问题描述
$ b
这可以通过一个简单的例子来证明吗?
编辑: c> ListT [] 有点不对,我错过了要求内部monad是可交换的。那么,是否有这个要求,还是存在另一个问题,是 ListT
越野车? (Haskell wiki上的全都使用 ListT IO
和 IO
显然是不可交换的。)
如何违反关联法:
v :: Int - > ListT [] Int
v 0 = ListT [[0,1]]
v 1 = ListT [[0],[1]]
main = do
print $ runListT $((v> => v)> = v)0
- = [[0,1,0,0,1],[0,1,1,0, 1],[0,1,0,0],[0,1,0,1],[0,1,1,0],[0,1,1,1]]
print $ runListT $(v> =>(v> = v))0
- = [[0,1,0,0,1],[0,1,0,0],[ 0,1,0,1],[0,1,1,0,1],[0,1,1,0],[0,1,1,1]]
更多示例(主要使用 IO
)以及如何解决 ListT
可以在中找到。
I've seen mentioned that
Can this be demonstrated by a simple example?
Edit: My idea with ListT []
is a bit wrong, I missed that the documentation requires the inner monad to be commutative. So, is ListT
buggy just in the sense that has this requirement, or is there another problem? (The examples at Haskell wiki all use ListT IO
and IO
is obviously not commutative.)
A simple example that shows how it fails the associativity law:
v :: Int -> ListT [] Int
v 0 = ListT [[0, 1]]
v 1 = ListT [[0], [1]]
main = do
print $ runListT $ ((v >=> v) >=> v) 0
-- = [[0,1,0,0,1],[0,1,1,0,1],[0,1,0,0],[0,1,0,1],[0,1,1,0],[0,1,1,1]]
print $ runListT $ (v >=> (v >=> v)) 0
-- = [[0,1,0,0,1],[0,1,0,0],[0,1,0,1],[0,1,1,0,1],[0,1,1,0],[0,1,1,1]]
More examples (mostly using IO
) and a solution how to fix ListT
can be found at ListT done right.
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