diff --git a/src/abstract_tree/assignment.rs b/src/abstract_tree/assignment.rs
index 5783122..0dd7425 100644
--- a/src/abstract_tree/assignment.rs
+++ b/src/abstract_tree/assignment.rs
@@ -74,20 +74,10 @@ impl AbstractTree for Assignment {
                     let identifier_type = identifier.expected_type(context)?;
 
                     if let Type::List(item_type) = type_definition.inner() {
-                        let item_type_definition = TypeDefinition::new(*item_type.clone());
+                        println!("{item_type}");
 
-                        item_type_definition.inner().check(
-                            &identifier_type,
-                            context,
-                            identifier_node,
-                            source,
-                        )?;
-                        item_type_definition.inner().check(
-                            &statement_type,
-                            context,
-                            statement_node,
-                            source,
-                        )?;
+                        item_type.check(&identifier_type, context, identifier_node, source)?;
+                        item_type.check(&statement_type, context, statement_node, source)?;
                     } else {
                         type_definition.inner().check(
                             &identifier_type,
@@ -121,7 +111,7 @@ impl AbstractTree for Assignment {
 
         let new_value = match self.operator {
             AssignmentOperator::PlusEqual => {
-                if let Some(mut previous_value) = context.variables()?.get(key).cloned() {
+                if let Some((mut previous_value, _)) = context.variables()?.get(key).cloned() {
                     previous_value += value;
                     previous_value
                 } else {
@@ -129,7 +119,7 @@ impl AbstractTree for Assignment {
                 }
             }
             AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = context.variables()?.get(key).cloned() {
+                if let Some((mut previous_value, _)) = context.variables()?.get(key).cloned() {
                     previous_value -= value;
                     previous_value
                 } else {
@@ -138,8 +128,11 @@ impl AbstractTree for Assignment {
             }
             AssignmentOperator::Equal => value,
         };
+        let new_value_type = new_value.r#type(context)?;
 
-        context.variables_mut()?.insert(key.clone(), new_value);
+        context
+            .variables_mut()?
+            .insert(key.clone(), (new_value, new_value_type));
 
         Ok(Value::Empty)
     }
@@ -151,7 +144,7 @@ impl AbstractTree for Assignment {
 
 #[cfg(test)]
 mod tests {
-    use crate::{evaluate, List, Value};
+    use crate::{evaluate, Error, List, Value};
 
     #[test]
     fn simple_assignment() {
@@ -180,4 +173,19 @@ mod tests {
 
         assert_eq!(Value::List(List::with_items(vec![Value::Integer(1)])), test);
     }
+
+    #[test]
+    fn list_add_wrong_type() {
+        let test = evaluate(
+            "
+            x <[str]> = []
+            x += 1
+            ",
+        );
+
+        if let Err(Error::TypeCheck { .. }) = test {
+        } else {
+            panic!()
+        }
+    }
 }
diff --git a/src/abstract_tree/for.rs b/src/abstract_tree/for.rs
index 057d2f5..65e7ce5 100644
--- a/src/abstract_tree/for.rs
+++ b/src/abstract_tree/for.rs
@@ -59,7 +59,7 @@ impl AbstractTree for For {
 
                 iter_context
                     .variables_mut()?
-                    .insert(key.clone(), value.clone());
+                    .insert(key.clone(), (value.clone(), value.r#type(context)?));
 
                 self.block.run(source, &mut iter_context).map(|_value| ())
             })?;
@@ -69,7 +69,7 @@ impl AbstractTree for For {
             for value in values.iter() {
                 loop_context
                     .variables_mut()?
-                    .insert(key.clone(), value.clone());
+                    .insert(key.clone(), (value.clone(), value.r#type(context)?));
 
                 self.block.run(source, &mut loop_context.clone())?;
             }
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 2444c9b..ea12a62 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -89,7 +89,7 @@ impl AbstractTree for FunctionCall {
 
                 let variables = context.variables()?;
 
-                if let Some(value) = variables.get(key) {
+                if let Some((value, _)) = variables.get(key) {
                     value.clone()
                 } else {
                     return Err(Error::FunctionIdentifierNotFound(identifier.clone()));
diff --git a/src/abstract_tree/identifier.rs b/src/abstract_tree/identifier.rs
index 78af224..5145189 100644
--- a/src/abstract_tree/identifier.rs
+++ b/src/abstract_tree/identifier.rs
@@ -34,7 +34,7 @@ impl AbstractTree for Identifier {
     }
 
     fn run(&self, _source: &str, context: &Map) -> Result<Value> {
-        if let Some(value) = context.variables()?.get(&self.0) {
+        if let Some((value, _)) = context.variables()?.get(&self.0) {
             Ok(value.clone())
         } else {
             Err(Error::VariableIdentifierNotFound(self.inner().clone()))
@@ -42,7 +42,7 @@ impl AbstractTree for Identifier {
     }
 
     fn expected_type(&self, context: &Map) -> Result<Type> {
-        if let Some(value) = context.variables()?.get(&self.0) {
+        if let Some((value, _)) = context.variables()?.get(&self.0) {
             value.r#type(context)
         } else {
             for built_in_function in BUILT_IN_FUNCTIONS {
diff --git a/src/abstract_tree/index.rs b/src/abstract_tree/index.rs
index 3e06fc7..e5abd18 100644
--- a/src/abstract_tree/index.rs
+++ b/src/abstract_tree/index.rs
@@ -61,12 +61,18 @@ impl AbstractTree for Index {
                 let value = if let Expression::Identifier(identifier) = &self.index {
                     let key = identifier.inner();
 
