Do not require expressions to have content

* Make parser return implicit toplevel node
 * Make `RootNode` return `Value::Empty` if it has no arguments instead of failing

Relates to #28

src/error/display.rs

@@ -29,11 +29,6 @@ impl fmt::Display for EvalexprError {
             },
             ExpectedTuple { actual } => write!(f, "Expected a Value::Tuple, but got {:?}.", actual),
             ExpectedEmpty { actual } => write!(f, "Expected a Value::Empty, but got {:?}.", actual),
-            EmptyExpression => write!(
-                f,
-                "Got an empty expression that cannot be parsed into a node tree, because it \
-                 returns nothing."
-            ),
             AppendedToLeafNode => write!(f, "Tried to append a node to a leaf node."),
             PrecedenceViolation => write!(
                 f,

src/error/mod.rs

@@ -74,9 +74,6 @@ pub enum EvalexprError {
         actual: Value,
     },
 
-    /// The given expression is empty
-    EmptyExpression,
-
     /// Tried to append a child to a leaf node.
     /// Leaf nodes cannot have children.
     AppendedToLeafNode,

src/operator/mod.rs

@@ -21,12 +21,12 @@ pub trait Operator: Debug + Display {
     /// True if this operator is a leaf, meaning it accepts no arguments.
     // Make this a const fn once #57563 is resolved
     fn is_leaf(&self) -> bool {
-        self.argument_amount() == 0
+        self.max_argument_amount() == 0
     }
 
-    /// Returns the amount of arguments required by this operator.
+    /// Returns the maximum amount of arguments required by this operator.
     // Make this a const fn once #57563 is resolved
-    fn argument_amount(&self) -> usize;
+    fn max_argument_amount(&self) -> usize;
 
     /// Evaluates the operator with the given arguments and context.
     fn eval(&self, arguments: &[Value], context: &dyn Context) -> EvalexprResult<Value>;
@@ -119,13 +119,16 @@ impl Operator for RootNode {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         1
     }
 
     fn eval(&self, arguments: &[Value], _context: &Context) -> EvalexprResult<Value> {
-        expect_operator_argument_amount(arguments.len(), 1)?;
-        Ok(arguments[0].clone())
+        if let Some(first) = arguments.first() {
+            Ok(first.clone())
+        } else {
+            Ok(Value::Empty)
+        }
     }
 }
 
@@ -138,7 +141,7 @@ impl Operator for Add {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -174,7 +177,7 @@ impl Operator for Sub {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -210,7 +213,7 @@ impl Operator for Neg {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         1
     }
 
@@ -240,7 +243,7 @@ impl Operator for Mul {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -276,7 +279,7 @@ impl Operator for Div {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -312,7 +315,7 @@ impl Operator for Mod {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -348,7 +351,7 @@ impl Operator for Exp {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -375,7 +378,7 @@ impl Operator for Eq {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -399,7 +402,7 @@ impl Operator for Neq {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -423,7 +426,7 @@ impl Operator for Gt {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -457,7 +460,7 @@ impl Operator for Lt {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -491,7 +494,7 @@ impl Operator for Geq {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -525,7 +528,7 @@ impl Operator for Leq {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -559,7 +562,7 @@ impl Operator for And {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -585,7 +588,7 @@ impl Operator for Or {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -611,7 +614,7 @@ impl Operator for Not {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         1
     }
 
@@ -636,7 +639,7 @@ impl Operator for Tuple {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         2
     }
 
@@ -673,7 +676,7 @@ impl Operator for Const {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         0
     }
 
@@ -693,7 +696,7 @@ impl Operator for VariableIdentifier {
         true
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         0
     }
 
@@ -717,7 +720,7 @@ impl Operator for FunctionIdentifier {
         false
     }
 
-    fn argument_amount(&self) -> usize {
+    fn max_argument_amount(&self) -> usize {
         1
     }
 

src/tree/mod.rs

@@ -264,8 +264,8 @@ impl Node {
         &self.operator
     }
 
-    fn has_correct_amount_of_children(&self) -> bool {
-        self.children().len() == self.operator().argument_amount()
+    fn has_enough_children(&self) -> bool {
+        self.children().len() == self.operator().max_argument_amount()
     }
 
     fn insert_back_prioritized(&mut self, node: Node, is_root_node: bool) -> EvalexprResult<()> {
@@ -275,7 +275,7 @@ impl Node {
         {
             if self.operator().is_leaf() {
                 Err(EvalexprError::AppendedToLeafNode)
-            } else if self.has_correct_amount_of_children() {
+            } else if self.has_enough_children() {
                 if self.children.last().unwrap().operator().precedence()
                     < node.operator().precedence()
                     // Right-to-left chaining
@@ -381,17 +381,8 @@ pub(crate) fn tokens_to_operator_tree(tokens: Vec<Token>) -> EvalexprResult<Node
 
     if root.len() > 1 {
         Err(EvalexprError::UnmatchedLBrace)
-    } else if let Some(mut root) = root.pop() {
-        if root.children().len() > 1 {
-            Err(EvalexprError::wrong_operator_argument_amount(
-                root.children().len(),
-                1,
-            ))
-        } else if let Some(child) = root.children.pop() {
-            Ok(child)
-        } else {
-            Err(EvalexprError::EmptyExpression)
-        }
+    } else if let Some(root) = root.pop() {
+        Ok(root)
     } else {
         Err(EvalexprError::UnmatchedRBrace)
     }