Add help menu

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## Next
 - Add support for non-US "metre" and "litre" spellings
+- Add help menu
 - Add `--version` flag
 - Freak out instead of ignoring unexpected arguments
 - Fix decimeter parsed as centimeter

src/main.rs

@@ -1,23 +1,48 @@
 use cpc::eval;
 use cpc::units::Unit;
 use std::process::exit;
+use std::env;
 
 const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
+fn print_help() {
+  println!(concat!(
+    "Usage: cpc '<expression>' [options]",
+    "\n",
+    "\nOptions:",
+    "\n    --verbose   Enable verbose logging",
+    "\n    --version   Show cpc version",
+    "\n    --help      Show this help page",
+  ));
+}
+
+fn get_args() -> env::Args {
+  let mut args = env::args().into_iter();
+  args.next(); // skip binary name
+  return args;
+}
+
 /// CLI interface
 fn main() {
-  use std::env;
-  let mut args = env::args().into_iter();
-  args.next();
-  let mut verbose = false;
-  let mut expression_opt = None;
-  for arg in args {
+  // parse these first so they work if there are unexpected args
+  for arg in get_args() {
     match arg.as_str() {
-      "-v" | "--verbose" => verbose = true,
       "--version" => {
         println!("{}", VERSION);
         exit(0);
       },
+      "--help" => {
+        print_help();
+        exit(0);
+      },
+      _ => {},
+    }
+  }
+  let mut verbose = false;
+  let mut expression_opt = None;
+  for arg in get_args() {
+    match arg.as_str() {
+      "-v" | "--verbose" => verbose = true,
       _ => {
         if expression_opt == None {
           expression_opt = Some(arg);
@@ -28,23 +53,22 @@ fn main() {
       }
     }
   }
-  match expression_opt {
-    Some(expression) => {
-      match eval(&expression, true, Unit::Celsius, verbose) {
-        Ok(answer) => {
-          if !verbose {
-            println!("{}", answer);
-          }
-        },
-        Err(e) => {
-          eprintln!("{}", e);
-          exit(1);
-        },
-      }
-    }
+  let expression = match expression_opt {
+    Some(expression) => expression,
     None => {
-      eprintln!("No argument supplied");
+      print_help();
+      exit(0);
+    },
+  };
+  match eval(&expression, true, Unit::Celsius, verbose) {
+    Ok(answer) => {
+      if !verbose {
+        println!("{}", answer);
+      }
+    },
+    Err(e) => {
+      eprintln!("{}", e);
       exit(1);
-    }
+    },
   }
 }