用 andThen 链接消息

andThen 函数允许更新调用组合。可以与管道运算符（|>）一起使用以进行链接更新。

示例：你正在制作文档编辑器，并且你希望将每个修改消息发送到文档，还要保存它：

import Update.Extra exposing (andThen)
import Update.Extra.Infix exposing (..)

-- type alias Model = [...]

type Message
    = ModifyDocumentWithSomeSettings
    | ModifyDocumentWithOtherSettings
    | SaveDocument

update : Model -> Message -> (Model, Cmd)
update model msg =
    case msg of
        ModifyDocumentWithSomeSettings ->
            -- make the modifications
            (modifiedModel, Cmd.none)
            |> andThen SaveDocument
        ModifyDocumentWithOtherSettings ->
            -- make other modifications
            (modifiedModel, Cmd.none)
            |> andThen SaveDocument
        SaveDocument ->
            -- save document code

如果你也导入 Update.Extra.Infix exposing (..)，你可以使用中缀运算符：

update : Model -> Message -> (Model, Cmd)
update model msg =
    case msg of
        ModifyDocumentWithSomeSettings ->
            -- make the modifications
            (modifiedModel, Cmd.none)
            :> andThen SaveDocument
        ModifyDocumentWithOtherSettings ->
            -- make other modifications
            (modifiedModel, Cmd.none)
            :> SaveDocument
        SaveDocument ->
            -- save document code