Refactor command-line handling into modules per subcommands

cli/src/lib.rs

@@ -0,0 +1,57 @@
+use clap::{App, AppSettings};
+use failure::Fallible;
+use std::ffi::OsString;
+
+mod cmd;
+use cmd::ArgMatchResult;
+pub use cmd::CommandInvocation;
+
+/// Parse the given command line and return an as-yet un-executed CommandInvocation.
+pub fn parse_command_line<I, T>(iter: I) -> Fallible<CommandInvocation>
+where
+    I: IntoIterator<Item = T>,
+    T: Into<OsString> + Clone,
+{
+    let subcommands = cmd::subcommands();
+
+    let mut app = App::new("TaskChampion")
+        .version("0.1")
+        .about("Personal task-tracking")
+        .setting(AppSettings::ColoredHelp);
+
+    for subcommand in subcommands.iter() {
+        app = subcommand.decorate_app(app);
+    }
+
+    let matches = app.get_matches_from_safe(iter)?;
+
+    for subcommand in subcommands.iter() {
+        match subcommand.arg_match(&matches) {
+            ArgMatchResult::Ok(invocation) => return Ok(CommandInvocation::new(invocation)),
+            ArgMatchResult::Err(err) => return Err(err),
+            ArgMatchResult::None => {}
+        }
+    }
+
+    // one of the subcommands also matches the lack of subcommands, so this never
+    // occurrs.
+    unreachable!()
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_parse_command_line_success() -> Fallible<()> {
+        // This just verifies that one of the subcommands works; the subcommands themselves
+        // are tested in their own unit tests.
+        parse_command_line(vec!["task", "pending"].iter())?;
+        Ok(())
+    }
+
+    #[test]
+    fn test_parse_command_line_failure() {
+        assert!(parse_command_line(vec!["task", "--no-such-arg"].iter()).is_err());
+    }
+}