iterator - 我如何在 rust 中使用 Iterator 的 #map 和 #fold？

Question

rustc 0.13.0-nightly (f168c12c5 2014-10-25 20:57:10 +0000)

我有以下代码。

fn main() {
  let nums = vec![1i,2,3];

  let strings = nums.iter()
  .map(|n| n.to_string())
  .fold(String::new, |a, b| a.push_str(b.as_slice()));

  assert_eq!("123", strings.as_slice());
}

它应该将 nums 中的整数转换为它们的字符串表示形式，并将它们连接成一个大字符串。

这是我从 rustc 得到的:

test2.rs:6:31: 6:53 error: type `fn() -> collections::string::String` does not implement any method in scope named `push_str`
test2.rs:6   .fold(String::new, |a, b| a.push_str(b.as_slice()));
                                         ^~~~~~~~~~~~~~~~~~~~~~
test2.rs:8:29: 8:39 error: type `fn() -> collections::string::String` does not implement any method in scope named `as_slice`
test2.rs:8   assert_eq!("123", strings.as_slice());
                                       ^~~~~~~~~~
<std macros>:6:23: 6:33 error: the type of this value must be known in this context
<std macros>:6                 if !((*given_val == *expected_val) &&
                                     ^~~~~~~~~~
<std macros>:1:1: 14:2 note: in expansion of assert_eq!
test2.rs:8:3: 8:41 note: expansion site
<std macros>:6:23: 6:33 error: the type of this value must be known in this context
<std macros>:6                 if !((*given_val == *expected_val) &&
                                     ^~~~~~~~~~
<std macros>:1:1: 14:2 note: in expansion of assert_eq!
test2.rs:8:3: 8:41 note: expansion site
<std macros>:6:37: 6:50 error: the type of this value must be known in this context
<std macros>:6                 if !((*given_val == *expected_val) &&
                                                   ^~~~~~~~~~~~~
<std macros>:1:1: 14:2 note: in expansion of assert_eq!
test2.rs:8:3: 8:41 note: expansion site
<std macros>:7:23: 7:36 error: the type of this value must be known in this context
<std macros>:7                      (*expected_val == *given_val)) {
                                     ^~~~~~~~~~~~~
<std macros>:1:1: 14:2 note: in expansion of assert_eq!
test2.rs:8:3: 8:41 note: expansion site
error: aborting due to 6 previous errors

所以它提示 fn() -> collections::string::String 类型，但我期望映射的结果只是一个普通的 collections::string::字符串。

我在这里做错了什么，还是不应该像我在示例中尝试的那样使用 map ？

编辑:

哈哈 好的我发现了错误，String::new 是一个 fn() -> collections::string::String，所以这是一个错字!

正确的是 .fold(String::new(), |a, b| a.push_string(b.as_slice()));

Answer 1

有3个问题:

1. String::new 缺少一个 ()，应该是 String::new()
2. 您必须从折叠闭包返回一个新状态，push_str 返回 ()。
3. 您必须使状态可变，以便您可以在其上使用 push_str。

---

fn main() {
  let nums = vec![1i,2,3];

  let strings = nums.iter()
  .map(|n| n.to_string())
  .fold(String::new(), |mut a, b| {
        a.push_str(b.as_slice());
        a
    });

  assert_eq!("123", strings.as_slice());
}