-                    map.variables()?.get(key).cloned().unwrap_or(Value::Empty)
+                    map.variables()?
+                        .get(key)
+                        .map(|(value, _)| value.clone())
+                        .unwrap_or_default()
                 } else {
                     let value = self.index.run(source, context)?;
                     let key = value.as_string()?;
 
-                    map.variables()?.get(key).cloned().unwrap_or(Value::Empty)
+                    map.variables()?
+                        .get(key)
+                        .map(|(value, _)| value.clone())
+                        .unwrap_or_default()
                 };
 
                 Ok(value)
diff --git a/src/abstract_tree/index_assignment.rs b/src/abstract_tree/index_assignment.rs
index a5588ff..61e7a77 100644
--- a/src/abstract_tree/index_assignment.rs
+++ b/src/abstract_tree/index_assignment.rs
@@ -64,7 +64,8 @@ impl AbstractTree for IndexAssignment {
 
         let new_value = match self.operator {
             AssignmentOperator::PlusEqual => {
-                if let Some(mut previous_value) = index_context.variables()?.get(index_key).cloned()
+                if let Some((mut previous_value, _)) =
+                    index_context.variables()?.get(index_key).cloned()
                 {
                     previous_value += value;
                     previous_value
@@ -73,7 +74,8 @@ impl AbstractTree for IndexAssignment {
                 }
             }
             AssignmentOperator::MinusEqual => {
-                if let Some(mut previous_value) = index_context.variables()?.get(index_key).cloned()
+                if let Some((mut previous_value, _)) =
+                    index_context.variables()?.get(index_key).cloned()
                 {
                     previous_value -= value;
                     previous_value
@@ -83,10 +85,11 @@ impl AbstractTree for IndexAssignment {
             }
             AssignmentOperator::Equal => value,
         };
+        let new_value_type = new_value.r#type(context)?;
 
         index_context
             .variables_mut()?
-            .insert(index_key.clone(), new_value);
+            .insert(index_key.clone(), (new_value, new_value_type));
 
         Ok(Value::Empty)
     }
diff --git a/src/abstract_tree/value_node.rs b/src/abstract_tree/value_node.rs
index 25bce08..e459274 100644
--- a/src/abstract_tree/value_node.rs
+++ b/src/abstract_tree/value_node.rs
@@ -132,8 +132,9 @@ impl AbstractTree for ValueNode {
 
                     for (key, statement) in key_statement_pairs {
                         let value = statement.run(source, context)?;
+                        let value_type = value.r#type(context)?;
 
-                        variables.insert(key.clone(), value);
+                        variables.insert(key.clone(), (value, value_type));
                     }
                 }
 
@@ -237,8 +238,11 @@ mod tests {
         {
             let mut variables = map.variables_mut().unwrap();
 
-            variables.insert("x".to_string(), Value::Integer(1));
-            variables.insert("foo".to_string(), Value::String("bar".to_string()));
+            variables.insert("x".to_string(), (Value::Integer(1), Type::Integer));
+            variables.insert(
+                "foo".to_string(),
+                (Value::String("bar".to_string()), Type::String),
+            );
         }
 
         assert_eq!(evaluate("{ x = 1, foo = 'bar' }"), Ok(Value::Map(map)));
diff --git a/src/built_in_functions/fs.rs b/src/built_in_functions/fs.rs
index d14554e..35b95ba 100644
--- a/src/built_in_functions/fs.rs
+++ b/src/built_in_functions/fs.rs
@@ -31,9 +31,16 @@ impl BuiltInFunction for Read {
                     let created = metadata.created()?.elapsed()?.as_secs() as i64;
                     let modified = metadata.modified()?.elapsed()?.as_secs() as i64;
 
