diff --git a/src/abstract_tree/assignment.rs b/src/abstract_tree/assignment.rs
index fe0b1fa..ac52999 100644
--- a/src/abstract_tree/assignment.rs
+++ b/src/abstract_tree/assignment.rs
@@ -1,3 +1,5 @@
+use std::ops::AddAssign;
+
 use serde::{Deserialize, Serialize};
 
 use crate::{
@@ -129,13 +131,12 @@ impl AbstractTree for Assignment {
     }
 
     fn run(&self, source: &str, context: &Context) -> Result<Value, RuntimeError> {
-        let value = self.statement.run(source, context)?;
+        let right = self.statement.run(source, context)?;
 
         let new_value = match self.operator {
             AssignmentOperator::PlusEqual => {
-                if let Some(mut previous_value) = context.get_value(&self.identifier)? {
-                    previous_value += value;
-                    previous_value
+                if let Some(left) = context.get_value(&self.identifier)? {
+                    left.add_assign(right)?
                 } else {
                     return Err(RuntimeError::VariableIdentifierNotFound(
                         self.identifier.clone(),
@@ -143,16 +144,15 @@ impl AbstractTree for Assignment {
                 }
             }
             AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = context.get_value(&self.identifier)? {
-                    previous_value -= value;
-                    previous_value
+                if let Some(mut left) = context.get_value(&self.identifier)? {
+                    left.sub_assign(right)?
                 } else {
                     return Err(RuntimeError::VariableIdentifierNotFound(
                         self.identifier.clone(),
                     ));
                 }
             }
-            AssignmentOperator::Equal => value,
+            AssignmentOperator::Equal => right,
         };
 
         context.set_value(self.identifier.clone(), new_value)?;
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index e2295c8..d82e6ae 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -108,12 +108,12 @@ impl AbstractTree for For {
 
         if let Value::List(list) = &expression_run {
             if self.is_async {
-                list.items().par_iter().try_for_each(|value| {
+                list.items()?.par_iter().try_for_each(|value| {
                     self.context.set_value(key.clone(), value.clone())?;
                     self.block.run(source, &self.context).map(|_value| ())
                 })?;
             } else {
-                for value in list.items().iter() {
+                for value in list.items()?.iter() {
                     self.context.set_value(key.clone(), value.clone())?;
                     self.block.run(source, &self.context)?;
                 }
diff --git a/src/abstract_tree/index.rs b/src/abstract_tree/index.rs
index c0a4cea..ec9a9cc 100644
--- a/src/abstract_tree/index.rs
+++ b/src/abstract_tree/index.rs
@@ -55,7 +55,7 @@ impl AbstractTree for Index {
         match value {
             Value::List(list) => {
                 let index = self.index.run(source, context)?.as_integer()? as usize;
-                let item = list.items().get(index).cloned().unwrap_or_default();
+                let item = list.items()?.get(index).cloned().unwrap_or_default();
 
                 Ok(item)
             }
diff --git a/src/abstract_tree/math.rs b/src/abstract_tree/math.rs
index aa315f5..7069d6d 100644
--- a/src/abstract_tree/math.rs
+++ b/src/abstract_tree/math.rs
@@ -49,12 +49,12 @@ impl AbstractTree for Math {
         let left = self.left.run(source, context)?;
         let right = self.right.run(source, context)?;
         let value = match self.operator {
-            MathOperator::Add => left + right,
-            MathOperator::Subtract => left - right,
-            MathOperator::Multiply => left * right,
-            MathOperator::Divide => left / right,
-            MathOperator::Modulo => left % right,
-        }?;
+            MathOperator::Add => left.add_assign(right)?,
+            MathOperator::Subtract => todo!(),
+            MathOperator::Multiply => todo!(),
+            MathOperator::Divide => todo!(),
+            MathOperator::Modulo => todo!(),
+        };
 
