问题描述
在Haskell中,你可以像下面这样使用绑定操作符(>> =
): repli :: [a] - > [a]
repli xs = xs>> = \ x - > [x,x]
* Main> repli [1,2,3]
[1,1,2,2,3,3]
我读过 flatMap
是Scala的绑定操作符:
def repli [A](xs:List [A]):List [A] =
xs.flatMap {x => List(x,x)}
scala> repli(List(1,2,3))
res0:List [Int] = List(1,2,2,3,3)
作为教学练习中,我试图增加对>支撑;> =
来的Scala:
class MyList [T](list:List [T]){
def>> = [U] (F:T =>列表[U]):列表[U] = list.flatMap(F)
}
隐式DEF list2mylist [T](列表:列表[T])=新MYLIST (列表)
def repNeNew [A](xs:List [A]):List [A] =
xs>> = {x:A => List(x,x)}
scala> repliNew(List(1,2,3))
res1:List [Int] = List(1,2,2,3,3)
这个功能完美,但仅适用于列表。我真的很想用 flatMap
方法来支持任何类。最好的方法是什么?
特征MA [M [_],A (隐含m:Monad [M]):M [B] =(f:A => M [B])
m.bind(value,f)
}
所有 M [_]
M [A]
至 MA [M,A]
/ code>和 A
:
隐式def ma [ M [_],A](m:=> M [A]):MA [M,A] =新MA [M,A] {
懒值值= m
}
您只需要一个特征 Monad
和一个它为你关心的每一个monad:
pre $ $ code $特性Monad [M [_]] {
def pure [A] (A:= A):M [A]
def bind [A,B](m:M [A],f:A => M [B]):M [B]
}
In Haskell, you can use the bind operator (>>=
) like this:
repli :: [a] -> [a]
repli xs = xs >>= \x -> [x,x]
*Main> repli [1,2,3]
[1,1,2,2,3,3]
I've read that flatMap
is Scala's bind operator:
def repli [A](xs: List[A]): List[A] =
xs.flatMap { x => List(x,x) }
scala> repli (List(1,2,3))
res0: List[Int] = List(1, 1, 2, 2, 3, 3)
As a pedagogic exercise, I'm trying to add support for >>=
to Scala:
class MyList[T](list: List[T]) {
def >>= [U](f: T => List[U]): List[U] = list.flatMap(f)
}
implicit def list2mylist[T](list: List[T]) = new MyList(list)
def repliNew [A](xs: List[A]): List[A] =
xs >>= { x: A => List(x,x) }
scala> repliNew (List(1,2,3))
res1: List[Int] = List(1, 1, 2, 2, 3, 3)
This works perfectly, but only for Lists. I really want to support any class with a flatMap
method. Whats the best way to go about this?
Scalaz does it as follows:
trait MA[M[_], A] {
def value: M[A]
def >>=(f: A => M[B])(implicit m: Monad[M]): M[B] =
m.bind(value, f)
}
With implicit conversions from M[A]
to MA[M, A]
for all M[_]
and A
:
implicit def ma[M[_], A](m: => M[A]): MA[M, A] = new MA[M, A] {
lazy val value = m
}
You just need a trait Monad
and an instance of it for every monad you care about:
trait Monad[M[_]] {
def pure[A](a: => A): M[A]
def bind[A, B](m: M[A], f: A => M[B]): M[B]
}
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