Idris2：依赖和类型歧视

Question

这可能是非常基本的依赖类型编程问题，但我找不到任何相关信息。问题是这样的：“我有一堆消息，根据某些配置，我可能需要处理也可能不需要处理。我如何区分需要处理的消息和不需要处理的消息？类型级别。

例如，我有一些配置和一些消息。

record Configuration where
  constructor MkConfiguration
  handlesOpenClose: Bool
  handlesWillSave: Bool

config : Configuration
config = MkConfiguration { openClose: True, willSave: False }

data Message = FileOpened -- Handle when openClose = True
             | FileClosed -- Handle when openClose = True
             | WillSave   -- Handle when willSave = True

我现在希望能够写出这样的东西：

GetMessagesForConfig : Configuration -> Type
GetMessagesForConfig config = {- 
 config.openClose = true so FileOpened and FileClosed have to be handled,
 config.willSave  = false so WillSave does not have to be handled
-}

MessagesForConfig : Type
MessagesForConfig = GetMessagesForConfig config

handleMessage : MessagesForConfig -> Response
handleMessage FileOpened = {- do something -}
handleMessage FileClosed = {- do something -}
handleMessage WillSave impossible

这或类似的可能吗？

Answer 1

一种无需执行类似开放联合的操作即可实现此目的的简单方法是：

data Message : (openClose : Bool) -> (willSave : Bool) -> Type where
    FileOpened : Message True a
    FileClosed : Message True a
    WillSave   : Message a True

handleMessage : Messages True False -> Response
handleMessage FileOpened = {- do something -}
handleMessage FileClosed = {- do something -}
handleMessage WillSave impossible

Idris2：依赖和类型歧视

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