rpn(1): better error handling

src/rpn.rs

@@ -45,7 +45,7 @@
 use std::{
 	collections::VecDeque,
 	env::args,
-	fmt,
+	fmt::{ self, Display, Formatter },
 	io::stdin,
 	process::ExitCode,
 };
@@ -70,19 +70,6 @@ enum CalcType {
 	Empty,
 }
 
-#[derive(Debug, Clone)]
-struct EvaluationError { pub message: String }
-
-impl fmt::Display for EvaluationError {
-	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-		write!(f, "{}", self.message)
-	}
-}
-
-// I’m no math nerd but I want the highest possible approximation of 0.9
-// repeating and it seems this can give it to me
-const PRECISION_MOD: f64 = 0.9 + f64::EPSILON * 100.0;
-
 impl From<&str> for CalcType {
 	fn from(value: &str) -> Self {
 		match value {
@@ -103,15 +90,48 @@ impl From<&str> for CalcType {
 	}
 }
 
+impl Display for CalcType {
+	fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+		let y: String;
+		write!(f, "{}", match self {
+			Add => "addition",
+			Subtract => "subtraction",
+			Multiply => "multiplication",
+			Divide => "division",
+			Power => "exponentiation",
+			Floor => "floor divion",
+			Modulo => "modulus",
+			Val(x) => {
+				y = x.to_string(); &y
+			},
+			Empty => "",
+		})
+	}
+}
+
+#[derive(Debug, Clone)]
+struct EvaluationError { pub message: String }
+
+impl Display for EvaluationError {
+	fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+		write!(f, "{}", self.message)
+	}
+}
+
+// I’m no math nerd but I want the highest possible approximation of 0.9
+// repeating and it seems this can give it to me
+const PRECISION_MOD: f64 = 0.9 + f64::EPSILON * 100.0;
+
 fn eval(
 	input: &str,
 	initial_stack: VecDeque<f64>,
-) -> Result<VecDeque<f64>, EvaluationError> {
+) -> Result<(VecDeque<f64>, bool), EvaluationError> {
 	let mut stack = initial_stack;
+	let mut oper = false;
 
 	if input.is_empty() {
 		stack.clear();
-		return Ok(stack);
+		return Ok((stack, oper));
 	}
 
 	// Split the input into tokens.
@@ -122,7 +142,7 @@ fn eval(
 		let x: CalcType = match CalcType::from(tok) {
 			Empty => {
 				return Err(EvaluationError {
-					message: format!("Invalid token: {}", tok),
+					message: format!("{}: Invalid token", tok),
 				})
 			},
 			x => x,
@@ -136,13 +156,28 @@ fn eval(
 
 	for op in &ops {
 		match op {
-			Val(v) => {
+			Val(_) => {
 				return Err(EvaluationError { message: format!(
 					"{}: Unexpected value in the operator stack.",
-					v
+					op,
 				)});
 			},
 			_ => {
+
+				if stack.len() < 2 {
+					return Err(EvaluationError {
+						message: format!("{}: Unexpected operation.", op)
+					})
+				}
+
+				if stack.len() > ops.len() + 1 {
+					return Err(
+						EvaluationError { message: format!("Syntax error.")}
+					);
+				}
+
+				oper = true;
+
 				let (x, y) = (
 					stack.pop_back().unwrap(),
 					stack.pop_back().unwrap(),
@@ -162,7 +197,7 @@ fn eval(
 		};
 	}
 
-	Ok(stack)
+	Ok((stack, oper))
 }
 
 // Round a float to the given precision level
@@ -183,15 +218,17 @@ fn main() -> ExitCode {
 		while let Ok(_) = stdin().read_line(&mut buf) {
 			match eval(&buf.trim(), stack) {
 				Ok(s) => {
-					stack = s.clone();
+					buf.clear();
+					stack = s.0.clone();
 
 					let val = match stack.iter().last() {
 						Some(v) => v,
 						None => break,
 					};
 
+					if s.1 == false { continue; }
+
 					println!("{}", round_precise(val, precision).to_string());
-					buf.clear();
 				},
 				Err(err) => {
 					eprintln!("{}: {}", argv[0], err.message);
@@ -209,7 +246,7 @@ fn main() -> ExitCode {
 
 		match eval(&input, stack) {
 			Ok(s) => {
-				stack = s.clone();
+				stack = s.0.clone();
 
 				let val = match stack.iter().last() {
 					Some(v) => v,