         Ok(value)
     }
diff --git a/src/built_in_functions/mod.rs b/src/built_in_functions/mod.rs
index 833dbdc..8a603c6 100644
--- a/src/built_in_functions/mod.rs
+++ b/src/built_in_functions/mod.rs
@@ -125,7 +125,7 @@ impl Callable for BuiltInFunction {
 
                 let value = arguments.first().unwrap();
                 let length = if let Ok(list) = value.as_list() {
-                    list.items().len()
+                    list.items()?.len()
                 } else if let Ok(map) = value.as_map() {
                     map.inner().len()
                 } else if let Ok(str) = value.as_string() {
@@ -163,7 +163,7 @@ impl Callable for BuiltInFunction {
                 let value = arguments.first().unwrap();
 
                 if let Ok(list) = value.as_list() {
-                    let items = list.items();
+                    let items = list.items()?;
 
                     if items.len() == 0 {
                         Ok(Value::none())
diff --git a/src/built_in_functions/str.rs b/src/built_in_functions/str.rs
index 08d215a..54fce79 100644
--- a/src/built_in_functions/str.rs
+++ b/src/built_in_functions/str.rs
@@ -334,7 +334,7 @@ impl Callable for StrFunction {
                 RuntimeError::expect_argument_amount(self.name(), 3, arguments.len())?;
 
                 let mut string = arguments.first().unwrap().as_string()?.clone();
-                let range = arguments.get(1).unwrap().as_list()?.items();
+                let range = arguments.get(1).unwrap().as_list()?.items()?;
                 let start = range[0].as_integer()? as usize;
                 let end = range[1].as_integer()? as usize;
                 let pattern = arguments.get(2).unwrap().as_string()?;
diff --git a/src/context.rs b/src/context.rs
index 9d25c24..f8f749c 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -117,7 +117,7 @@ impl Context {
         Ok(())
     }
 
-    /// Modify a context to take the functions and type definitions of another.
+    /// Modify a context to take all the information of another.
     ///
     /// In the case of the conflict, the inherited value will override the previous
     /// value.
@@ -173,7 +173,7 @@ impl Context {
     pub fn get_type(&self, identifier: &Identifier) -> Result<Option<Type>, RwLockError> {
         if let Some(value_data) = self.inner.read()?.get(identifier) {
             match value_data {
-                ValueData::Value { inner, .. } => return Ok(Some(inner.r#type())),
+                ValueData::Value { inner, .. } => return Ok(Some(inner.r#type()?)),
                 ValueData::TypeHint { inner, .. } => return Ok(Some(inner.clone())),
                 ValueData::TypeDefinition(_) => todo!(),
             }
@@ -181,7 +181,7 @@ impl Context {
 
         for built_in_value in all_built_in_values() {
             if built_in_value.name() == identifier.inner().as_ref() {
-                return Ok(Some(built_in_value.get().r#type()));
+                return Ok(Some(built_in_value.get().r#type()?));
             }
         }
 
diff --git a/src/error/runtime_error.rs b/src/error/runtime_error.rs
index 8137af7..b60f493 100644
--- a/src/error/runtime_error.rs
+++ b/src/error/runtime_error.rs
@@ -136,6 +136,12 @@ impl RuntimeError {
     }
 }
 
+impl From<ValidationError> for RuntimeError {
+    fn from(error: ValidationError) -> Self {
+        RuntimeError::ValidationFailure(error)
+    }
+}
+
 impl From<csv::Error> for RuntimeError {
     fn from(error: csv::Error) -> Self {
         RuntimeError::Csv(error.to_string())
@@ -184,12 +190,6 @@ impl From<FromUtf8Error> for RuntimeError {
     }
 }
 
-impl Display for RuntimeError {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "{self:?}")
-    }
-}
-
 impl From<RwLockError> for RuntimeError {
     fn from(error: RwLockError) -> Self {
         RuntimeError::RwLock(error)
@@ -201,3 +201,9 @@ impl<T> From<PoisonError<T>> for RuntimeError {
         RuntimeError::RwLock(RwLockError)
     }
 }
+
+impl Display for RuntimeError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        write!(f, "{self:?}")
+    }
+}
diff --git a/src/error/validation_error.rs b/src/error/validation_error.rs
index 25a9876..c4d32f5 100644
--- a/src/error/validation_error.rs
+++ b/src/error/validation_error.rs
@@ -18,6 +18,18 @@ pub enum ValidationError {
     /// The 'assert' macro did not resolve successfully.
     AssertFailed { position: SourcePosition },
 
+    /// Two value are incompatible for addition.
+    CannotAdd { left: Value, right: Value },
+
+    /// Two value are incompatible for subtraction.
+    CannotSubtract { left: Value, right: Value },
+
+    /// Two value are incompatible for multiplication.
+    CannotMultiply { left: Value, right: Value },
+
+    /// Two value are incompatible for dividing.
+    CannotDivide { left: Value, right: Value },
+
     /// The attempted conversion is impossible.
     ConversionImpossible {
         initial_type: Type,
diff --git a/src/value/list.rs b/src/value/list.rs
index ee15f56..6213366 100644
--- a/src/value/list.rs
+++ b/src/value/list.rs
@@ -1,4 +1,8 @@
-use std::fmt::{self, Display, Formatter};
+use std::{
+    cmp::Ordering,
+    fmt::{self, Display, Formatter},
+    sync::{Arc, RwLock, RwLockReadGuard, RwLockWriteGuard},
+};
 
 use stanza::{
     renderer::{console::Console, Renderer},
@@ -6,10 +10,10 @@ use stanza::{
     table::{Cell, Content, Row, Table},
 };
 
