Implement user interface

src/main.rs

@@ -1,8 +1,39 @@
 use metrify::run;
+use std::env;
+use std::io;
+use std::io::Write;
+use std::process;
 
 fn main() {
-	match run("5′7″") {
+	let args: Vec<String> = env::args().collect();
+	let name = args[0].clone();
+	let args = args[1..].join(" ");
+
+	let mut input = args;
+
+	if input.len() == 0 {
+		print!("> ");
+		match io::stdout().flush() {
+			Ok(_) => {}
+			Err(err) => {
+				eprintln!("{name}: Error: {err}");
+				process::exit(1);
+			}
+		}
+		match io::stdin().read_line(&mut input) {
+			Ok(_) => {}
+			Err(err) => {
+				eprintln!("{name}: Error: {err}");
+				process::exit(1);
+			}
+		}
+	}
+
+	match run(&input) {
 		Ok(str) => println!("{str}"),
-		Err(err) => eprintln!("{err}"),
+		Err(err) => {
+			eprintln!("{name}: Error: {err}");
+			process::exit(1);
+		}
 	}
 }