如何在Haskell中简化此功能？

由顽固不化发布于 2020-05-09 14:32:37

我认为以这种方式编写代码是多余的。无论类型构造器是什么，返回值都相同。有没有办法一次写一次返回值？

data End = Leftend (Int,Int) | Rightend (Int, Int)
            deriving (Eq, Ord, Show)


cmp:: End->End->Ordering
cmp (Leftend (l, h1))  (Rightend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Leftend (l, h1))  (Leftend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Rightend (l, h1))  (Rightend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Rightend (l, h1))  (Leftend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT

ut_et

2020-05-09 14:32:37

我猜...

import Data.Ord

discard :: End -> (Int, Int)
discard (Leftend v) = v
discard (Rightend v) = v

cmp :: End -> End -> Ordering
cmp = comparing (fst . discard)

zsed

2020-05-09 14:32:37

我不确定这是否是您要找的答案。但是我会做的就是这样。

data End = Leftend (Int,Int) | Rightend (Int, Int)
            deriving (Eq, Ord, Show)

first :: End -> Int
first x = case x of
  Rightend (val, _) -> val
  Leftend (val, _) -> val

data Ordering = LT | EQ | GT deriving (Eq, Show)

cmp:: End->End->Ordering
cmp end1 end2
  | x < y = LT
  | x == y = EQ
  | x > y = GT
  where
    x = first end1
    y = first end2

我更喜欢Ocaml之类的东西，其中类型系统可以让您做一些神奇的事情，并且我假设您正在寻找的解决方案。

cmp :: End -> End -> Ordering
cmp (_ (x, _)) (_ (y, _))
  | x < y = LT
  | x == y = EQ
  | x > y = GT

顽固不化

这家伙很懒，什么都没留下

积分
0
话题
0
评论
3106
注册排名
2467