如何使用printf自定义自定义类型的输出? [英] How do I customize output of a custom type using printf?

问题描述

我已经阅读了很多 Expert F#并正在构建一个实际的应用程序.调试时，我已经习惯于传递这样的fsi命令以使内容在repl窗口中清晰可见:

I've read through a good chunk of Expert F# and am working on building an actual application. While debugging, I've grown accustomed to passing fsi commands like this to make things legible in the repl window:

fsi.AddPrinter(fun (x : myType) -> myType.ToString())

我想将此扩展到与printf格式化程序一起使用，所以我可以输入例如.

I would like to extend this to work with the printf formatter, so I could type e.g.

printf "%A" instanceOfMyType 

并控制自定义类型的输出.该书暗示可以做到这一点(第93页，通用结构格式可以扩展为可与任何用户定义的数据类型一起使用， F#网站上涵盖的一个主题")，但我没有找到有关如何实际完成此操作的任何参考.有人知道如何吗?甚至有可能吗?

and control the output for a custom type. The book implies that this can be done (p 93, "Generic structural formatting can be extended to work with any user-defined data types, a topic covered on the F# website"), but I have failed to find any references as to how to actually accomplish this. Does anyone know how? Is it even possible?

我应该包括一个代码示例，它是我正在处理的一种记录类型，例如

I should have included a code sample, it's a record type that I'm dealing with, e.g.

type myType = 
    {a: int}        
    override m.ToString() = "hello"

let t = {a=5}
printfn "%A" t
printfn "%A" (box t)  

两个打印语句均产生:

{a = 5;}

推荐答案

在F#2.0中，看起来正确的方法是使用

It looks like the Right Way to do this in F# 2.0 is by using the StructuredFormatDisplay attribute, for example:

[<StructuredFormatDisplay("hello {a}")>]
type myType = {a: int}

在此示例中，您将获得hello 42，而不是默认的{a = 42;}.

In this example, instead of the default {a = 42;}, you would get hello 42.

对于对象，记录和联合类型，其工作方式相同.尽管模式必须为"PreText {PropertyName} PostText"格式( PreText 和 PostText 是可选的)，但实际上它比ToString()更强大，因为:

This works the same way for object, record, and union types. And although the pattern must be of the format "PreText {PropertyName} PostText" (PreText and PostText being optional), this is actually more powerful than ToString() because:

1. PropertyName可以是任何类型的属性.如果不是字符串，则也将进行结构化格式设置.

1. PropertyName can be a property of any type. If it is not a string, then it will also be subject to structured formatting. Don Syme's blog gives an example of recursively formatting a tree in this way.

它可能是计算所得的属性.因此，您实际上可以使ToString()适用于记录和联合类型，尽管采用的是一种相当简单的方法:

It may be a calculated property. So you could actually get ToString() to work for record and union types, though in a rather round-about way:

[<StructuredFormatDisplay("{AsString}")>]
type myType = 
    {a: int}
    override m.ToString() = "hello"
    member m.AsString = m.ToString()  // a property that calls a method

顺便说一句，如果您调用printfn "%O"而不是printfn "%A"，则将始终使用ToString()(甚至用于记录和联合类型).

By the way, ToString() will always be used (even for record and union types) if you call printfn "%O" instead of printfn "%A".

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