问题描述
我正在尝试将 combineReducers
从 Redux 移植到 Scala.这个想法是每个函数控制它的一小部分状态,combineReducers
创建一个控制整个状态的函数.我无法弄清楚应该像这样工作的函数所需的签名:
I'm trying to port combineReducers
from Redux to Scala. The idea is that each function controls it's small part of the state and combineReducers
creates a function that controls the whole state. I can't figure out required signature for the function that should work something like this:
sealed trait Event
case class Create() extends Event
case class Save() extends Event
object Reducers {
def combineReducers[E, S1, S2](fTag: String, f: (E, S1) => S1, gTag: String, g: (E, S2) => S2) = ???
type IntState = Int
type StringState = String
def intReducer(e: Event, state: IntState): IntState = state + 1
def stringReducer(e: Event, state: StringState): StringState = state + e.toString
val reducer = combineReducers("count", intReducer, "names", stringReducer)
val initialState = ("count" ->> 0 ) :: ("names" ->> "") :: HNil
val newState = reducer(Create(), initialState) // returns ("count" ->> 1) :: ("names" ->> "Create()") :: HNil
reducer(Save(), newState) // returns ("count" ->> 2) :: ("names" ->> "Create()Save()") :: HNil
}
推荐答案
尝试
import shapeless.labelled.{FieldType, field}
import shapeless.{::, HNil}
import shapeless.syntax.singleton._
sealed trait Event
case class Create() extends Event
case class Save() extends Event
object Reducers {
type Reducer[E, S] = (E, S) => S
def combineReducers[K1 <: String, E, S1, K2 <: String, S2](
fTag: K1,
f: Reducer[E, S1],
gTag: K2,
g: Reducer[E, S2]
): Reducer[E, FieldType[K1, S1] :: FieldType[K2, S2] :: HNil] =
{ case (e, v1 :: v2 :: HNil) => field[K1](f(e, v1)) :: field[K2](g(e, v2)) :: HNil }
type IntState = Int
type StringState = String
def intReducer(e: Event, state: IntState): IntState = state + 1
def stringReducer(e: Event, state: StringState): StringState = state + e.toString
val reducer: Reducer[Event, FieldType[Witness.`"count"`.T, IntState] :: FieldType[Witness.`"names"`.T, StringState] :: HNil] =
combineReducers("count".narrow, intReducer, "names".narrow, stringReducer)
val initialState = ("count" ->> 0 ) :: ("names" ->> "") :: HNil
val newState = reducer(Create(), initialState) // returns ("count" ->> 1) :: ("names" ->> "Create()") :: HNil
val newState1 = reducer(Save(), newState) // returns ("count" ->> 2) :: ("names" ->> "Create()Save()") :: HNil
}
我们应该用显式类型注释reducer
,因为编译器无法推断combineReducers
中的E
.如果您将签名更改为
We should annotate reducer
with explicit type because compiler cant't infer E
in combineReducers
. If you change signature to
def combineReducers[K1 <: String, S1, K2 <: String, S2](
fTag: K1,
f: Reducer[Event, S1],
gTag: K2,
g: Reducer[Event, S2]
): Reducer[Event, FieldType[K1, S1] :: FieldType[K2, S2] :: HNil] = ...
然后你就可以写了
val reducer = combineReducers("count".narrow, intReducer, "names".narrow, stringReducer)
为了组合任意数量的 reducer,你可以创建类型类
For combining arbitrary number of reducers you can create type class
trait CombineReducers[Keys <: HList, Reducers <: HList] {
type Record <: HList
def apply(keys: Keys, reducers: Reducers): Reducer[Event, Record]
}
object CombineReducers {
type Aux[Keys <: HList, Reducers <: HList, Record0 <: HList] = CombineReducers[Keys, Reducers] { type Record = Record0 }
def instance[Keys <: HList, Reducers <: HList, Record0 <: HList](f: (Keys, Reducers) => Reducer[Event, Record0]): Aux[Keys, Reducers, Record0] = new CombineReducers[Keys, Reducers] {
type Record = Record0
override def apply(keys: Keys, reducers: Reducers): Reducer[Event, Record0] = f(keys, reducers)
}
implicit val hnilCombineReducers: Aux[HNil, HNil, HNil] = instance { case (HNil, HNil) => { case (_, HNil) => HNil }}
implicit def hconsCombineReducers[K <: String, Ks <: HList, S, Rs <: HList](implicit
cr: CombineReducers[Ks, Rs]): Aux[K :: Ks, Reducer[Event, S] :: Rs, FieldType[K, S] :: cr.Record] = instance {
case (k :: ks, r :: rs) => {
case (e, s :: ss) => field[K](r(e, s)) :: cr(ks, rs)(e, ss)
}
}
}
并使用它
def combineReducers[Keys <: HList, Reducers <: HList](keys: Keys, reducers: Reducers)(implicit
cr: CombineReducers[Keys, Reducers]): Reducer[Event, cr.Record] = cr(keys, reducers)
val reducer =
combineReducers("count".narrow :: "names".narrow :: HNil, (intReducer: (Event, IntState) => IntState) :: (stringReducer : (Event, StringState) => StringState) :: HNil)
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