-use crate::Value;
+use crate::{error::rw_lock_error::RwLockError, Value};
 
-#[derive(Debug, Clone, Eq, PartialEq, PartialOrd, Ord)]
-pub struct List(Vec<Value>);
+#[derive(Debug, Clone)]
+pub struct List(Arc<RwLock<Vec<Value>>>);
 
 impl Default for List {
     fn default() -> Self {
@@ -19,28 +23,29 @@ impl Default for List {
 
 impl List {
     pub fn new() -> Self {
-        List(Vec::new())
+        List(Arc::new(RwLock::new(Vec::new())))
     }
 
     pub fn with_capacity(capacity: usize) -> Self {
-        List(Vec::with_capacity(capacity))
+        List(Arc::new(RwLock::new(Vec::with_capacity(capacity))))
     }
 
     pub fn with_items(items: Vec<Value>) -> Self {
-        List(items)
+        List(Arc::new(RwLock::new(items)))
     }
 
-    pub fn items(&self) -> &Vec<Value> {
-        &self.0
+    pub fn items(&self) -> Result<RwLockReadGuard<Vec<Value>>, RwLockError> {
+        Ok(self.0.read()?)
     }
 
-    pub fn items_mut(&mut self) -> &mut Vec<Value> {
-        &mut self.0
+    pub fn items_mut(&self) -> Result<RwLockWriteGuard<Vec<Value>>, RwLockError> {
+        Ok(self.0.write()?)
     }
 
     pub fn as_text_table(&self) -> Table {
         let cells: Vec<Cell> = self
             .items()
+            .unwrap()
             .iter()
             .map(|value| {
                 if let Value::List(list) = value {
@@ -76,3 +81,43 @@ impl Display for List {
         f.write_str(&renderer.render(&self.as_text_table()))
     }
 }
+
+impl Eq for List {}
+
+impl PartialEq for List {
+    fn eq(&self, other: &Self) -> bool {
+        if let Ok(left) = self.items() {
+            if let Ok(right) = other.items() {
+                if left.len() != right.len() {
+                    return false;
+                } else {
+                    for (i, j) in left.iter().zip(right.iter()) {
+                        if i != j {
+                            return false;
+                        }
+                    }
+                }
+            }
+        }
+
+        false
+    }
+}
+
+impl PartialOrd for List {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for List {
+    fn cmp(&self, other: &Self) -> Ordering {
+        if let Ok(left) = self.items() {
+            if let Ok(right) = other.items() {
+                return left.cmp(&right);
+            }
+        }
+
+        Ordering::Equal
+    }
+}
diff --git a/src/value/mod.rs b/src/value/mod.rs
index 7249521..92f6bb4 100644
--- a/src/value/mod.rs
+++ b/src/value/mod.rs
@@ -1,6 +1,8 @@
 //! Types that represent runtime values.
 use crate::{
-    built_in_values::BuiltInValue, error::RuntimeError, Identifier, Type, TypeSpecification,
+    built_in_values::BuiltInValue,
+    error::{rw_lock_error::RwLockError, RuntimeError, ValidationError},
+    Identifier, Type, TypeSpecification,
 };
 
 use serde::{
@@ -68,17 +70,17 @@ impl Value {
         Value::Range(start..=end)
     }
 
-    pub fn r#type(&self) -> Type {
-        match self {
+    pub fn r#type(&self) -> Result<Type, RwLockError> {
+        let r#type = match self {
             Value::List(list) => {
                 let mut previous_type = None;
 
-                for value in list.items().iter() {
+                for value in list.items()?.iter() {
                     let value_type = value.r#type();
 
                     if let Some(previous) = &previous_type {
                         if &value_type != previous {
-                            return Type::List(Box::new(Type::Any));
+                            return Ok(Type::List(Box::new(Type::Any)));
                         }
                     }
 
