Calc man page

members/builtins/src/calc.rs

 use calculate::{eval, eval_polish, CalcError, Value};
 use std::io::{self, Write};
 
+const REPL_GUIDE: &'static str = r#"ion-calc
+Type in expressions to have them evaluated.
+Type "help" for help."#;
+
+pub const MAN_CALC: &str = r#"NAME
+    calc - Floating point calculator
+
+SYNOPSIS
+    calc [EXPRESSION]
+
+DESCRIPTION
+    Evaluates arithmetic expressions
+
+SPECIAL EXPRESSIONS
+    help (only in interactive mode)
+        prints this help text
+
+    --help (only in non-interactive mode)
+        prints this help text
+
+    exit (only in interactive mode)
+        exits the program
+
+NOTATIONS
+    infix notation 
+        e.g. 3 * 4 + 5
+
+    polish notation 
+        e.g. + * 3 4 5
+
+EXAMPLES
+    Add two plus two in infix notation
+        calc 2+2
+
+    Add two plus two in polish notation
+        calc + 2 2
+
+AUTHOR
+    Written by Hunter Goldstein.
+"#;
+
 fn calc_or_polish_calc(args: &str) -> Result<Value, CalcError> {
     match eval(&args) {
         Ok(t) => Ok(t),
@@ -11,35 +52,47 @@ fn calc_or_polish_calc(args: &str) -> Result<Value, CalcError> {
 pub fn calc(args: &[String]) -> Result<(), String> {
     let stdout = io::stdout();
     let mut stdout = stdout.lock();
-    if !args.is_empty() {
-        let result = calc_or_polish_calc(&args.join(" "));
-        let _ = match result {
-            Ok(v) => writeln!(stdout, "{}", v),
-            Err(e) => writeln!(stdout, "{}", e),
-        };
-    } else {
-        let prompt = b"ion-calc: ";
-        loop {
-            let _ = stdout.write(prompt);
-            let _ = stdout.flush();
-            let mut input = String::new();
-            let _ = io::stdin().read_line(&mut input);
-            if input.is_empty() {
-                break;
-            } else {
-                match input.trim() {
-                    "" => (),
-                    "exit" => break,
-                    s => {
-                        let result = calc_or_polish_calc(s);
-                        let _ = match result {
-                            Ok(v) => writeln!(stdout, "{}", v),
-                            Err(e) => writeln!(stdout, "{}", e),
-                        };
+    match args.first() {
+        Some(ref s) if (*s == "--help") => {
+            // "--help" only makes sense if it is the first option. Only look for it
+            // in the first position.
+            println!("{}", MAN_CALC);
+            Ok(())
+        }
+        Some(_) => {
+            let result = calc_or_polish_calc(&args.join(" "));
+            let _ = match result {
+                Ok(v) => writeln!(stdout, "{}", v),
+                Err(e) => writeln!(stdout, "{}", e),
+            };
+            Ok(())
+        }
+        None => {
+            let prompt = b"ion-calc: ";
+            println!("{}", REPL_GUIDE);
+            loop {
+                let _ = stdout.write(prompt);
+                let _ = stdout.flush();
+                let mut input = String::new();
+                let _ = io::stdin().read_line(&mut input);
+                if input.is_empty() {
+                    break;
+                } else {
+                    match input.trim() {
+                        "" => (),
+                        "exit" => break,
+                        "help" => println!("{}", MAN_CALC),
+                        s => {
+                            let result = calc_or_polish_calc(s);
+                            let _ = match result {
+                                Ok(v) => writeln!(stdout, "{}", v),
+                                Err(e) => writeln!(stdout, "{}", e),
+                            };
+                        }
                     }
                 }
             }
+            Ok(())
         }
     }
-    Ok(())
 }