-                    file_data_variables.insert("name".to_string(), Value::String(name));
-                    file_data_variables.insert("created".to_string(), Value::Integer(created));
-                    file_data_variables.insert("modified".to_string(), Value::Integer(modified));
+                    file_data_variables
+                        .insert("name".to_string(), (Value::String(name), Type::String));
+                    file_data_variables.insert(
+                        "created".to_string(),
+                        (Value::Integer(created), Type::Integer),
+                    );
+                    file_data_variables.insert(
+                        "modified".to_string(),
+                        (Value::Integer(modified), Type::Integer),
+                    );
                 }
 
                 files.items_mut().push(Value::Map(file_data));
diff --git a/src/value/function.rs b/src/value/function.rs
index ec2be97..63cc4ab 100644
--- a/src/value/function.rs
+++ b/src/value/function.rs
@@ -76,6 +76,7 @@ impl Function {
 
         for ((identifier, argument_type), expression) in parameter_argument_pairs {
             let value = expression.run(source, context)?;
+            let value_type = value.r#type(context)?;
 
             match argument_type {
                 Type::Any => {}
@@ -110,7 +111,9 @@ impl Function {
 
             let key = identifier.inner().clone();
 
-            function_context.variables_mut()?.insert(key, value);
+            function_context
+                .variables_mut()?
+                .insert(key, (value, value_type));
         }
 
         let return_value = self.body.run(source, &function_context)?;
diff --git a/src/value/map.rs b/src/value/map.rs
index 82a972c..1d71bb0 100644
--- a/src/value/map.rs
+++ b/src/value/map.rs
@@ -6,7 +6,7 @@ use std::{
     sync::{Arc, RwLock, RwLockReadGuard, RwLockWriteGuard},
 };
 
-use crate::{value::Value, List, Result, Table};
+use crate::{value::Value, Result, Type};
 
 /// A collection dust variables comprised of key-value pairs.
 ///
@@ -14,7 +14,7 @@ use crate::{value::Value, List, Result, Table};
 /// to one another.
 #[derive(Clone, Debug)]
 pub struct Map {
-    variables: Arc<RwLock<BTreeMap<String, Value>>>,
+    variables: Arc<RwLock<BTreeMap<String, (Value, Type)>>>,
 }
 
 impl Map {
@@ -28,8 +28,8 @@ impl Map {
     pub fn clone_from(other: &Self) -> Result<Self> {
         let mut new_map = BTreeMap::new();
 
-        for (key, value) in other.variables()?.iter() {
-            new_map.insert(key.clone(), value.clone());
+        for (key, (value, r#type)) in other.variables()?.iter() {
+            new_map.insert(key.clone(), (value.clone(), r#type.clone()));
         }
 
         Ok(Map {
@@ -37,11 +37,11 @@ impl Map {
         })
     }
 
-    pub fn variables(&self) -> Result<RwLockReadGuard<BTreeMap<String, Value>>> {
+    pub fn variables(&self) -> Result<RwLockReadGuard<BTreeMap<String, (Value, Type)>>> {
         Ok(self.variables.read()?)
     }
 
-    pub fn variables_mut(&self) -> Result<RwLockWriteGuard<BTreeMap<String, Value>>> {
+    pub fn variables_mut(&self) -> Result<RwLockWriteGuard<BTreeMap<String, (Value, Type)>>> {
         Ok(self.variables.write()?)
     }
 }
@@ -87,30 +87,13 @@ impl Display for Map {
 
         let variables = self.variables.read().unwrap().clone().into_iter();
 
-        for (key, value) in variables {
+        for (key, (value, _)) in variables {
             writeln!(f, "  {key} = {value}")?;
         }
         write!(f, "}}")
     }
 }
 
-impl From<&Table> for Result<Map> {
-    fn from(value: &Table) -> Result<Map> {
-        let map = Map::new();
-
-        for (row_index, row) in value.rows().iter().enumerate() {
-            map.variables_mut()?
-                .insert(
-                    row_index.to_string(),
-                    Value::List(List::with_items(row.clone())),
-                )
-                .unwrap();
-        }
-
-        Ok(map)
-    }
-}
-
 impl Serialize for Map {
     fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
     where
diff --git a/src/value/table.rs b/src/value/table.rs
index c3a6dff..76cbb4d 100644
--- a/src/value/table.rs
+++ b/src/value/table.rs
@@ -282,7 +282,7 @@ impl From<Map> for Result<Table> {
     fn from(map: Map) -> Self {
         let variables = map.variables()?;
         let keys = variables.keys().cloned().collect();
-        let values = variables.values().cloned().collect();
+        let values = variables.values().map(|(value, _)| value.clone()).collect();
         let mut table = Table::new(keys);
 
         table.insert(values)?;
@@ -295,7 +295,7 @@ impl From<&Map> for Result<Table> {
     fn from(map: &Map) -> Self {
         let variables = map.variables()?;
         let keys = variables.keys().cloned().collect();
-        let values = variables.values().cloned().collect();
+        let values = variables.values().map(|(value, _)| value.clone()).collect();
         let mut table = Table::new(keys);
 
         table.insert(values)?;
@@ -308,7 +308,7 @@ impl From<&mut Map> for Result<Table> {
     fn from(map: &mut Map) -> Self {
         let variables = map.variables()?;
         let keys = variables.keys().cloned().collect();
-        let values = variables.values().cloned().collect();
+        let values = variables.values().map(|(value, _)| value.clone()).collect();
         let mut table = Table::new(keys);
 
         table