Add structopt for arg parsing

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/main.rs

@@ -3,21 +3,21 @@
 mod common;
 mod vm;
 
-use std::{env, fs, io, process};
+use structopt::StructOpt;
+use std::{fs, io, process, path::{Path, PathBuf}};
+
+#[derive(StructOpt, Debug)]
+struct Options {
+    // maybe some other options:
+    // * debug
+    #[structopt(name = "FILE", parse(from_os_str))]
+    input: PathBuf,
+}
 
 type Result<T> = std::result::Result<T, Box<dyn std::error::Error>>;
 
-fn get_input_string() -> io::Result<String> {
-    let args: Vec<String> = env::args().collect();
-    if args.len() <= 1 {
-        println!(
-            "usage: {} file.asm",
-            args.get(0).map(String::as_str).unwrap_or("rasp")
-        );
-        process::exit(1);
-    } else {
-        fs::read_to_string(&args[1])
-    }
+fn get_input_string(path: impl AsRef<Path>) -> io::Result<String> {
+    fs::read_to_string(path.as_ref())
 }
 
 fn main() -> Result<()> {
@@ -28,7 +28,8 @@ fn main() -> Result<()> {
             syn::{lexer, parser},
         }
     };
-    let text = get_input_string()?;
+    let opt = Options::from_args();
+    let text = get_input_string(&opt.input)?;
     let lexerdef = lexer::lexerdef();
     let lexer = lexerdef.lexer(&text);
     let (res, errors) = parser::parse(&lexer);