如何使用格式的动态格式字符串！宏？ [英] How can I use a dynamic format string with the format! macro?

问题描述

我刚开始学习Rust，我正在做一些小工具来帮助我理解语言。我有一个问题使用格式！宏来格式化 String 。作为格式！需要一个文字，我不能将我的字符串传递给它。我想这样做动态添加字符串到当前字符串在视图引擎中使用。

  let test = String :: from（测试：{}）; 
 let test2 = String :: from（Not working！）; 
 println！（test，test2）; 
  

实际上我想实现的是下面的例子，其中main.html包含 {content} 。

 使用std :: io :: prelude :: * 
使用std :: fs :: File; 
使用std :: io; 
 
 fn main（）{
 let mut buffer = String :: new（）; 
 read_from_file_using_try（& mut buffer）; 
 
 println！（& buffer，content =content）; 
} 
 
 fn read_from_file_using_try（buffer：& mut String） - >结果<（），io :: Error> {
 let mut file = try！（File :: open（main.html））; 
 try！（file.read_to_string（buffer））; 
 OK（（））
} 
  

所以我想打印内容

解决方案

简单回答：不能这样做。

长答案：格式！宏（及其衍生物）需要一个字符串字面值，即编译时已知的字符串。为了交换这个要求，如果提供的参数与格式不匹配，就会出现编译错误。

---

正在寻找被称为模板引擎。 Rust模板引擎列表没有特定的顺序：

• 把手
  
• Rustache


• Maud
>

• Horrorshow


• fomat-macros


• ...
>

模板引擎具有不同的特性，并且在编译时或运行时发生验证的程度和灵活性有很大不同（我似乎回想一下，Maud是非常以HTML为中心的）。这是由你找到一个最适合你的用例。

I just started learning Rust, and I'm making some small tools to help me understand the language. I have a problem formatting a String using the format! macro. As format! takes a literal, I am not able pass my string to it. I want to do this to dynamically add strings into the current string for use in a view engine. I'm open for suggestions if there might be a better way to do it.

    let test = String::from("Test: {}");
    let test2 = String::from("Not working!");
    println!(test, test2);

What I actually want to achieve is the below example, where main.html contains {content}.

use std::io::prelude::*;
use std::fs::File;
use std::io;

fn main() {
    let mut buffer = String::new();
    read_from_file_using_try(&mut buffer);

    println!(&buffer, content="content");
}

fn read_from_file_using_try(buffer: &mut String) -> Result<(), io::Error> {
    let mut file = try!(File::open("main.html"));
    try!(file.read_to_string(buffer));
    Ok(())
}

So I want to print the contents of main.html after I formatted it.

解决方案

Short answer: it cannot be done.

---

Long answer: the format! macro (and its derivatives) requires a string literal, that is a string known at compilation-time. In exchange for this requirement, if the arguments provided do not match the format, a compilation error is raised.

---

What you are looking for is known as a template engine. A non-exhaustive list of Rust template engines in no particular order:

• Handlebars
• Rustache
• Maud
• Horrorshow
• fomat-macros
• ...

Template engines have different characteristics, and notably differ by the degree of validation occurring at compile-time or run-time and their flexibility (I seem to recall that Maud was very HTML-centric, for example). It's up to you to find the one most fitting for your use case.

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