我是一名有用的助手,可以为您翻译文本。
这很合理且易于使用,但是设计中有一件事情让我感到困扰。为什么
因为
我目前正在学习使用Idris进行类型驱动开发这本书。
我有两个问题与第6章中示例数据存储的设计有关。该数据存储是一个命令行应用程序,允许用户设置存储在其中的数据类型,然后添加新数据。
以下是相关代码的部分内容(稍作简化)。 您可以在Github上查看完整代码:
module Main
import Data.Vect
infixr 4 .+.
-- This datatype is to define what sorts of data can be contained in the data store.
data Schema
= SString
| SInt
| (.+.) Schema Schema
-- This is a type-level function that translates a Schema to an actual type.
SchemaType : Schema -> Type
SchemaType SString = String
SchemaType SInt = Int
SchemaType (x .+. y) = (SchemaType x, SchemaType y)
-- This is the data store. It can potentially be storing any sort of schema.
record DataStore where
constructor MkData
schema : Schema
size : Nat
items : Vect size (SchemaType schema)
-- This adds new data to the datastore, making sure that the new data is
-- the same type that the DataStore holds.
addToStore
: (dataStore : DataStore) -> SchemaType (schema dataStore) -> DataStore
addToStore (MkData schema' size' store') newitem =
MkData schema' _ (addToData store')
where
addToData
: Vect size' (SchemaType schema') -> Vect (size' + 1) (SchemaType schema')
addToData xs = xs ++ [newitem]
-- These are commands the user can use on the command line. They are able
-- to change the schema, and add new data.
data Command : Schema -> Type where
SetSchema : (newSchema : Schema) -> Command schema
Add : SchemaType schema -> Command schema
-- Given a Schema, this parses input from the user into a Command.
parse : (schema : Schema) -> String -> Maybe (Command schema)
-- This is the main workhorse of the application. It parses user
-- input, turns it into a command, then evaluates the command and
-- returns an updated DataStore.
processInput
: (dataStore : DataStore) -> String -> Maybe (String, DataStore)
processInput dataStore@(MkData schema' size' items') input =
case parse schema' input of
Nothing => Just ("Invalid command\n", dataStore)
Just (SetSchema newSchema) =>
Just ("updated schema and reset datastore\n", MkData newSchema _ [])
Just (Add item) =>
let newStore = addToStore (MkData schema' size' items') item
in Just ("ID " ++ show (size dataStore) ++ "\n", newStore)
-- This kicks off processInput with an empty datastore and a simple Schema
-- of SString.
main : IO ()
main = replWith (MkData SString _ []) "Command: " processInput
这很合理且易于使用,但是设计中有一件事情让我感到困扰。为什么
DataStore
包含一个Schema
而不是由一个Schema
进行索引?因为
DataStore
没有被Schema
索引,所以我们可能会编写一个不正确的addToStore
函数,例如:addToStore
: (dataStore : DataStore) -> SchemaType (schema dataStore) -> DataStore
addToStore _ newitem =
MkData SInt _ []
这是我想象中更加类型安全的代码。你可以在Github上看到完整代码:
module Main
import Data.Vect
infixr 4 .+.
data Schema
= SString
| SInt
| (.+.) Schema Schema
SchemaType : Schema -> Type
SchemaType SString = String
SchemaType SInt = Int
SchemaType (x .+. y) = (SchemaType x, SchemaType y)
record DataStore (schema : Schema) where
constructor MkData
size : Nat
items : Vect size (SchemaType schema)
addToStore
: (dataStore : DataStore schema) ->
SchemaType schema ->
DataStore schema
addToStore {schema} (MkData size' store') newitem =
MkData _ (addToData store')
where
addToData
: Vect size' (SchemaType schema) -> Vect (size' + 1) (SchemaType schema)
addToData xs = xs ++ [newitem]
data Command : Schema -> Type where
SetSchema : (newSchema : Schema) -> Command schema
Add : SchemaType schema -> Command schema
parse : (schema : Schema) -> String -> Maybe (Command schema)
processInput
: (schema : Schema ** DataStore schema) ->
String ->
Maybe (String, (newschema ** DataStore newschema))
processInput (schema ** (MkData size' items')) input =
case parse schema input of
Nothing => Just ("Invalid command\n", (_ ** MkData size' items'))
Just (SetSchema newSchema) =>
Just ("updated schema and reset datastore\n", (newSchema ** MkData _ []))
Just (Add item) =>
let newStore = addToStore (MkData size' items') item
msg = "ID " ++ show (size newStore) ++ "\n"
in Just (msg, (schema ** newStore))
main : IO ()
main = replWith (SString ** MkData _ []) "Command: " processInput
这里有两个问题:
为什么这本书没有使用更安全的
DataStore
类型(由Schema
索引)?使用不太安全的版本(只包含一个Schema
)是否会有所收获?一个类型被另一个类型索引与包含有何理论区别?这种区别是否取决于您正在使用的编程语言?
例如,我想在 Idris 中可能没有太大的区别,但在 Haskell 中可能有很大的区别。(是吗...?)
我刚开始尝试 Idris(并不是特别熟悉 Haskell 中单例或 GADTs 的用法),所以我很难理解这个问题。如果您可以指向任何论文来讨论这个问题,我会非常感兴趣。
addToStore
不能更改输出数据存储的模式。要更改模式,您需要使用一些外部机制,例如依赖对,这样可以明确您更改模式的意图,而在代码的先前版本中并非如此。 - Anton Trunov