Add preliminary help subcommand

pineappl_cli/Cargo.toml

@@ -15,6 +15,7 @@ version.workspace = true
 [dependencies]
 anyhow = "1.0.48"
 clap = { features = ["deprecated", "derive"], version = "4.0.32" }
+clap_mangen = "0.2.7"
 enum_dispatch = "0.3.7"
 flate2 = { optional = true, version = "1.0.22" }
 git-version = "0.3.5"

pineappl_cli/src/help.rs

@@ -0,0 +1,49 @@
+use super::helpers::{GlobalConfiguration, Subcommand};
+use anyhow::Result;
+use clap::{CommandFactory, Parser};
+use clap_mangen::Man;
+use std::io::Write;
+use std::process::{Command, ExitCode, Stdio};
+
+/// Display a manpage for selected subcommands.
+#[derive(Parser)]
+pub struct Opts {
+    /// Name of the (chain of) subcommand(s) to read the manpage of.
+    subcommand: Vec<String>,
+}
+
+impl Subcommand for Opts {
+    fn run(&self, _: &GlobalConfiguration) -> Result<ExitCode> {
+        let cmd = crate::Opts::command();
+        let subcmd = if self.subcommand.is_empty() {
+            cmd
+        } else {
+            let mut iter = self.subcommand.iter();
+            let mut cmd = cmd;
+            while let Some(s) = iter.next() {
+                // TODO: if `unwrap` fails the arguments to `help` are wrong
+                cmd = cmd.find_subcommand(s).unwrap().clone();
+            }
+            cmd
+        };
+        let man = Man::new(subcmd);
+        let mut buffer = Vec::new();
+
+        man.render(&mut buffer)?;
+
+        // TODO: check remaining `unwrap`s
+
+        // TODO: is this the best way? What do we do when `man` isn't available (Windows, ...)?
+        let mut child = Command::new("man")
+            .args(["-l", "-"])
+            .stdin(Stdio::piped())
+            .stdout(Stdio::inherit())
+            .spawn()
+            .unwrap();
+
+        child.stdin.take().unwrap().write_all(&buffer).unwrap();
+        child.wait().unwrap();
+
+        Ok(ExitCode::SUCCESS)
+    }
+}

pineappl_cli/src/main.rs

@@ -6,6 +6,7 @@ mod convolute;
 mod delete;
 mod diff;
 mod evolve;
+mod help;
 mod helpers;
 mod import;
 mod info;
@@ -59,6 +60,7 @@ enum SubcommandEnum {
     Delete(delete::Opts),
     Diff(diff::Opts),
     Evolve(evolve::Opts),
+    Help(help::Opts),
     Import(import::Opts),
     Info(info::Opts),
     Merge(merge::Opts),