Either load a script or run the REPL

src/main.rs

@@ -1,3 +1,41 @@
-fn main() {
-    println!("Hello, world!");
+use std::{env, fs, io::{self, Write}};
+
+/// Execute a script.
+fn run(_source: String) {
+    todo!();
+}
+
+/// Run the REPL.
+fn run_prompt() {
+    let mut buffer = String::new();
+    let stdin = io::stdin();
+    let mut stdout = io::stdout();
+    loop {
+        print!("slox> ");
+        stdout.flush().unwrap();
+        let n_read = stdin.read_line(&mut buffer).expect("Failed to read from stdin");
+        match n_read {
+            0 => return,
+            _ => run(buffer.clone()),
+        }
+    }
+}
+
+/// Load a file and run the script it contains.
+fn run_file(file: &str) {
+    let contents = fs::read_to_string(file).expect(&format!("Could not load {}", file));
+    run(contents)
+}
+
+/// Main program. Either load a script from a file, or start the REPL.
+fn main() {
+    let args: Vec<String> = env::args().skip(1).collect();
+    let n_args = args.len();
+    if n_args == 0 {
+        run_prompt()
+    } else if n_args == 1 {
+        run_file(&args[0]);
+    } else {
+        println!("Usage: slox [script]");
+    }
 }