我很困惑。我可以这样写:
import Control.Monad
main = print $ head $ (foldr (.) id [f, g]) [3]
where f = (1:)
g = undefined
输出是1
。这是有道理的,因为它简化为:
main = print $ head $ ((1:) . undefined . id) [3]
main = print $ head $ (1:) ((undefined . id) [3])
main = print $ head $ 1 : ((undefined . id) [3])
main = print $ 1
但如果我使用一种模糊相似的单子技术,它的工作原理就不一样了:
import Control.Monad
main = print $ (foldr (<=<) return [f, g]) 3
where f = const Nothing
g = undefined
这命中prelude.Undefined
。这很奇怪,因为我希望它会减少:
main = print $ ((const Nothing) <=< undefined <=< return) 3
main = print $ return 3 >>= undefined >>= (\_ -> Nothing)
main = print $ Nothing -- nope! instead, undefined makes this blow up
但是,翻转合成顺序:
import Control.Monad
main = print $ (foldr (>=>) return [f, g]) 3
where f = const Nothing
g = undefined
确实完成了预期的短路并产生Nothing
.
main = print $ (const Nothing >=> undefined >=> return) 3
main = print $ (const Nothing 3) >>= undefined >>= return
main = print $ Nothing >>= undefined >>= return
main = print $ Nothing
我想比较这两种方法可能是比较苹果和橙子,但你能解释一下区别吗?我认为这f <=< g
是 monadic 的类似物f . g
,但它们显然不像我想象的那样类似。你能解释一下为什么吗?