Accept arbitrary filenames in roc-cli

cli/src/main.rs

@@ -9,7 +9,11 @@ use roc::parse;
 use std::io;
 
 fn main() -> std::io::Result<()> {
-    let mut file = File::open("Example.roc")?;
+    let argv = std::env::args().into_iter().collect::<Vec<String>>();
+
+    match argv.get(1) {
+        Some(filename) => {
+            let mut file = File::open(filename)?;
             let mut contents = String::new();
 
             file.read_to_string(&mut contents)?;
@@ -17,6 +21,13 @@ fn main() -> std::io::Result<()> {
             let expr = parse::parse_string(contents.as_str()).unwrap();
 
             process_task(eval(expr))
+        },
+        None => {
+            println!("Usage: roc FILENAME.roc");
+
+            Ok(())
+        }
+    }
 }
 
 fn process_task(evaluated: Evaluated) -> std::io::Result<()> {