Question

当我用GHC(使用 < code>-Wall 旗)编集下列代码时:

module Main where

data Tree a = EmptyTree | Node a (Tree a) (Tree a) deriving (Show)

insert :: (Ord a) => a -> Tree a -> Tree a
insert x EmptyTree = Node x EmptyTree EmptyTree
insert x (Node a left right)
    | x == a = Node a left right
    | x < a = Node a (insert x left) right
    | x > a = Node a left (insert x right)

main :: IO()
main = do
    let nums = [1..10]::[Int]
    print . foldr insert EmptyTree $ nums

GHC抱怨说, Infement 中匹配的图案并非详尽无遗:

test.hs|6| 1:
||     Warning: Pattern match(es) are non-exhaustive
||              In an equation for `insert : Patterns not matched: _ (Node _ _ _)

GHC为何发出这一警告? 很明显, GHC 抱怨的模式在 < code> Indig x (左键右键) 中处理。

Answer 1

Riccardo是对的,GHC没有推断你的警卫不可能全都是假的。所以请接受他的回答。

我要去跳跃和谈论编码风格。

您不使用 其它 的动机可能是它看起来不明智 :

insert :: (Ord a) => a -> Tree a -> Tree a
insert x EmptyTree = Node x EmptyTree EmptyTree
insert x (Node a left right)
    | x == a    = Node a left right
    | x < a     = Node a (insert x left) right
    | otherwise = Node a left (insert x right)

阅读此代码时, 读者必须亲自确认, 最后卫兵会接受 < code>x & gt; a 的情况。

我们可以写成这样:

insert :: (Ord a) => a -> Tree a -> Tree a
insert x EmptyTree = Node x EmptyTree EmptyTree
insert x (Node a left right) = case x `compare` a of
    EQ -> Node a left right
    LT -> Node a (insert x left) right
    GT -> Node a left (insert x right)

ordering type by compare 返回的 compare 中只有三个值 E/code>, LT 和 GT ,因此GHC可以确认您覆盖了所有可能性,而人类读者可以很容易地看到您正确覆盖了这些可能性。



这也是一种更有效率的代码:我们称 compare  一次,而不是拨,然后可能又拨 。


现在我还要再去谈一些懒惰的事

您可能也写了类似此函数的函数 :

contains :: (Ord a) => a -> Tree a -> Bool
contains _ EmptyTree = False
contains x (Node a left right) = case x `compare` a of
    EQ -> True
    ...


当x =  = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =

但是现在回顾一下我对上面  插入  的定义。 当它给定的树是  Node  时, 它总是返回第一个参数总是  node  的 < code> 。 但是它没有在前面说明这一点: 它评估了  x`compare` a  。

我们可以重写 插入 ,以进行尽可能晚的比较:

insert :: (Ord a) => a -> Tree a -> Tree a
insert x EmptyTree = Node x EmptyTree EmptyTree
insert x (Node a left right) = Node a newLeft newRight
  where comparison = x `compare` a
        newLeft  = if comparison == LT then insert x left  else left
        newRight = if comparison == GT then insert x right else right


现在,我们尽快返回  node a  位数 -- -- 即使比较出错! -- -- 我们最多只能做一次比较 。

Answer 2

这是因为图案匹配不完整。不能保证 xsuble a , x<>a , 或 x>a 中有一个是 NaN。例如, 如果类型是 Double 和 , 那么其中没有一个是 True 。

Answer 3

GHC is not able to infer whether your three guards in the insert x (Node a left right) cover all possible cases, and consequently there will be no body to be associated with insert x (Node a left right). Try replacing the last condition x > a with otherwise (a synonim for True). In this specific case however, it s true that the guards do not cover all cases, see augustss example about double numbers.

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