Hinku is a small library for writing token stream based parsers with a focus on error handling ergonomics and informative error messages. The crate and it's API is under heavy development and experimentation — do not expect any kind of stability from version to version.

Parsers written with hinku are built on backtrack-capable token streams. These streams are forked whenever a parsing subroutine is called. If a parsing operation is successful, progress on the token stream is committed to it's parent stream. Alternatively, when an operation fails, the stream can be backtracked to an earlier point in the stream and an alternative operation can be attempted. When a sub-stream is dropped, it's parent stream is automatically backtracked to it's position before the fork or to the point of latest commit.

Parsing operations are functions with the following signature:

FnOnce(stream: &mut dyn TokenStream<T>) -> ParseResult<R, E>

They can be called either directly passing a stream as an argument, or by passing the function to TokenStreamExt::take, which handles forking and committing on your behalf. A simple parser for matchematical expressions consisting of +, -, *, / and number literals can be written as follows:

fn factor(mut stream: &mut dyn TokenStream<Token>) -> Result<Factor> {
  let lhs = stream.take(literal)?;
  
  let operation = 
  
  if let Some(operation) = stream.take(either(multiplication, division)).optional() {
    Ok(Factor::Operation {
      lhs,
      operation: operation.merge(),
      rhs: Box::new(stream.take(factor)?),
    })
  } else {
    Ok(Factor::Literal(lhs))
  }
}

fn expr(mut stream: &mut dyn TokenStream<Token>) -> Result<Expr> {
  let lhs = stream.take(factor)?;
  
  if let Some(operation) = stream.take(either(addition, subtraction)).optional() {
    Ok(Expr::Operation {
      lhs,
      operation: operation.merge(),
      rhs: Box::new(stream.take(expr)?),
    })
  } else {
    Ok(Expr::Factor(lhs))
  }
}

A complete version of this example and others can be found from the tests/ subdirectory.