I love match

rust/rpn-calculator/src/lib.rs

@@ -9,42 +9,29 @@ pub enum CalculatorInput {
 
 pub fn evaluate(inputs: &[CalculatorInput]) -> Option<i32> {
     let mut stack: Vec<i32> = Vec::new();
-    let last_result = inputs.iter().fold(None, |_, entry| match entry {
-        CalculatorInput::Value(x) => {
-            stack.push(*x);
-            Some(*x)
-        }
-        CalculatorInput::Add => {
-            let val1 = stack.pop()?;
-            let val2 = stack.pop()?;
-            let result = val2 + val1;
-            stack.push(result);
-            Some(result)
+    let last_result = inputs.iter().fold(None, |current: Option<i32>, entry| {
+        match (entry, stack.pop()) {
+            (CalculatorInput::Value(x), None) => {
+                stack.push(current.unwrap());
+                if let Some(y) = current {
+                    stack.push(y);
                 }
-        CalculatorInput::Subtract => {
-            let val1 = stack.pop()?;
-            let val2 = stack.pop()?;
-            let result = val2 - val1;
-            stack.push(result);
-            Some(result)
+                Some(*x)
             }
-        CalculatorInput::Multiply => {
-            let val1 = stack.pop()?;
-            let val2 = stack.pop()?;
-            let result = val2 * val1;
-            stack.push(result);
-            Some(result)
+            (CalculatorInput::Value(x), Some(y)) => {
+                stack.push(y);
+                stack.push(current.unwrap());
+                Some(*x)
             }
-        CalculatorInput::Divide => {
-            let val1 = stack.pop()?;
-            let val2 = stack.pop()?;
-            let result = val2 / val1;
-            stack.push(result);
-            Some(result)
+            (CalculatorInput::Add, Some(x)) => Some(current.unwrap() + x),
+            (CalculatorInput::Subtract, Some(x)) => Some(x - current.unwrap()),
+            (CalculatorInput::Multiply, Some(x)) => Some(x * current.unwrap()),
+            (CalculatorInput::Divide, Some(x)) => Some(x / current.unwrap()),
+            _ => None,
         }
     });
 
-    if stack.len() == 1 {
+    if stack.is_empty() {
         last_result
     } else {
         // too many operands