Struct ParseFallback 

pub struct ParseFallback<P, T> {
    pub(crate) inner: P,
    pub(crate) value: T,
    pub(crate) value_str: String,
}

Parser that substitutes missing value but not parse failure, created with fallback.

Fields

inner: P

value: T

value_str: String

Implementations

impl<P, T: Display> ParseFallback<P, T>

pub fn display_fallback(self) -> Self

Show fallback value in --help using Display representation

Combinatoric example

#[derive(Debug, Clone)]
pub struct Options {
    jobs: usize,
}

pub fn options() -> OptionParser<Options> {
    let jobs = long("jobs")
        .help("Number of jobs")
        .argument("JOBS")
        .fallback(42)
        .display_fallback();
    construct!(Options { jobs }).to_options()
}

fn main() {
    println!("{:?}", options().run())
}

Derive example

#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
#[allow(dead_code)]
pub struct Options {
    /// Number of jobs
    #[bpaf(argument("JOBS"), fallback(42), display_fallback)]
    jobs: usize,
}

fn main() {
    println!("{:?}", options().run())
}

Output

fallback changes parser to fallback to a default value used when argument is not specified

$ app
Options { jobs: 42 }

If value is present - fallback value is ignored

$ app --jobs 10
Options { jobs: 10 }

Parsing errors are preserved and presented to the user

$ app --jobs ten
Error: couldn't parse ten: invalid digit found in string

With display_fallback, debug_fallback, and format_fallback, you can make it so the default value is visible in the --help output.

$ app --help

Usage: app [--jobs=JOBS]

Available options:

--jobs=JOBS

Number of jobs

[default: 42]

-h, --help

Prints help information

impl<P, T: Debug> ParseFallback<P, T>

pub fn debug_fallback(self) -> Self

Show fallback value in --help using Debug representation

Combinatoric example

fn try_to_get_version() -> Result<usize, &'static str> {
    Ok(42)
}

#[derive(Debug, Clone)]
pub struct Options {
    version: usize,
}

pub fn options() -> OptionParser<Options> {
    let version = long("version")
        .help("Specify protocol version")
        .argument("VERS")
        .fallback_with(try_to_get_version)
        .debug_fallback();
    construct!(Options { version }).to_options()
}

fn main() {
    println!("{:?}", options().run())
}

Derive example

fn try_to_get_version() -> Result<usize, &'static str> {
    Ok(42)
}

#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
pub struct Options {
    #[bpaf(argument("VERS"), fallback_with(try_to_get_version), debug_fallback)]
    /// Specify protocol version
    version: usize,
}

fn main() {
    println!("{:?}", options().run())
}

Output

fallback_with changes parser to fallback to a value that comes from a potentially failing computation when argument is not specified

$ app
Options { version: 42 }

If value is present - fallback value is ignored

$ app --version 10
Options { version: 10 }

Parsing errors are preserved and presented to the user

$ app --version ten
Error: couldn't parse ten: invalid digit found in string

bpaf encases parsers with fallback value of some sort in usage with []

$ app --help

Usage: app [--version=VERS]

Available options:

--version=VERS

Specify protocol version

[default: 42]

-h, --help

Prints help information

impl<P, T> ParseFallback<P, T>

pub fn format_fallback( self, format: impl Fn(&T, &mut Formatter<'_>) -> Result, ) -> Self

Show fallback value in --help using the provided formatting function.

Combinatoric example

use std::{fmt::Display as _, path::PathBuf};
#[derive(Debug, Clone)]
pub struct Options {
    log_file: PathBuf,
}

pub fn options() -> OptionParser<Options> {
    let log_file = long("log-file")
        .help("Path to log file")
        .argument::<PathBuf>("FILE")
        .guard(
            |log_file| !log_file.is_dir(),
            "The log file can't be a directory",
        )
        .fallback(PathBuf::from("logfile.txt"))
        .format_fallback(|path, f| path.display().fmt(f));
    construct!(Options { log_file }).to_options()
}

fn main() {
    println!("{:?}", options().run())
}

Derive example

use std::{fmt::Display as _, path::PathBuf};
#[derive(Debug, Clone, Bpaf)]
#[bpaf(options)]
#[allow(dead_code)]
pub struct Options {
    /// Path to log file
    #[bpaf(
        argument("FILE"),
        guard(|log_file| !log_file.is_dir(), "The log file can't be a directory"),
        fallback(PathBuf::from("logfile.txt")),
        format_fallback(|path, f| path.display().fmt(f)),
    )]
    log_file: PathBuf,
}

fn main() {
    println!("{:?}", options().run())
}

Output

fallback changes parser to fallback to a default value used when argument is not specified