@@ -86,7 +88,7 @@ impl Value {
                 }
 
                 if let Some(previous) = previous_type {
-                    Type::List(Box::new(previous))
+                    Type::List(Box::new(previous?))
                 } else {
                     Type::List(Box::new(Type::Any))
                 }
@@ -97,7 +99,7 @@ impl Value {
                 for (key, value) in map.inner() {
                     identifier_types.push((
                         Identifier::new(key.inner()),
-                        TypeSpecification::new(value.r#type()),
+                        TypeSpecification::new(value.r#type()?),
                     ));
                 }
 
@@ -111,7 +113,9 @@ impl Value {
             Value::Range(_) => todo!(),
             Value::Struct(_) => todo!(),
             Value::Enum(_) => todo!(),
-        }
+        };
+
+        Ok(r#type)
     }
 
     pub fn is_string(&self) -> bool {
@@ -150,7 +154,8 @@ impl Value {
         self == &Value::none()
     }
 
-    /// Borrows the value stored in `self` as `&String`, or returns `Err` if `self` is not a `Value::String`.
+    /// Borrows the value stored in `self` as `&String`, or returns `Err` if
+    /// `self` is not a `Value::String`.
     pub fn as_string(&self) -> Result<&String, RuntimeError> {
         match self {
             Value::String(string) => Ok(string),
@@ -160,7 +165,8 @@ impl Value {
         }
     }
 
-    /// Copies the value stored in `self` as `i64`, or returns `Err` if `self` is not a `Value::Int`
+    /// Copies the value stored in `self` as `i64`, or returns `Err` if `self`
+    /// is not a `Value::Int`
     pub fn as_integer(&self) -> Result<i64, RuntimeError> {
         match self {
             Value::Integer(i) => Ok(*i),
@@ -170,7 +176,8 @@ impl Value {
         }
     }
 
-    /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self` is not a `Primitive::Float`.
+    /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self`
+    /// is not a `Primitive::Float`.
     pub fn as_float(&self) -> Result<f64, RuntimeError> {
         match self {
             Value::Float(f) => Ok(*f),
@@ -180,8 +187,11 @@ impl Value {
         }
     }
 
-    /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self` is not a `Primitive::Float` or `Value::Int`.
-    /// Note that this method silently converts `i64` to `f64`, if `self` is a `Value::Int`.
+    /// Copies the value stored in  `self` as `f64`, or returns `Err` if `self`
+    /// is not a `Primitive::Float` or `Value::Int`.
+    ///
+    /// Note that this method silently converts `i64` to `f64`, if `self` is
+    /// a `Value::Int`.
     pub fn as_number(&self) -> Result<f64, RuntimeError> {
         match self {
             Value::Float(f) => Ok(*f),
@@ -192,7 +202,8 @@ impl Value {
         }
     }
 
-    /// Copies the value stored in  `self` as `bool`, or returns `Err` if `self` is not a `Primitive::Boolean`.
+    /// Copies the value stored in  `self` as `bool`, or returns `Err` if `self`
+    /// is not a `Primitive::Boolean`.
     pub fn as_boolean(&self) -> Result<bool, RuntimeError> {
         match self {
             Value::Boolean(boolean) => Ok(*boolean),
@@ -202,7 +213,8 @@ impl Value {
         }
     }
 
-    /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::List`.
+    /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if
+    /// `self` is not a `Value::List`.
     pub fn as_list(&self) -> Result<&List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
@@ -212,7 +224,8 @@ impl Value {
         }
     }
 
-    /// Takes ownership of the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::List`.
+    /// Takes ownership of the value stored in `self` as `Vec<Value>`, or
+    /// returns `Err` if `self` is not a `Value::List`.
     pub fn into_inner_list(self) -> Result<List, RuntimeError> {
         match self {
             Value::List(list) => Ok(list),
@@ -222,7 +235,8 @@ impl Value {
         }
     }
 
