列表、IEnumerable 等上的 F# 扩展方法

发布于 2024-10-10 04:46:44 字数 517 浏览 0 评论 0原文

在 C# 中，如果我有一个小部件定义，比如说：

class widget
{
    public string PrettyName() { ... do stuff here }
}

并且我想允许轻松打印小部件列表，我可能会这样做：

namespace ExtensionMethods
{
    public static PrintAll( this IEnumerable<Widget> widgets, TextWriter writer )
    {
        foreach(var w in widgets) { writer.WriteLine( w.PrettyName() ) }
    }  
}

如何使用记录类型和集合（最好是 List 或 Seq）完成类似的事情F#）。我希望有一个 Widgest 列表，并且能够直接在集合上调用执行类似操作的函数。假设（因为它是 F#）该函数不会更改它所附加到的集合的状态，而是返回一些新值。

原文

In C#, if I had a widget definition, say:

class widget
{
    public string PrettyName() { ... do stuff here }
}

and I wanted to allow for easy printing of a list of Widgets, I might do this:

namespace ExtensionMethods
{
    public static PrintAll( this IEnumerable<Widget> widgets, TextWriter writer )
    {
        foreach(var w in widgets) { writer.WriteLine( w.PrettyName() ) }
    }  
}

How would I accomplish something similar with a record type and a collection (List or Seq preferrably in F#). I'd love to have a list of Widgest and be able to call a function right on the collection that did something like this. Assume (since it's F#) that the function would not be changing the state of the collection that it's attached to, but returning some new value.

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剑心龙吟 2024-10-17 04:46:44

F# 中不可能有完全类似的解决方案。 F# 扩展成员只能像原始类型上的成员一样进行定义，因此您可以在泛型类型 IEnumerable<'T> 上定义 F# 扩展，但不能在特定实例化（例如 IEnumerable。

在 C# 中，扩展方法通常用于提供更流畅的编码风格（例如 myWidgets.PrintAll(tw)）。在 F# 中，您通常只需定义一个 let 绑定函数并使用管道运算符即可实现类似的效果。例如：

module Widget =
  let printAll (tw:TextWriter) s =
    for (w:Widget) in s do
      writer.WriteLine(w.PrettyName())

open Widget
let widgets = // generate a sequence of widgets somehow
let tw = TextWriter()
widgets |> printAll tw

An exactly analogous solution isn't possible in F#. F# extension members can only be defined as if they were members on the original type, so you can define F# extensions on the generic type IEnumerable<'T>, but not on a specific instantiations such as IEnumerable<Widget>.

In C#, extension methods are often used to provide a more fluent coding style (e.g. myWidgets.PrintAll(tw)). In F#, you'd typically just define a let-bound function and use the pipeline operator to achieve a similar effect. For example:

module Widget =
  let printAll (tw:TextWriter) s =
    for (w:Widget) in s do
      writer.WriteLine(w.PrettyName())

open Widget
let widgets = // generate a sequence of widgets somehow
let tw = TextWriter()
widgets |> printAll tw

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