为什么下面两个表达式有不同的结果

Question

fibs = 1:1:[x+y|x <- fibs, y <- tail fibs]

返回

[1,1,2,3,4,5,6,7,8,9]

fibs = 1:1:[x+y|(x, y) <- zip fibs (tail fibs)]

返回

[1,1,2,3,5,8,13,21,34,55...]

Answer 1

第一种是评估的笛卡尔乘积（即，每一个组合）fibs和tail fibs，而第二个是评估所述成对配对。

Prelude> [(x,y) | x <- [1,2,3], y <- [4,5,6]]
[(1,4),(1,5),(1,6),(2,4),(2,5),(2,6),(3,4),(3,5),(3,6)]

Prelude> [(x,y) | (x,y) <- [1,2,3] `zip` [4,5,6]]
[(1,4),(2,5),(3,6)]