diff --git a/crates/rue-compiler/src/compiler/item/type_alias_item.rs b/crates/rue-compiler/src/compiler/item/type_alias_item.rs
index 60ffc28..86a9ef2 100644
--- a/crates/rue-compiler/src/compiler/item/type_alias_item.rs
+++ b/crates/rue-compiler/src/compiler/item/type_alias_item.rs
@@ -1,6 +1,7 @@
 use rue_parser::TypeAliasItem;
+use rue_typing::{Alias, Type, TypeId};
 
-use crate::{compiler::Compiler, value::Type, ErrorKind, TypeId};
+use crate::compiler::Compiler;
 
 impl Compiler<'_> {
     /// Define a type for an alias in the current scope, but leave it as unknown for now.
@@ -22,22 +23,11 @@ impl Compiler<'_> {
             .map_or(self.ty.std().unknown, |ty| self.compile_type(ty));
 
         // Set the alias type to the resolved type.
-        *self.db.ty_mut(alias_type_id) = Type::Alias(type_id);
-
-        // A cycle between type aliases has been detected.
-        // We set it to unknown to prevent stack overflow issues later.
-        if self.db.is_cyclic(alias_type_id) {
-            let name = type_alias
-                .name()
-                .expect("the name should exist if it's in a cyclic reference");
-
-            self.db.error(
-                ErrorKind::RecursiveTypeAlias(name.to_string()),
-                name.text_range(),
-            );
-
-            *self.db.ty_mut(alias_type_id) = Type::Unknown;
-        }
+        *self.ty.get_mut(alias_type_id) = Type::Alias(Alias {
+            original_type_id: None,
+            type_id,
+            generic_types: Vec::new(),
+        });
 
         self.type_definition_stack.pop().unwrap();
     }
diff --git a/crates/rue-typing/src/difference.rs b/crates/rue-typing/src/difference.rs
index 73a1a4a..0f2f01a 100644
--- a/crates/rue-typing/src/difference.rs
+++ b/crates/rue-typing/src/difference.rs
@@ -3,15 +3,16 @@ use std::collections::HashSet;
 use num_bigint::BigInt;
 use num_traits::One;
 
-use crate::{bigint_to_bytes, Enum, StandardTypes, Struct, Type, TypeId, TypeSystem, Variant};
+use crate::{bigint_to_bytes, Enum, Struct, Type, TypeId, TypeSystem, Variant};
 
 pub(crate) fn difference_type(
     types: &mut TypeSystem,
-    std: &StandardTypes,
     lhs: TypeId,
     rhs: TypeId,
     visited: &mut HashSet<(TypeId, TypeId)>,
 ) -> TypeId {
+    let std = types.std();
+
     if !visited.insert((lhs, rhs)) {
         return lhs;
     }
@@ -198,8 +199,8 @@ pub(crate) fn difference_type(
             let (lhs_first, lhs_rest) = (*lhs_first, *lhs_rest);
             let (rhs_first, rhs_rest) = (*rhs_first, *rhs_rest);
 
-            let first = difference_type(types, std, lhs_first, rhs_first, visited);
-            let rest = difference_type(types, std, lhs_rest, rhs_rest, visited);
+            let first = difference_type(types, lhs_first, rhs_first, visited);
+            let rest = difference_type(types, lhs_rest, rhs_rest, visited);
 
             if matches!(types.get(first), Type::Never) || matches!(types.get(first), Type::Never) {
                 std.never
@@ -216,7 +217,7 @@ pub(crate) fn difference_type(
             let mut result = Vec::new();
 
             for item in &items {
-                let item = difference_type(types, std, *item, rhs, visited);
+                let item = difference_type(types, *item, rhs, visited);
                 if matches!(types.get(item), Type::Never) {
                     continue;
                 }
@@ -238,17 +239,17 @@ pub(crate) fn difference_type(
             let items = items.clone();
             let mut lhs = lhs;
             for item in items {
-                lhs = difference_type(types, std, lhs, item, visited);
+                lhs = difference_type(types, lhs, item, visited);
             }
             lhs
         }
 
-        (Type::Alias(alias), _) => difference_type(types, std, alias.type_id, rhs, visited),
-        (_, Type::Alias(alias)) => difference_type(types, std, lhs, alias.type_id, visited),
+        (Type::Alias(alias), _) => difference_type(types, alias.type_id, rhs, visited),
+        (_, Type::Alias(alias)) => difference_type(types, lhs, alias.type_id, visited),
 
