Reduce unwrap()s.

Relates to #14

src/context/mod.rs

@@ -101,7 +101,7 @@ impl Context for HashMapContext {
 /// let ctx = evalexpr::context_map! {
 ///     "x" => 8,
 ///     "f" => Function::new(Box::new(|_| Ok(42.into()) ))
-/// }.unwrap();
+/// }.unwrap(); // Do proper error handling here
 ///
 /// assert_eq!(eval_with_context("x + f()", &ctx), Ok(50.into()));
 /// ```

src/tree/iter.rs

@@ -19,24 +19,19 @@ impl<'a> Iterator for NodeIter<'a> {
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            let mut pop_stack = false;
             let mut result = None;
 
             if let Some(last) = self.stack.last_mut() {
                 if let Some(next) = last.next() {
                     result = Some(next);
                 } else {
-                    pop_stack = true;
+                    // Can not fail because we just borrowed last.
+                    self.stack.pop().unwrap();
                 }
             } else {
                 return None;
             }
 
-            if pop_stack {
-                // Can not fail because we borrowed last before.
-                self.stack.pop().unwrap();
-            }
-
             if let Some(result) = result {
                 self.stack.push(result.children.iter());
                 return Some(result);

src/tree/mod.rs

@@ -389,13 +389,17 @@ impl Node {
             if self.operator().is_leaf() {
                 Err(EvalexprError::AppendedToLeafNode)
             } else if self.has_enough_children() {
-                if self.children.last().unwrap().operator().precedence()
+                // Unwrap cannot fail because of has_enough_children
+                let last_child_operator = self.children.last().unwrap().operator();
+
+                if last_child_operator.precedence()
                     < node.operator().precedence()
                     // Right-to-left chaining
-                    || (self.children.last().unwrap().operator().precedence()
-                    == node.operator().precedence() && !self.children.last().unwrap().operator().is_left_to_right() && !node.operator().is_left_to_right())
+                    || (last_child_operator.precedence()
+                    == node.operator().precedence() && !last_child_operator.is_left_to_right() && !node.operator().is_left_to_right())
                 {
                     // println!("Recursing into {:?}", self.children.last().unwrap().operator());
+                    // Unwrap cannot fail because of has_enough_children
                     self.children
                         .last_mut()
                         .unwrap()
@@ -406,6 +410,7 @@ impl Node {
                         return Err(EvalexprError::AppendedToLeafNode);
                     }
 
+                    // Unwrap cannot fail because of has_enough_children
                     let last_child = self.children.pop().unwrap();
                     self.children.push(node);
                     let node = self.children.last_mut().unwrap();