一般写一个函数的例子 [英] Generically write cases of a function

问题描述

这是一个函数：

pre $ data $ AnyControlFrame =
PingFrame_ACF PingFrame
| SynStream_ACF SynStreamFrame
| RstStreamFrame_ACF RstStreamFrame
| SettingsFrame_ACF SettingsFrame
| WindowUpdateFrame_ACF WindowUpdateFrame
| Ignored_ACF LB.ByteString


writeControlFrame :: AnyControlFrame - >把
writeControlFrame（A）写入一个
writeControlFrame（RstStreamFrame_ACF a）=放一个
writeControlFrame（SettingsFrame_ACF a）=放一个
writeControlFrame（WindowUpdateFrame_ACF a）=放一个
....

以及案例不断涌现。有没有一种方法可以泛泛地写出这个函数，即抽象出所有的情况？

编辑：构造函数是用具体但不同类型参数化的。我认为，我可能有这个设计的错误....如果是这样，请指出！ 解决方案

你自己定义 AnyControlFrame 吗？如果是这样，那么你可以做到这一点：

$ b

  data AnyControlFrame = PingFrame_ACF {someValue :: SomeDataType} 
 | RstStreamFrame_ACF {someValue :: SomeDataType} 
 | SettingsFrame_ACF {someValue :: SomeDataType} 
 | WindowUpdateFrame_ACF {someValue :: SomeDataType} 
 | .... 
 
 writeControlFrame :: AnyControlFrame  - >把
 writeControlFrame = put。 someValue 
  

这是因为 someValue 是 AnyControlFrame - > SomeDataType 和 put的类型是 SomeDataType - >把。因此你可以编写它们。

如果你自己没有定义 AnyControlFrame ）你可以做的是自己定义 someValue ：

  someValue :: AnyControlFrame  - > SomeDataType 
 writeControlFrame（PingFrame_ACF a）= a 
 writeControlFrame（RstStreamFrame_ACF a）= a 
 writeControlFrame（SettingsFrame_ACF a）= a 
 writeControlFrame（WindowUpdateFrame_ACF a）= a 
。 ... 
 
 writeControlFrame :: AnyControlFrame  - >把
 writeControlFrame = put。 someValue 
  

使用 someValue :: AnyControlFrame - > SomeDataType 使你的代码更加通用 （尽管这不是我用来描述它的词）。如果你想编写几个函数，比如 writeControlFrame ，那么使用 someValue 可以简单得多。

Here is a function:

data AnyControlFrame =
   PingFrame_ACF PingFrame
   |SynStream_ACF SynStreamFrame
   |RstStreamFrame_ACF RstStreamFrame
   |SettingsFrame_ACF SettingsFrame
   |WindowUpdateFrame_ACF WindowUpdateFrame
   |Ignored_ACF LB.ByteString


writeControlFrame :: AnyControlFrame -> Put 
writeControlFrame (PingFrame_ACF a)         = put a
writeControlFrame (RstStreamFrame_ACF a)    = put a
writeControlFrame (SettingsFrame_ACF a)     = put a
writeControlFrame (WindowUpdateFrame_ACF a) = put a
....

and the cases keep coming. Is there a way to write this function "generically", that is, to abstract over all the cases?

EDIT: The constructors are parametrized with concrete, but different types. I reckon that I may have got something wrong with this "design".... if so, please point it out!

解决方案

Are you defining AnyControlFrame yourself? If so then you can do this:

data AnyControlFrame = PingFrame_ACF         { someValue :: SomeDataType }
                     | RstStreamFrame_ACF    { someValue :: SomeDataType }
                     | SettingsFrame_ACF     { someValue :: SomeDataType }
                     | WindowUpdateFrame_ACF { someValue :: SomeDataType }
                     | ....

writeControlFrame :: AnyControlFrame -> Put
writeControlFrame = put . someValue

This works because the type of someValue is AnyControlFrame -> SomeDataType and the type of put is SomeDataType -> Put. Hence you can compose them.

If you are not defining AnyControlFrame yourself then the only other thing that (I believe) you can do is define someValue yourself:

someValue :: AnyControlFrame -> SomeDataType
writeControlFrame (PingFrame_ACF a)         = a
writeControlFrame (RstStreamFrame_ACF a)    = a
writeControlFrame (SettingsFrame_ACF a)     = a
writeControlFrame (WindowUpdateFrame_ACF a) = a
....

writeControlFrame :: AnyControlFrame -> Put
writeControlFrame = put . someValue

Either way, using someValue :: AnyControlFrame -> SomeDataType makes your code more “generic” (although that's not the word I would use to describe it). If you want to write several functions like writeControlFrame then using someValue makes it much easier.

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