当使用带有模式保护的模式匹配并且所有警告都打开时,我观察到一个有趣的行为
{-# OPTIONS_GHC -Wall #-}
module Mood where
data Mood = Happy
| Indifferent
| Sad
deriving Show
flipMood :: Mood -> Mood
flipMood Happy = Sad
flipMood Indifferent = Indifferent
flipMood Sad = Happy
flipMood' :: Mood -> Mood
flipMood' mood
| Happy <- mood = Sad
| Indifferent <- mood = Indifferent
| Sad <- mood = Happy
即使flipMood
并且flipMood'
几乎在做同样的事情,我也会收到以下错误消息:
Mood.hs:15:1: Warning:
Pattern match(es) are non-exhaustive
In an equation for ‘flipMood'’: Patterns not matched: _
Ok, modules loaded: Mood.
因此需要添加一个 catch all case 比如
| otherwise = mood
来满足穷举检查器。
Core 似乎很好,这两个函数的行为相同:
flipMood =
\ ds_dTh ->
case ds_dTh of _ {
Happy -> Sad;
Indifferent -> Indifferent;
Sad -> Happy
}
flipMood' = flipMood
关闭优化后,我得到以下核心输出,这似乎可以解释这种行为:
flipMood' =
\ mood_axV ->
case mood_axV of wild_X9 {
__DEFAULT ->
case wild_X9 of _ {
Indifferent -> Indifferent;
Sad -> Happy
};
Happy -> Sad
}
为什么会这样?我错过了什么吗?
亲切的问候, raichoo