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如果我有一个祖父母，一个子组件和一个孙子组件，祖父母可以请求孩子的状态吗？我试过像这里一样使用“请求”，但是当你请求一个也有自己的孩子的孩子的状态时，类型不匹配。当我请求没有孩子的孩子的状态时，指南中的示例可以正常工作。

错误是：

Could not match type

  Query

with type

  Coproduct (Coproduct Query (ChildF AnswerSlot Query)) Query
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1 回答 1

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当然是。您可能只是left在对孩子的查询中遗漏了 a - 这是必需的，因为孩子的查询代数将Coproduct f (ChildF p f')具有自己的孩子的形式。

相应地，您也可以从祖父母那里查询孙子，方法是使用right和构造ChildF带有适当值的孙子。

我整理了一个查询孩子和孙子的人为示例，希望可以使事情更清楚：

module Main where

import Prelude

import Data.Functor.Coproduct (Coproduct, left, right)
import Data.Maybe (Maybe(..), fromMaybe)

import Debug.Trace (traceA) -- from purescript-debug

import Halogen as H
import Halogen.HTML as HH

--------------------------------------------------------------------------------

type GrandState = Unit
data GrandQuery a = AskGrandChild (String -> a)

grandchild :: forall g. H.Component GrandState GrandQuery g
grandchild = H.component { render, eval }
  where
  render :: GrandState -> H.ComponentHTML GrandQuery
  render _ = HH.div_ []
  eval :: GrandQuery ~> H.ComponentDSL GrandState GrandQuery g
  eval (AskGrandChild k) = pure $ k "Hello from grandchild"

--------------------------------------------------------------------------------

type ChildState = Unit
data ChildQuery a = AskChild (String -> a)
type GrandSlot = Unit

type ChildState' g = H.ParentState ChildState GrandState ChildQuery GrandQuery g GrandSlot
type ChildQuery' = Coproduct ChildQuery (H.ChildF GrandSlot GrandQuery)

child :: forall g. Functor g => H.Component (ChildState' g) ChildQuery' g
child = H.parentComponent { render, eval, peek: Nothing }
  where
  render :: ChildState -> H.ParentHTML GrandState ChildQuery GrandQuery g GrandSlot
  render _ = HH.slot unit \_ -> { component: grandchild, initialState: unit }
  eval :: ChildQuery ~> H.ParentDSL ChildState GrandState ChildQuery GrandQuery g GrandSlot
  eval (AskChild k) = pure $ k "Hello from child"

--------------------------------------------------------------------------------

type ParentState = Unit
data ParentQuery a = Something a
type ChildSlot = Unit

type ParentState' g = H.ParentState ParentState (ChildState' g) ParentQuery ChildQuery' g ChildSlot
type ParentQuery' = Coproduct ParentQuery (H.ChildF ChildSlot ChildQuery')

parent :: forall g. Functor g => H.Component (ParentState' g) ParentQuery' g
parent = H.parentComponent { render, eval, peek: Nothing }
  where
  render :: ParentState -> H.ParentHTML (ChildState' g) ParentQuery ChildQuery' g ChildSlot
  render _ = HH.slot unit \_ -> { component: child, initialState: H.parentState unit }
  eval :: ParentQuery ~> H.ParentDSL ParentState (ChildState' g) ParentQuery ChildQuery' g ChildSlot
  eval (Something next) = do

    -- note the `left` here
    childAnswer <- H.query unit $ left $ H.request AskChild
    traceA $ fromMaybe "child not found" $ childAnswer

    grandAnswer <- H.query unit $ right $ H.ChildF unit $ H.request AskGrandChild
    traceA $ fromMaybe "grandchild not found" $ grandAnswer

    pure next
于 2016-07-15T10:20:36.850 回答