列表的所有元素*列表

Question

我正在尝试用2个这样的列表来做元组列表：

["a", "b", "c"] ["a", "b", "c"]

对此：

[("a", "a"), ("a", "b"),("a", "c")], [("b", "a"), ("b", "b"), ("b", "c"), ("c", "a"), ("c", "b"), ("a", "c")]

所以我正在尝试，但是我不知道该如何做：

f [] [] = [[]]
f (x:xs) y =
         zip (repeat x) y: f xs y

Answer 1

要进行显式递归，很难击败：

f :: [a] -> [a] -> [[(a, a)]]
f [] _       = []
f _ []       = []
f (x:xs) yss = go x yss : f xs yss
  where
  go _ []     = []
  go x (y:ys) = (x, y): go x ys

但是没有理由这样做。这是列表理解的用例！

f xs ys = [[(x, y) | y <- ys] | x <- xs]