refactor(tazjin/rlox): Add Interpreter trait for switching impls

Change-Id: Iae28d64ce879014c5e5d7e145c536c1f16ad307d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2418
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/main.rs

@@ -7,9 +7,64 @@ use std::process;
 mod bytecode;
 mod treewalk;
 
+/// Trait for making the different interpreters callable in the same
+/// way.
+pub trait Lox {
+    type Value;
+    type Error: std::fmt::Display;
+
+    fn create() -> Self;
+    fn interpret(&mut self, source: String) -> Result<Self::Value, Vec<Self::Error>>;
+}
+
 fn main() {
+    let mut args = env::args();
+    if args.len() > 2 {
+        println!("Usage: rlox [script]");
+        process::exit(1);
+    }
+
     match env::var("LOX_INTERPRETER").as_ref().map(String::as_str) {
-        Ok("treewalk") => treewalk::main(),
-        _ => bytecode::main(),
+        Ok("treewalk") => pick::<treewalk::interpreter::Interpreter>(args.nth(1)),
+        _ => pick::<bytecode::Interpreter>(args.nth(1)),
+    }
+}
+
+fn pick<I: Lox>(file_arg: Option<String>) {
+    if let Some(file) = file_arg {
+        run_file::<I>(&file);
+    } else {
+        run_prompt::<I>();
+    }
+}
+
+// Run Lox code from a file and print results to stdout
+fn run_file<I: Lox>(file: &str) {
+    let contents = fs::read_to_string(file).expect("failed to read the input file");
+    let mut lox = I::create();
+    run(&mut lox, contents);
+}
+
+// Evaluate Lox code interactively in a shitty REPL.
+fn run_prompt<I: Lox>() {
+    let mut line = String::new();
+    let mut lox = I::create();
+
+    loop {
+        print!("> ");
+        io::stdout().flush().unwrap();
+        io::stdin()
+            .read_line(&mut line)
+            .expect("failed to read user input");
+        run(&mut lox, std::mem::take(&mut line));
+        line.clear();
+    }
+}
+
+fn run<I: Lox>(lox: &mut I, code: String) {
+    if let Err(errors) = lox.interpret(code) {
+        for error in errors {
+            eprintln!("{}", error);
+        }
     }
 }