Haskell趣学指南-学习笔记（3）-1

ghci> let xxs = [[1,3,5,2,3,1,2,4,5],[1,2,3,4,5,6,7,8,9],[1,2,4,2,1,6,3,1,3,2,3,6]]
ghci> [ [ x | x <- xs, even x ] | xs <- xxs]
[[2,2,4],[2,4,6,8],[2,4,2,6,2,6]]

分解：

项	含义，内容
let	定义一个常量
xxs	[[ 1 , 3 , 5 , 2 , 3 , 1 , 2 , 4 , 5 ],[ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 ],[ 1 , 2 , 4 , 2 , 1 , 6 , 3 , 1 , 3 , 2 , 3 , 6 ]]1个整体
xs	[1,3,5,2,3,1,2,4,5]，[1,2,3,4,5,6,7,8,9]，[1,2,4,2,1,6,3,1,3,2,3,6]3个个体
x	[1,3,5,2,3,1,2,4,5]，[1,2,3,4,5,6,7,8,9]，[1,2,4,2,1,6,3,1,3,2,3,6]3个个体中的每一个数值
even	判断是否为偶数