Move semantics and argument parsing

September 20, 2020

In my initial design for meap, the Parser trait contained this method which extracts typed values from an untyped low-level parser output:

fn parse_low_level(
    self,
    ll: &mut low_level::LowLevelParserOutput,
) -> Result<Self::Item, Box<dyn error::Error>>;

Note that the parser is consumed by this method; After calling this method on a parser, the parser is destroyed. This allows the parser to contain values which move into its parsed output. Two examples:

a parser wrapping another parser, giving it a default value. The default value stored in the parser can be moved into the result in the case where it’s needed, preventing the need to copy the data from the parser into its output.
the Map combinator, which wraps a parser, and calls a function on its output. I want to make the function a FnOnce - the most permissive of rust’s function traits - but calling a FnOnce consumes it. If the mapped function was in a field of the Map combinator, the entire combinator would need to be consumed in order to call the FnOnce.

This worked perfectly fine up until I added help messages. If the argument parser finds that its input is invalid, it stops parsing and prints a help message, including details on arguments that it accepts. E.g.

Usage: sand [OPTIONS] PERIOD

Args:
    PERIOD     how long to wait

Options:
    [-i, --interval DURATION]     how frequently to update the display (Default: 1s)
    [-h, --help]                  print help message

The problem is that if we’re part-way through parsing, and encounter an error, part of the parser has already been consumed by the parsing process, and so can’t be re-traversed to generate a help message.

This requirement led to me changing the type of the method above:

fn parse_low_level(
    &mut self,
    ll: &mut low_level::LowLevelParserOutput,
) -> Result<Self::Item, Box<dyn error::Error>>;

Now the parser is mutably borrowed, so it remains in scope after parsing. This is slightly unfortunate as now some errors that were previously caught at compile-time are now caught at runtime. In particular, for parsers that contain some value that is moved out during parsing, running the parser twice will panic as the necessary values are gone. Previously it was not possible to run the parser twice, as it was destroyed by running.

Note that running the parser twice is a mistake, but now it’s possible (though still not easy!) to make this mistake.

The Map combinator is now defined as:

pub struct Map<T, U, F: FnOnce(T) -> U, PT: Parser<Item = T>> {
    f: Option<F>,
    parser_t: PT,
}

Its implementation of parse_low_level:

fn parse_low_level(
    &mut self,
    ll: &mut low_level::LowLevelParserOutput,
) -> Result<Self::Item, Box<dyn error::Error>> {
    Ok((self.f.take().expect("function has already been called"))(
        self.parser_t.parse_low_level(ll)?,
    ))
}

A similar technique is employed for default combinators. Put the “moving out” data inside an Option, and move the data out of the Option when needed, setting the Option to None in the process, and crashing if it was already None.