add eval command

src/commands/eval/mod.rs

@@ -0,0 +1,23 @@
+use crate::common::{Context, Error};
+
+mod tokenizer;
+mod parse;
+
+fn evaluate(expr: &str) -> Result<f64, Error> {
+    let tokens = tokenizer::Token::tokenize(expr)?;
+    let mut tokens = tokens.iter();
+
+    let tree = parse::ParseTree::new(&mut tokens)?;
+
+    Ok(tree.evaluate())
+}
+
+/// Evaluates an expression (uses Polish Notation)
+#[poise::command(slash_command, prefix_command)]
+pub async fn eval(ctx: Context<'_>,
+                  #[rest]
+                  expr: String) -> Result<(), Error>
+{
+    ctx.reply(format!("{}", evaluate(&expr)?)).await?;
+    Ok(())
+}