Higher Order Parser

HOP is a lightweight, higher-order parser in Rust.

Hi-Parser provides a more Haskell-like parser style, and explores the ? syntax of Result to have a do-block-like monadic notation. This isn't as good as a proper do notation, but avoids identation hell. For example, a lambda, in a functional language, can be parsed as:

// Parses a λ-term in the syntax: `λx => body`
pub fn parse_lam(state: parser::State) -> parser::Answer<Option<Box<Term>>> {
  let (state, _)    = parser::there_take_exact("λ")?;
  let (state, name) = parser::there_nonempty_name(state)?;
  let (state, _)    = parser::there_take_exact("=>")?;
  let (state, body) = parse_term(state)?;
  Ok((state, Box::new(Term::Lam { name, body })))
}

A Parser is defined simply as:

Answer<A> = Result<(State, A), String>
Parser<A> = Fn(State) -> Answer<A>>

That is, a Parser is a function that receives a State and returns an Answer. That answer is either an updated state, and the parse result (if it succeeds), or an error message, as a simple string, if it fails. Note that there are two ways to fail:

1. Recoverable. Return an Ok with a Bool, or an Option:

- Ok((new_state, Some(result))) if it succeeds
- Ok((old_state, None))         if it fails

This backtracks, and can be used to implement alternatives. For example, if you're parsing an AST, "Animal", with 2 constructors, dog and cat, then you could implement:

parse_dog    : Parser<Option<Animal>>
parse_cat    : Parser<Option<Animal>>
parse_animal : Parser<Animal>

2. Irrecoverable. Return an Err with a message:

- Err(error_message)

This will abort the entire parser, like a "throw", and return the error message. Use this when you know that only one parsing branch can reach this location, yet the source is wrong.

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This parser is used by HVM.

Note: this parser is in very early stage and provides very few features.

TODO: add examples, improve documentation.