docs: Add REPL example

This is to help in cases like clap-rs#3668 and clap-rs#3673

Cargo.toml

@@ -143,6 +143,7 @@ rustversion = "1"
 trycmd = { version = "0.13", default-features = false, features = ["color-auto", "diff", "examples"] }
 humantime = "2"
 snapbox = "0.2.9"
+shlex = "1.1.0"
 
 [[example]]
 name = "demo"
@@ -182,6 +183,11 @@ name = "hostname"
 path = "examples/multicall-hostname.rs"
 required-features = ["unstable-multicall"]
 
+[[example]]
+name = "repl"
+path = "examples/repl.rs"
+required-features = ["unstable-multicall"]
+
 [[example]]
 name = "01_quick"
 path = "examples/tutorial_builder/01_quick.rs"

examples/README.md

@@ -26,6 +26,9 @@
   - hostname: [builder](multicall-hostname.md)
     - Topics:
       - Subcommands
+- repl: [builder](repl.rs)
+  - Topics:
+    - Read-Eval-Print Loops / Custom command lines
 
 ## Contributing
 

examples/multicall-hostname.rs

@@ -4,14 +4,13 @@ use clap::Command;
 
 fn main() {
     let cmd = Command::new(env!("CARGO_CRATE_NAME"))
+        .multicall(true)
         .arg_required_else_help(true)
         .subcommand_value_name("APPLET")
         .subcommand_help_heading("APPLETS")
         .subcommand(Command::new("hostname").about("show hostname part of FQDN"))
         .subcommand(Command::new("dnsdomainname").about("show domain name part of FQDN"));
 
-    let cmd = cmd.multicall(true);
-
     match cmd.get_matches().subcommand_name() {
         Some("hostname") => println!("www"),
         Some("dnsdomainname") => println!("example.com"),

examples/repl.rs

@@ -0,0 +1,95 @@
+// Note: this requires the `unstable-multicall` feature
+
+use std::io::Write;
+
+use clap::Command;
+
+fn main() -> Result<(), String> {
+    loop {
+        let line = readline()?;
+        let line = line.trim();
+        if line.is_empty() {
+            continue;
+        }
+
+        match respond(line) {
+            Ok(quit) => {
+                if quit {
+                    break;
+                }
+            }
+            Err(err) => {
+                write!(std::io::stdout(), "{}", err).map_err(|e| e.to_string())?;
+                std::io::stdout().flush().map_err(|e| e.to_string())?;
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn respond(line: &str) -> Result<bool, String> {
+    let args = shlex::split(line).ok_or("error: Invalid quoting")?;
+    let matches = cli()
+        .try_get_matches_from(&args)
+        .map_err(|e| e.to_string())?;
+    match matches.subcommand() {
+        Some(("ping", _matches)) => {
+            write!(std::io::stdout(), "Pong").map_err(|e| e.to_string())?;
+            std::io::stdout().flush().map_err(|e| e.to_string())?;
+        }
+        Some(("quit", _matches)) => {
+            write!(std::io::stdout(), "Exiting ...").map_err(|e| e.to_string())?;
+            std::io::stdout().flush().map_err(|e| e.to_string())?;
+            return Ok(true);
+        }
+        Some((name, _matches)) => unimplemented!("{}", name),
+        None => unreachable!("subcommand required"),
+    }
+
+    Ok(false)
+}
+
+fn cli() -> Command<'static> {
+    // strip out usage
+    const PARSER_TEMPLATE: &str = "\
+        {all-args}
+    ";
+    // strip out name/version
+    const APPLET_TEMPLATE: &str = "\
+        {about-with-newline}\n\
+        {usage-heading}\n    {usage}\n\
+        \n\
+        {all-args}{after-help}\
+    ";
+
+    Command::new("repl")
+        .multicall(true)
+        .arg_required_else_help(true)
+        .disable_help_flag(true)
+        .subcommand_required(true)
+        .subcommand_value_name("APPLET")
+        .subcommand_help_heading("APPLETS")
+        .help_template(PARSER_TEMPLATE)
+        .subcommand(
+            Command::new("ping")
+                .about("Get a response")
+                .help_template(APPLET_TEMPLATE),
+        )
+        .subcommand(
+            Command::new("quit")
+                .alias("exit")
+                .about("Quit the REPL")
+                .help_template(APPLET_TEMPLATE),
+        )
+}
+
+fn readline() -> Result<String, String> {
+    write!(std::io::stdout(), "$ ").map_err(|e| e.to_string())?;
+    std::io::stdout().flush().map_err(|e| e.to_string())?;
+    let mut buffer = String::new();
+    std::io::stdin()
+        .read_line(&mut buffer)
+        .map_err(|e| e.to_string())?;
+    Ok(buffer)
+}