[英]Scala: Bad inferred type for Option composed with StateT monad transformer
I'm somewhat familiar with Haskell monad transformers, but new to Scalaz (version 7). 我对Haskell monad变换器有点熟悉,但对Scalaz(版本7)不熟悉。 I made (what I thought was) a straightforward translation from the following Haskell code:
我从以下Haskell代码中做了(我认为是)一个简单的翻译:
import Control.Monad.State
type Pass a = StateT String Maybe a
monadTest :: Pass String
monadTest = do
s <- get
return s
to this Scala code: 这个Scala代码:
import scalaz._
import Scalaz._
object StateTest {
type Pass[A] = StateT[Option, String, A]
def monadTest: Pass[String] =
for {
s <- get[String]
} yield s
}
The Haskell code compiles. Haskell代码编译。 The Scala fails to compile, with the following error:
Scala无法编译,出现以下错误:
[error] .../StateTest.scala:9: type mismatch;
[error] found : scalaz.IndexedStateT[scalaz.Id.Id,String,String,String]
[error] required: StateTest.Pass[String]
[error] (which expands to) scalaz.IndexedStateT[Option,String,String,String]
[error] s <- get[String]
[error] ^
First, it seems scalaz implements StateT
in terms of IndexedStateT
. 首先,它似乎scalaz实现
StateT
来讲IndexedStateT
。 Ok. 好。 But, it seems that the
get[String]
monadic value is inferred to have type StateT[Id, String, String]
instead of StateT[Option, String, String]
. 但是,似乎
get[String]
StateT[Id, String, String]
值被推断为具有类型StateT[Id, String, String]
而不是StateT[Option, String, String]
。 Why? 为什么?
I'm using Scala 2.10.1, scalaz 7.0.0. 我正在使用Scala 2.10.1,scalaz 7.0.0。
In your example, the call to get[String]
is calling the get
method of StateFunctions
, reproduced here: 在您的示例中,对
get[String]
的调用是调用StateFunctions
的get
方法,在此处再现:
def get[S]: State[S, S] = init
Where State[S, A]
is an alias for StateT[Id, S, A]
which is an alias for IndexedStateT[Id, S, S, A]
. 其中
State[S, A]
是StateT[Id, S, A]
的别名,它是IndexedStateT[Id, S, S, A]
的别名。
Because you are using StateT
, you need to call get
on an instance of StateTMonadState[S, F]
, or StateTMonadState[String, Option]
in your case. 由于您使用
StateT
,你需要调用get
对一个实例StateTMonadState[S, F]
或StateTMonadState[String, Option]
你的情况。 The working example is: 工作示例是:
import scalaz._
import Scalaz._
object StateTest {
type Pass[A] = StateT[Option, String, A]
val sm = StateT.stateTMonadState[String, Option]
def monadTest: Pass[String] =
for {
s <- sm.get
} yield s
}
The MonadState
instance can also be resolved implicitly via MonadState[F[_, _], S]
but it is less convenient to use due to the type lambda required. MonadState
实例也可以通过MonadState[F[_, _], S]
隐式解析MonadState[F[_, _], S]
但由于需要lambda类型,因此使用起来不太方便。 See MonadState.scala and StateT.scala for more info. 有关详细信息,请参阅MonadState.scala和StateT.scala。
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.