[英]What is the name of the monadic “versions” of fmap?
Having a background in powershell scripting i first thought it natural to think of function composition in a pipe kind of way. 拥有powershell脚本的背景我首先想到以管道方式思考函数组合是很自然的。 That means the syntax for composition should be
fun1 | fun2 | fun3
这意味着合成的语法应该是
fun1 | fun2 | fun3
fun1 | fun2 | fun3
fun1 | fun2 | fun3
in a psudocode kind of way. fun1 | fun2 | fun3
以一种psudocode的方式。 (where fun[i]
is the i
'th function to apply in order). (
fun[i]
是i
按顺序申请的功能)。 This order of functions is also what you find in haskell monadic binds. 这个函数的顺序也是你在haskell monadic绑定中找到的。
fun1 >>= fun2 >>= fun3
. fun1 >>= fun2 >>= fun3
。
But in other occasions in haskell, the order of functions is more math'y, such as fun3 . fun2 . fun1
但在haskell的其他场合,函数的顺序更像是数学,比如
fun3 . fun2 . fun1
fun3 . fun2 . fun1
fun3 . fun2 . fun1
, or in the functorial setting fmap fun3 . fmap fun2 . fmap fun1
fun3 . fun2 . fun1
,或在functorial设置fmap fun3 . fmap fun2 . fmap fun1
fmap fun3 . fmap fun2 . fmap fun1
fmap fun3 . fmap fun2 . fmap fun1
. fmap fun3 . fmap fun2 . fmap fun1
。
I am very much aware that the functions have different signature in the two examples, but it puzzles me that the structure is reversed, still. 我非常清楚这些函数在两个例子中有不同的签名,但令我感到困惑的是,结构仍然是相反的。 My workaround is to sometimes define a function
mmap = flip (>>=)
such that I can write mmap fun3 . mmap fun2 . mmap fun1
我的解决方法是有时定义一个函数
mmap = flip (>>=)
这样我就可以编写mmap fun3 . mmap fun2 . mmap fun1
mmap fun3 . mmap fun2 . mmap fun1
mmap fun3 . mmap fun2 . mmap fun1
. mmap fun3 . mmap fun2 . mmap fun1
。
So to the question: 那么问题是:
mmap
already defined? mmap
? What is is called? Is there a
mmap
already defined?是否已经定义了
mmap
? What is is called?叫什么叫?
Hoogle is your friend. Hoogle是你的朋友。 The type signature of
(>>=)
is: (>>=)
的类型签名是:
(>>=) :: Monad m => m a -> (a -> m b) -> m b
Hence, you're looking for a function with the type signature: 因此,您正在寻找具有类型签名的函数:
flip (>>=) :: Monad m => (a -> m b) -> m a -> m b
This is in fact the =<<
function. 这实际上是
=<<
函数。 Hence, you can write fun3 =<< fun2 =<< fun1
. 因此,你可以写
fun3 =<< fun2 =<< fun1
。
Why is bind defined as an operator with arguments in an order that feels backwards?
为什么绑定被定义为具有参数的运算符,其顺序感觉倒退?
It's because monadic code looks a lot like imperative code. 这是因为monadic代码看起来很像命令式代码。 For example, consider the following:
例如,请考虑以下事项:
permute2 :: [a] -> [[a]]
permute2 xs = do
x <- xs
xs <- map return xs
return (x:xs)
Without the syntactic sugar of do
it would be written as: 如果没有的语法糖
do
会被写成:
permute2 :: [a] -> [[a]]
permute2 xs =
xs >>= \x ->
map return xs >>= \xs ->
return (x:xs)
See the similarity? 看到相似度? If we used
=<<
instead it would look like: 如果我们使用
=<<
而不是它看起来像:
permute2 :: [a] -> [[a]]
permute2 xs = (\x -> (\xs -> return (x:xs)) =<< map return xs) =<< xs
Not very readable is it? 不是很可读吗?
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.