需要为Yesod路径定义什么类型的类？ [英] What typeclasses need to be defined for a Yesod path?

问题描述

在我的应用程序中，我的数据模型有几个不同的使用Integer或String作为标识符的实例。为了安全起见，我已经将这些标识符封装到newtype声明中，如下所示：

  newtype DocId = DocId Integer 
 newtype GroupName = GroupName String 
 newtype UserName = UserName String 
  

当我设置我的Yesod路径，我发现我必须为其中的每一个创建至少三个实例，并且实例几乎总是相同的

  instance Read DocId其中
 readsPrec prec val = case读取val 
（i，）：_  - > [（DocId i，）] 
 []  - > [] 
 
实例B.ToMarkup DocId其中
 toMarkup（DocId val）= B.toMarkup val 
 
实例PathPiece DocId其中
 toPathPiece（DocId i ）= T.pack $ show i 
 fromPathPiece s = 
 case reads 
（i，）的$ T.unpack s：_  - >只需
 []  - > Nothing 
  

这段文字一遍又一遍。

为了在URL中呈现我的数据类型（例如@ {ViewDocument docId}）并能够解析这些URL，我真的需要设置什么？

GeneralizedNewtypeDeriving ，那么你可以添加派生PathPiece >在每个新数据类型下，或者派生实例PathPiece DocId ，如果你不能直接派生数据类型。

对于要成为路由一部分的每种数据类型，您都需要Read，Show和PathPiece实例。

In my application, my data model has several different instances of using an Integer or a String for some identifier. For safety, I've gone ahead and wrapped those identifiers into newtype declarations like so:

newtype DocId = DocId Integer
newtype GroupName = GroupName String
newtype UserName = UserName String

When I'm setting up my Yesod paths, I'm discovering that I have to create at least three instances for each of these, and the instances are almost always identical

instance Read DocId where
    readsPrec prec val = case reads val of
        (i, ""):_ -> [(DocId i, "")]
        [] -> []

instance B.ToMarkup DocId where
    toMarkup (DocId val) = B.toMarkup val

instance PathPiece DocId where
    toPathPiece (DocId i) = T.pack $ show i
    fromPathPiece s =
        case reads $ T.unpack s of
            (i, ""):_ -> Just i
            [] -> Nothing

This text, over and over again.

What do I really need to set up in order to both render my data type in URLs (like @{ViewDocument docId}) and be able to parse those URLs?

解决方案

If you turn on GeneralizedNewtypeDeriving, then you can just add deriving PathPiece under each new datatype, or deriving instance PathPiece DocId if you can't derive directly on the datatype.

You will need Read, Show, and PathPiece instances for every datatype that is to be part of a route.

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