Question

最后三行产生错误。如何在不出错的情况下获取这些表达式的类型？拜托，谢谢你。

--
-- pairs of Int
--

data Pair = P Int Int deriving (Show, Eq)

pairFst (P x y ) = x
pairSnd (P x y ) = y

instance Ord Pair where
  compare (P x1 y1) (P x2 y2) = 
    case compare x1 x2 of EQ -> compare y1 y2
                          LT -> LT
                          GT -> GT
--
-- pairs of things of type a
--

data Pair a = P a a deriving (Show, Eq)

pairFst (P x y ) = x
pairSnd (P x y ) = y

-- ordering on same kinds of pairs pairs of the same type of thing

instance Ord a => Ord (Pair a) where
  compare (P x1 y1) (P x2 y2) = 
    case compare x1 x2 of EQ -> compare y1 y2
                          LT -> LT
                          GT -> GT
--
-- pairs of things of type a and b
--

data Pair a b = P a b deriving (Show, Eq)

pairFst (P x y ) = x
pairSnd (P x y ) = y

-- ordering on same kinds of pairs pairs of the same type of thing

instance (Ord a, Ord b) => Ord (Pair a b) where
  compare (P x1 y1) (P x2 y2) = 
    case compare x1 x2 of EQ -> compare y1 y2
                          LT -> LT
                          GT -> GT
--
-- pairs of things of type a and b, automatic deriving
--

data Pair a b = P a b deriving (Show, Eq, Ord)

pairFst (P x y ) = x
pairSnd (P x y ) = y

:t (<) P 1 2
:t (<) P 1 'a'
:t (<) P 1 "a"

Answer 1

您无法让编译器本身在编译时轻松地显示类型，并且类型已在运行时擦除。相反，使用GHCi。从文件中删除有问题的:t行，然后运行

$ ghci WhateverFile.hs
 *> :t (<) P 1 2
 ...

作为一个提示，答案是“没什么，这是一个类型错误”，因为你忘记了parens。我想你想要

(<) (P 1 2)

应该具有

类型

 (Num a, Num b) => P a b -> Bool

或它的一些不太通用的版本。

Haskell类型导致“输入中的解析错误：'”

1 个答案: