Add flattening of operator chains when building operator tree

Relates to #44

src/operator/mod.rs

@@ -91,6 +91,15 @@ impl Operator {
         }
     }
 
+    /// Returns true if chains of this operator should be flattened into one operator with many arguments.
+    // Make this a const fn once #57563 is resolved
+    fn is_flatten_chains(&self) -> bool {
+        match self {
+            Operator::Chain => true,
+            _ => false,
+        }
+    }
+
     /// True if this operator is a leaf, meaning it accepts no arguments.
     // Make this a const fn once #57563 is resolved
     pub(crate) fn is_leaf(&self) -> bool {

src/tree/mod.rs

@@ -383,6 +383,9 @@ impl Node {
                         .last_mut()
                         .unwrap()
                         .insert_back_prioritized(node, false)
+                } else if self.children.last().unwrap().operator().id() == node.operator().id() && node.operator().is_flatten_chains() && !self.children.last().unwrap().has_enough_children() {
+                    // The operators will be chained together, and the next value will be added to this nodes last child.
+                    Ok(())
                 } else {
                     if node.operator().is_leaf() {
                         return Err(EvalexprError::AppendedToLeafNode);

src/value/mod.rs

@@ -194,6 +194,10 @@ impl From<Value> for EvalexprResult<Value> {
     }
 }
 
+impl From<()> for Value {
+    fn from(_: ()) -> Self { Value::Empty }
+}
+
 #[cfg(test)]
 mod tests {
     use value::{TupleType, Value};

tests/integration.rs

@@ -605,3 +605,17 @@ fn test_serde() {
         assert_eq!(manual_tree.eval(), serde_tree.eval());
     }
 }
+
+#[test]
+fn test_tuple_definitions() {
+    assert_eq!(eval_empty("()"), Ok(()));
+    assert_eq!(eval_int("(3)"), Ok(3));
+    assert_eq!(eval_tuple("(3, 4)"), Ok(vec![Value::from(3), Value::from(4)]));
+    assert_eq!(eval_tuple("2, (5, 6)"), Ok(vec![Value::from(2), Value::from(vec![Value::from(5), Value::from(6)])]));
+    assert_eq!(eval_tuple("1, 2"), Ok(vec![Value::from(1), Value::from(2)]));
+    assert_eq!(eval_tuple("1, 2, 3, 4"), Ok(vec![Value::from(1), Value::from(2), Value::from(3), Value::from(4)]));
+    assert_eq!(eval_tuple("(1, 2, 3), 5, 6, (true, false, 0)"), Ok(vec![Value::from(vec![Value::from(1), Value::from(2), Value::from(3)]), Value::from(5), Value::from(6), Value::from(vec![Value::from(true), Value::from(false), Value::from(0)])]));
+    assert_eq!(eval_tuple("1, (2)"), Ok(vec![Value::from(1), Value::from(2)]));
+    assert_eq!(eval_tuple("1, ()"), Ok(vec![Value::from(1), Value::from(())]));
+    assert_eq!(eval_tuple("1, ((2))"), Ok(vec![Value::from(1), Value::from(2)]));
+}