Support custom containers for many functions

[epage]

Prerequisites

Here are a few things you should provide to help me understand the issue:

• Rust version : 1.55.0
• nom version : 7.0.0
• nom compilation features used: default

Test case

I wanted to experiment with porting toml_edit to nom, one thing I especially found missing was custom collection support. If I use many0, it only supports Vec. The naive approach in code is to do

fn ml_body(input: &str) -> IResult<&str, String> {
    many(substr).map(|s| s.join(""))
}

Leading me to allocate a Vec<String> and then merge them. combine defaults to Vec but allows any Extend type, allowing

fn ml_body(input: &str) -> IResult<&str, String> {
    many(substr)
}

which will collect directly into the String, saving on a lot of allocations

I'm also looking at optimizing a section of my parser where collect into a Vec and then turn it into a HashMap with errors by creating a custom type that wraps HashMap with my error type and stores off errors on Extend, allowing me to bypass the Vec allocation.

I could do all of this with fold_ variants but it will be a bit messier.

[Stargateur]

I guess this can be link to #1393.

I think we can use https://doc.rust-lang.org/std/iter/trait.Extend.html to allow this for many.

I could do all of this with fold_ variants but it will be a bit messier.

You can add your own parser for reuse in the waiting, something like that (I'm more used to nom 6 so probably mistake):

fn many0_string(f: impl Parser) -> impl Parser {
  |input| {
    fold_many0(
      f,
      String::new,
      |mut acc: String, item| {
        string.push_str(item);
        acc
      }
    )(input)
  }
}

combine defaults to Vec but allows any Extend type, allowing

What is combine ? I don't follow here.

I'm also looking at optimizing a section of my parser where collect into a Vec and then turn it into a HashMap with errors by creating a custom type that wraps HashMap with my error type and stores off errors on Extend, allowing me to bypass the Vec allocation.

Some sort of partition_result() ?

[epage]

What is combine ? I don't follow here.

Sorry, combine is another parser combinator.

e.g. https://docs.rs/combine/4.6.1/combine/fn.many.html has the collection set to F: Extend<P::Output> + Default,

Some sort of partition_result() ?

More like try_fold

So speaking of, maybe we should have try_fold_many* :)

[Stargateur]

I think we can clearly add a try_fold and I will try update my PR to include extend.

[epage]

Oh, thanks for looking to include Extend!

I've split out try_fold into #1411.

[Geal]

oh, using Extend is a great suggestion! I was wondering about doing that too