feat(cli): create proper cli

made a proper cli using clap

src/cli/mod.rs

@@ -0,0 +1,4 @@
+mod structure;
+
+pub use structure::Action;
+pub use structure::Cli;

src/cli/structure.rs

@@ -0,0 +1,21 @@
+use clap::{Args, Parser, Subcommand};
+
+#[derive(Parser)]
+#[command(version, about = "a cli for math", long_about = None)]
+pub struct Cli {
+    #[command(subcommand)]
+    pub action: Option<Action>,
+}
+
+#[derive(Subcommand)]
+pub enum Action {
+    /// evaluate an expression
+    #[command(aliases = ["eval", "e"])]
+    Evaluate(EvaluateArgs),
+}
+
+#[derive(Args)]
+pub struct EvaluateArgs {
+    #[arg(trailing_var_arg = true)]
+    pub expression: Vec<String>,
+}

src/main.rs

@@ -1,8 +1,28 @@
 use std::io::{self, IsTerminal, Write};
 
+use clap::Parser;
+
+mod cli;
 mod math;
 
 fn main() -> io::Result<()> {
+    let cli = cli::Cli::parse();
+
+    match cli.action {
+        Some(cli::Action::Evaluate(args)) => {
+            let expr = args.expression.join(" ");
+            let expr = math::parse(&expr);
+            println!("{}", math::eval(expr));
+            Ok(())
+        }
+        None => repl(),
+    }
+}
+
+fn repl() -> io::Result<()> {
+    if io::stdin().is_terminal() {
+        println!("mcl: math command line | ctrl + d to exit");
+    }
     prompt()?;
 
     for line in io::stdin().lines() {