$ app
Options { log_file: "logfile.txt" }

If value is present - fallback value is ignored

$ app --log-file output.txt
Options { log_file: "output.txt" }

Parsing errors are preserved and presented to the user

$ app --log-file /
Error: /: The log file can't be a directory

With display_fallback, debug_fallback, and format_fallback, you can make it so the default value is visible in the --help output.

$ app --help

Usage: app [--log-file=FILE]

Available options:

--log-file=FILE

Path to log file

[default: logfile.txt]

-h, --help

Prints help information

Trait Implementations

impl<P, T> Parser<T> for ParseFallback<P, T>

where P: Parser<T>, T: Clone,

fn many(self) -> ParseMany<Self>

where Self: Sized,

Consume zero or more items from a command line and collect them into a Vec

fn collect<C>(self) -> ParseCollect<Self, C, T>

where C: FromIterator<T>, Self: Sized,

Transform parser into a collection parser

fn some(self, message: &'static str) -> ParseSome<Self>

where Self: Sized + Parser<T>,

Consume one or more items from a command line and collect them into a Vec

fn optional(self) -> ParseOptional<Self>

where Self: Sized + Parser<T>,

Turn a required argument into an optional one

fn count(self) -> ParseCount<Self, T>

where Self: Sized + Parser<T>,

Count how many times the inner parser succeeds, and return that number.

fn last(self) -> ParseLast<Self>

where Self: Sized + Parser<T>,

Apply the inner parser as many times as it succeeds, return the last value

fn parse<F, R, E>(self, f: F) -> ParseWith<T, Self, F, E, R>

where Self: Sized + Parser<T>, F: Fn(T) -> Result<R, E>, E: ToString,

Apply a failing transformation to a contained value

fn map<F, R>(self, map: F) -> ParseMap<T, Self, F, R>

where Self: Sized + Parser<T>, F: Fn(T) -> R + 'static,

Apply a pure transformation to a contained value

fn guard<F>(self, check: F, message: &'static str) -> ParseGuard<Self, F>

where Self: Sized + Parser<T>, F: Fn(&T) -> bool,

Validate or fail with a message

fn fallback(self, value: T) -> ParseFallback<Self, T>

where Self: Sized + Parser<T>,

Use this value as default if the value isn’t present on a command line

fn fallback_with<F, E>(self, fallback: F) -> ParseFallbackWith<T, Self, F, E>

where Self: Sized + Parser<T>, F: Fn() -> Result<T, E>, E: ToString,

Use value produced by this function as default if the value isn’t present

fn hide(self) -> ParseHide<Self>

where Self: Sized + Parser<T>,

Ignore this parser during any sort of help generation

fn hide_usage(self) -> ParseUsage<Self>

where Self: Sized + Parser<T>,

Ignore this parser when generating a usage line

fn custom_usage<M>(self, usage: M) -> ParseUsage<Self>

where M: Into<Doc>, Self: Sized + Parser<T>,

Customize how this parser looks like in the usage line

fn group_help<M: Into<Doc>>(self, message: M) -> ParseGroupHelp<Self>

where Self: Sized + Parser<T>,

Attach a help message to a complex parser

fn with_group_help<F>(self, f: F) -> ParseWithGroupHelp<Self, F>

where Self: Sized + Parser<T>, F: Fn(MetaInfo<'_>) -> Doc,

Make a help message for a complex parser from its MetaInfo

fn to_options(self) -> OptionParser<T>

where Self: Sized + Parser<T> + 'static,

Transform Parser into OptionParser to get ready to run it

fn run(self) -> T

where Self: Sized + Parser<T> + 'static,

Finalize and run the parser

fn boxed(self) -> Box<dyn Parser<T>>

where Self: Sized + Parser<T> + 'static,

Create a boxed representation for a parser