高阶函数，解析模式错误：xs

Question

这是我的代码：

-- combine lists with binary operation

clwbo :: (Num a, Num b, Num c) => (a -> b -> c) -> [a] -> [b] -> [c] 
clwbo fps lista listb
    |length lista /= length listb = []
    |length lista ==length listb = case lista listb of
                                       [] [] -> []
                                       x:xs y:ys -> [fps x y] ++ clwbo fps xs ys
                                       otherwise -> error "get off"

这是来自终端的错误消息：

test.hs:8:42: Parse error in pattern: xs
Failed, modules loaded: none.

有人愿意告诉我我的代码有什么问题吗？

Answer 1

您不能像在case 表达式中那样将多个模式并排放置。要一次匹配多个模式，请将它们放在一个元组中：

case (lista, listb) of
   ([], [])     -> ...
   (x:xs, y:ys) -> ...
   otherwise    -> ...