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我对nom7.1 版进行了这个测试：

use nom::bytes::complete::tag;

#[test]
fn test() {
    let (s, t) = tag("1")("123").unwrap();
}

运行cargo test给

error[E0283]: type annotations needed
  --> src/main.rs:5:18
   |
5  |     let (s, t) = tag("1")("123").unwrap();
   |                  ^^^ cannot infer type for type parameter `Error` declared on the function `tag`
   |
   = note: cannot satisfy `_: ParseError<&str>`
note: required by a bound in `nom::bytes::complete::tag`
  --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/nom-7.1.0/src/bytes/complete.rs:32:29
   |
32 | pub fn tag<T, Input, Error: ParseError<Input>>(
   |                             ^^^^^^^^^^^^^^^^^ required by this bound in `nom::bytes::complete::tag`
help: consider specifying the type arguments in the function call
   |
5  |     let (s, t) = tag::<T, Input, Error>("1")("123").unwrap();
   |                     +++++++++++++++++++

它建议我指定tag::<T, Input, Error>()

我该如何处理？我还没有完全理解为什么会出现这个问题。

我尝试从以下指定一些类型nom：

use nom::bytes::complete::tag;

#[test]
fn test() {
    let (s, t) = (tag::<_, &str, nom::error::ParseError<&str>>("1")("123")).unwrap();
}

error[E0782]: trait objects must include the `dyn` keyword
 --> src/main.rs:5:34
  |
5 |     let (s, t) = (tag::<_, &str, nom::error::ParseError<&str>>("1")("123")).unwrap();
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
help: add `dyn` keyword before this trait
  |
5 |     let (s, t) = (tag::<_, &str, dyn nom::error::ParseError<&str>>("1")("123")).unwrap();
  |                                  +++

error[E0277]: the size for values of type `dyn ParseError<&str>` cannot be known at compilation time
  --> src/main.rs:5:19
   |
5  |     let (s, t) = (tag::<_, &str, nom::error::ParseError<&str>>("1")("123")).unwrap();
   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `dyn ParseError<&str>`
note: required by a bound in `nom::bytes::complete::tag`
  --> ~/.cargo/registry/src/github.com-1ecc6299db9ec823/nom-7.1.0/src/bytes/complete.rs:32:22
   |
32 | pub fn tag<T, Input, Error: ParseError<Input>>(
   |                      ^^^^^ required by this bound in `nom::bytes::complete::tag`

error[E0038]: the trait `ParseError` cannot be made into an object
 --> src/main.rs:5:34
  |
5 |     let (s, t) = (tag::<_, &str, nom::error::ParseError<&str>>("1")("123")).unwrap();
  |                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `ParseError` cannot be made into an object
  |
  = note: the trait cannot be made into an object because it requires `Self: Sized`
  = note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
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1 回答 1

2

Nom有一个通用的错误类型：

解析器对其错误类型是通用的，要求它实现error::ParseError<Input>特征。

这意味着您的代码需要指定所需的错误类型；问题中的代码没有。

正如编译器建议的那样：考虑在函数调用中指定类型参数。的实施者之一ParseError将是一个很好的起点。Nom 提供两个：

选一：

use nom::{bytes::complete::tag, error::Error};

#[test]
fn test() {
    let (_s, _t) = tag::<_, _, Error<_>>("1")("123").unwrap();
}

也可以看看：

于 2021-12-22T01:56:04.683 回答