如何在 F# 中定义 printfn 等价物 [英] How to define printfn equivalent in F#

问题描述

由于我使用 F# 进行研究(特别是使用 F# 交互式)，因此我希望拥有可切换的调试时打印"功能.

Since I do research with F# (in particular, using F# interactive), I'd like to have switchable "print-when-in-debug" function.

我可以

let dprintfn = printfn

F# 互动说

val dprintfn : (Printf.TextWriterFormat<'a> -> 'a)

我可以使用

dprintfn "myval1 = %d, other val = %A" a b

只要我想在我的脚本中.

whenever I want in my scripts.

现在我想以不同的方式定义 dprintfn，这样它就会忽略它的所有参数，但与 printfn 语法兼容.怎么样?

Now I'd like to define dprintfn differently, so that it would ignore all its arguments yet being syntax-compatible with printfn. How?

我想到的最接近(但无法正常工作)的变体是:

The closest (yet non-working) variant I have in mind is:

let dprintfn (arg: (Printf.TextWriterFormat<'a> -> 'a)) = ()

但是它下面没有编译然后dprintfn "%A" "Hello"，导致error FS0003: This value is not a function and cannot be applied.

but it the following doesn't compile then dprintfn "%A" "Hello", resulting in error FS0003: This value is not a function and cannot be applied.

PS 我目前使用 Debug.WriteLine(...) 的别名作为变通方法，但这个问题对于理解 F# 的类型系统仍然很有趣.

P.S. I currently use an alias for Debug.WriteLine(...) as work-around, but the question is still interesting for understading F#'s type system.

推荐答案

您可以使用 kprintf 函数，该函数使用标准语法格式化字符串，然后调用您指定的 (lambda) 函数打印格式化的字符串.

You can use the kprintf function, which formats a string using the standard syntax, but then calls a (lambda) function you specify to print the formatted string.

例如，如果设置了 debug，则以下内容将打印字符串，否则不执行任何操作:

For example, the following prints the string if debug is set and otherwise does nothing:

let myprintf fmt = Printf.kprintf (fun str -> 
  // Output the formatted string if 'debug', otherwise do nothing
  if debug then printfn "%s" str) fmt

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