true
```

理解`(reduce f a)`的一种方式: 把列表中的`cons`换成`f`, `nil`换成`a`

例如列表[1,2,3]

```haskell
cons 1 (cons 2 (cons 3 nil))
-- reduce add 0
add 1 (add 2 (cons 3 0))
-- reduce multiply 1
multiply 1 (multiply 2 (multiply 3 1))
```