         (Type::Struct(ty), _) => {
             let ty = ty.clone();
-            let type_id = difference_type(types, std, ty.type_id, rhs, visited);
+            let type_id = difference_type(types, ty.type_id, rhs, visited);
 
             types.alloc(Type::Struct(Struct {
                 original_type_id: Some(ty.original_type_id.unwrap_or(lhs)),
@@ -260,7 +261,7 @@ pub(crate) fn difference_type(
         }
         (_, Type::Struct(ty)) => {
             let ty = ty.clone();
-            let type_id = difference_type(types, std, lhs, ty.type_id, visited);
+            let type_id = difference_type(types, lhs, ty.type_id, visited);
 
             types.alloc(Type::Struct(Struct {
                 original_type_id: Some(ty.original_type_id.unwrap_or(rhs)),
@@ -273,7 +274,7 @@ pub(crate) fn difference_type(
 
         (Type::Enum(ty), _) => {
             let ty = ty.clone();
-            let type_id = difference_type(types, std, ty.type_id, rhs, visited);
+            let type_id = difference_type(types, ty.type_id, rhs, visited);
 
             types.alloc(Type::Enum(Enum {
                 original_type_id: Some(ty.original_type_id.unwrap_or(lhs)),
@@ -284,7 +285,7 @@ pub(crate) fn difference_type(
         }
         (_, Type::Enum(ty)) => {
             let ty = ty.clone();
-            let type_id = difference_type(types, std, lhs, ty.type_id, visited);
+            let type_id = difference_type(types, lhs, ty.type_id, visited);
 
             types.alloc(Type::Enum(Enum {
                 original_type_id: Some(ty.original_type_id.unwrap_or(rhs)),
@@ -296,7 +297,7 @@ pub(crate) fn difference_type(
 
         (Type::Variant(variant), _) => {
             let variant = variant.clone();
-            let type_id = difference_type(types, std, variant.type_id, rhs, visited);
+            let type_id = difference_type(types, variant.type_id, rhs, visited);
 
             types.alloc(Type::Variant(Variant {
                 original_type_id: Some(variant.original_type_id.unwrap_or(lhs)),
@@ -310,7 +311,7 @@ pub(crate) fn difference_type(
         }
         (_, Type::Variant(variant)) => {
             let variant = variant.clone();
-            let type_id = difference_type(types, std, lhs, variant.type_id, visited);
+            let type_id = difference_type(types, lhs, variant.type_id, visited);
 
             types.alloc(Type::Variant(Variant {
                 original_type_id: Some(variant.original_type_id.unwrap_or(rhs)),
diff --git a/crates/rue-typing/src/semantic_types.rs b/crates/rue-typing/src/semantic_types.rs
index 6024b23..bd8b67d 100644
--- a/crates/rue-typing/src/semantic_types.rs
+++ b/crates/rue-typing/src/semantic_types.rs
@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use indexmap::IndexSet;
 use num_bigint::BigInt;
 
-use crate::TypeId;
+use crate::{Comparison, Type, TypeId, TypeSystem};
 