-    /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if `self` is not a `Value::Map`.
+    /// Borrows the value stored in `self` as `Vec<Value>`, or returns `Err` if
+    /// `self` is not a `Value::Map`.
     pub fn as_map(&self) -> Result<&Map, RuntimeError> {
         match self {
             Value::Map(map) => Ok(map),
@@ -242,6 +256,35 @@ impl Value {
             }),
         }
     }
+
+    pub fn add_assign(self, other: Self) -> Result<Value, ValidationError> {
+        match (self, other) {
+            (Value::Float(_), Value::Float(_)) => todo!(),
+            (Value::Float(_), Value::Integer(_)) => todo!(),
+            (Value::Integer(left), Value::Float(right)) => Ok(Value::Float((left as f64) + right)),
+            (Value::Integer(left), Value::Integer(right)) => Ok(Value::Integer(left + right)),
+            (Value::List(list), value) | (value, Value::List(list)) => {
+                list.items_mut()?.push(value);
+
+                Ok(Value::List(list))
+            }
+            (Value::Map(_), _) | (_, Value::Map(_)) => todo!(),
+            (Value::String(_), Value::String(_)) => todo!(),
+            (left, right) => Err(ValidationError::CannotAdd { left, right }),
+        }
+    }
+
+    pub fn sub_assign(self, other: Self) -> Result<Value, ValidationError> {
+        match (self, other) {
+            (Value::Float(_), Value::Float(_)) => todo!(),
+            (Value::Float(_), Value::Integer(_)) => todo!(),
+            (Value::Integer(left), Value::Float(right)) => Ok(Value::Float((left as f64) + right)),
+            (Value::Integer(left), Value::Integer(right)) => Ok(Value::Integer(left + right)),
+            (Value::Map(_), _) | (_, Value::Map(_)) => todo!(),
+            (Value::String(_), Value::String(_)) => todo!(),
+            (left, right) => Err(ValidationError::CannotSubtract { left, right }),
+        }
+    }
 }
 
 impl Default for Value {
@@ -360,7 +403,6 @@ impl AddAssign for Value {
             (Value::Float(left), Value::Float(right)) => *left += right,
             (Value::Float(left), Value::Integer(right)) => *left += right as f64,
             (Value::String(left), Value::String(right)) => *left += &right,
-            (Value::List(list), value) => list.items_mut().push(value),
             _ => {}
         }
     }
@@ -372,17 +414,6 @@ impl SubAssign for Value {
             (Value::Integer(left), Value::Integer(right)) => *left -= right,
             (Value::Float(left), Value::Float(right)) => *left -= right,
             (Value::Float(left), Value::Integer(right)) => *left -= right as f64,
-            (Value::List(list), value) => {
-                let index_to_remove = list
-                    .items()
-                    .iter()
-                    .enumerate()
-                    .find_map(|(i, list_value)| if list_value == &value { Some(i) } else { None });
-
-                if let Some(index) = index_to_remove {
-                    list.items_mut().remove(index);
-                }
-            }
             _ => {}
         }
     }
@@ -465,7 +496,12 @@ impl Serialize for Value {
             Value::Integer(inner) => serializer.serialize_i64(*inner),
             Value::Boolean(inner) => serializer.serialize_bool(*inner),
             Value::List(inner) => {
-                let items = inner.items();
+                let items = if let Ok(items) = inner.items() {
+                    items
+                } else {
+                    return Err(serde::ser::Error::custom("failed to obtain a read lock"));
+                };
+
                 let mut list = serializer.serialize_tuple(items.len())?;
 
                 for value in items.iter() {
diff --git a/src/value/struct_instance.rs b/src/value/struct_instance.rs
index e7bc9dc..24239a0 100644
--- a/src/value/struct_instance.rs
+++ b/src/value/struct_instance.rs
@@ -21,7 +21,7 @@ impl Display for StructInstance {
         writeln!(f, "{{")?;
 
         for (key, value) in self.map.inner() {
-            writeln!(f, "  {key} <{}> = {value}", value.r#type())?;
+            writeln!(f, "  {key} <{}> = {value}", value.r#type().unwrap())?;
         }
 
         write!(f, "}}")
diff --git a/tests/for_loop.rs b/tests/for_loop.rs
index dbe200f..b712136 100644
--- a/tests/for_loop.rs
+++ b/tests/for_loop.rs
@@ -42,12 +42,12 @@ fn modify_list() {
     );
 
     assert_eq!(
+        result,
         Ok(Value::List(List::with_items(vec![
             Value::Integer(1),
             Value::Integer(2),
             Value::Integer(3),
         ]))),
-        result
     );
 }
 
@@ -74,7 +74,7 @@ fn modify_map() {
 }
 
 #[test]
-fn do_not_modify_list_values() {
+fn modify_list_values() {
     let result = interpret(
         "
         list = [1 2 3]
@@ -84,12 +84,12 @@ fn do_not_modify_list_values() {
     );
 
     assert_eq!(
+        result,
         Ok(Value::List(List::with_items(vec![
-            Value::Integer(1),
             Value::Integer(2),
             Value::Integer(3),
+            Value::Integer(4),
         ]))),
-        result
     );
 }