[英]What does “ap” of \/ in Scalaz do?
I am looking at disjunction type of scalaz and I noticed method ap
我在看脱节型scalaz的,我注意到法
ap
/** Apply a function in the environment of the right of this disjunction. */ def ap[AA >: A, C](f: => AA \/ (B => C)): (AA \/ C) = f flatMap (ff => map(ff(_)))
I guess I understand what it does. 我想我明白它的作用。 Now I wonder when and why one should actually use it ?
现在我想知道何时以及为什么要实际使用它? Are there any examples of using this
ap
function ? 有没有使用这个
ap
功能的例子?
The disjunction you are looking for: 您正在寻找的分离:
import scalaz.{ \/, -\/ \/-, EitherT }
import scalaz.syntax.ToIdOps
object Testing extends ToIdOps // left and right methods come from there {
// say you have the following method
def someMethod(flag: Boolean): \/[Exception, SomeObject] {
if (flag) someObj.right else new Exception("this is a sample").left
}
}
// pattern matching
val x = someMethod match {
case \/-(right) => // this is someObject
case -\/(err) => // deal with the error
}
// catamorphism
def methodThatDealsWithObj(obj: someObject)
def methodThatDealsWithErr(err: Exception)
someMethod.fold(methodThatDealsWithObj)(methodThatDealsWithErr)
// for comprehensions
// ap behaves just like EitherT.
for {
correctResponse <- EitherT(someMethod)
}
Update 更新
To understand how EitherT
and ap
works, think of an Option
, which has Some
and None
and potential matches. 要了解
EitherT
和ap
工作原理,请考虑一个Option
,它有Some
和None
以及潜在的匹配。 With an Option
, you would do: 使用
Option
,您可以:
for {
a <- someOption
} yield ..
With scalaz.\\/
, you usually put an Exception
on the left and a "correct" return type on the right. 使用
scalaz.\\/
,您通常在左侧放置一个Exception
,在右侧放置一个“正确”的返回类型。 ap
is a function that says apply this if the either has the correct type. ap
是一个函数,如果要么具有正确的类型,则应用此函数。
for {
correctResponse <- ap(someEitherReturnMethod)
}
Use cases 用例
The most common things I can think off where I use them avidly is complex asynchronous flows, such as OAuth1 or OAuth2, where I care about fine grained chaining of errors. 我能想到的最常见的事情就是复杂的异步流程,例如OAuth1或OAuth2,我关心错误的细粒度链接。
You can use \\/
as the return of a Future
: 您可以使用
\\/
作为Future
的回报:
def someComplexThirdPartyApiCall: Future[\/[Exception, CorrectReturn]] = {
}
Because you can flatMap
over futures, you can chain a couple methods like the above, collect and propagate errors. 因为你可以对期货进行
flatMap
,你可以链接一些像上面这样的方法,收集和传播错误。
Example 例
def method1: Future[\/[Exception, String]]
def method2(result: String): Future[\/[Exception, String]]
def chainExample: Future[\/[Exception, Int]] = {
for {
firstResult <- EitherT(method1)
secondResult <- EitherT(method2(firstResult))
} yield secondResult.toInt
}
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