 /// The kind of ending that a list has.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -86,3 +86,193 @@ pub struct Variant {
     /// The discriminant value.
     pub discriminant: BigInt,
 }
+
+/// Constructs a structural type consisting of the items in a list.
+pub fn construct_items(
+    db: &mut TypeSystem,
+    items: impl DoubleEndedIterator<Item = TypeId>,
+    rest: Rest,
+) -> TypeId {
+    let mut result = db.std().nil;
+    for (i, item) in items.rev().enumerate() {
+        if i == 0 {
+            match rest {
+                Rest::Spread => {
+                    result = item;
+                    continue;
+                }
+                Rest::Optional => {
+                    result = db.alloc(Type::Pair(item, result));
+                    result = db.alloc(Type::Union(vec![result, db.std().nil]));
+                    continue;
+                }
+                Rest::Nil => {}
+            }
+        }
+        result = db.alloc(Type::Pair(item, result));
+    }
+    result
+}
+
+/// Deconstructs a structural type into a list of items and a rest value.
+pub fn deconstruct_items(
+    db: &mut TypeSystem,
+    type_id: TypeId,
+    length: usize,
+    rest: Rest,
+) -> Option<Vec<TypeId>> {
+    let mut items = Vec::with_capacity(length);
+    let mut current = type_id;
+
+    for i in (0..length).rev() {
+        if i == 0 {
+            match rest {
+                Rest::Spread => {
+                    items.push(current);
+                    break;
+                }
+                Rest::Optional => {
+                    if db.compare(db.std().nil, current) > Comparison::Assignable {
+                        return None;
+                    }
+
+                    let non_nil = db.difference(current, db.std().nil);
+                    let (first, rest) = db.get_pair(non_nil)?;
+
+                    if db.compare(rest, db.std().nil) > Comparison::Equal {
+                        return None;
+                    }
+
+                    items.push(first);
+                    break;
+                }
+                Rest::Nil => {
+                    let (first, rest) = db.get_pair(current)?;
+                    items.push(first);
+                    if db.compare(rest, db.std().nil) > Comparison::Equal {
+                        return None;
+                    }
+                    break;
+                }
+            }
+        }
+        let (first, rest) = db.get_pair(current)?;
+        items.push(first);
+        current = rest;
+    }
+
+    Some(items)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_construct_int_nil() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int].into_iter(), Rest::Nil);
+        let items = deconstruct_items(&mut db, type_id, 1, Rest::Nil);
+        assert_eq!(items, Some(vec![std.int]));
+    }
+
+    #[test]
+    fn test_construct_int() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int].into_iter(), Rest::Spread);
+        let items = deconstruct_items(&mut db, type_id, 1, Rest::Spread);
+        assert_eq!(items, Some(vec![std.int]));
+    }
+
+    #[test]
+    fn test_construct_int_optional() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int].into_iter(), Rest::Optional);
+        let items = deconstruct_items(&mut db, type_id, 1, Rest::Optional);
+        assert_eq!(items, Some(vec![std.int]));
+    }
+
+    #[test]
+    fn test_construct_empty_nil() {
+        let mut db = TypeSystem::new();
+        let type_id = construct_items(&mut db, [].into_iter(), Rest::Nil);
+        let items = deconstruct_items(&mut db, type_id, 0, Rest::Nil);
+        assert_eq!(items, Some(vec![]));
+    }
+
+    #[test]
+    fn test_construct_empty() {
+        let mut db = TypeSystem::new();
+        let type_id = construct_items(&mut db, [].into_iter(), Rest::Spread);
+        let items = deconstruct_items(&mut db, type_id, 0, Rest::Spread);
+        assert_eq!(items, Some(vec![]));
+    }
+
+    #[test]
+    fn test_construct_empty_optional() {
+        let mut db = TypeSystem::new();
+        let type_id = construct_items(&mut db, [].into_iter(), Rest::Optional);
+        let items = deconstruct_items(&mut db, type_id, 0, Rest::Optional);
+        assert_eq!(items, Some(vec![]));
+    }
+
+    #[test]
+    fn test_construct_int_int_nil() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int, std.int].into_iter(), Rest::Nil);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Nil);
+        assert_eq!(items, Some(vec![std.int, std.int]));
+    }
+
+    #[test]
+    fn test_construct_int_int() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int, std.int].into_iter(), Rest::Spread);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Spread);
+        assert_eq!(items, Some(vec![std.int, std.int]));
+    }
+
+    #[test]
+    fn test_construct_int_int_optional() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let type_id = construct_items(&mut db, [std.int, std.int].into_iter(), Rest::Optional);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Optional);
+        assert_eq!(items, Some(vec![std.int, std.int]));
+    }
+
+    #[test]
+    fn test_construct_bytes32_pair_nil() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let pair = db.alloc(Type::Pair(std.bytes32, std.nil));
+        let type_id = construct_items(&mut db, [std.bytes32, pair].into_iter(), Rest::Nil);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Nil);
+        assert_eq!(items, Some(vec![std.bytes32, pair]));
+    }
+
+    #[test]
+    fn test_construct_bytes32_pair() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let pair = db.alloc(Type::Pair(std.bytes32, std.nil));
+        let type_id = construct_items(&mut db, [std.bytes32, pair].into_iter(), Rest::Spread);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Spread);
+        assert_eq!(items, Some(vec![std.bytes32, pair]));
+    }
+
+    #[test]
+    fn test_construct_bytes32_pair_optional() {
+        let mut db = TypeSystem::new();
+        let std = db.std();
+        let pair = db.alloc(Type::Pair(std.bytes32, std.nil));
+        let type_id = construct_items(&mut db, [std.bytes32, pair].into_iter(), Rest::Optional);
+        let items = deconstruct_items(&mut db, type_id, 2, Rest::Optional);
+        assert_eq!(items, Some(vec![std.bytes32, pair]));
+    }
+}
diff --git a/crates/rue-typing/src/type_system.rs b/crates/rue-typing/src/type_system.rs
index 462e51a..b36ea13 100644
--- a/crates/rue-typing/src/type_system.rs
+++ b/crates/rue-typing/src/type_system.rs
@@ -103,16 +103,16 @@ impl TypeSystem {
         }
     }
 
-    pub fn pair_first(&self, type_id: TypeId) -> Option<TypeId> {
+    pub fn get_pair(&self, type_id: TypeId) -> Option<(TypeId, TypeId)> {
         match self.get(type_id) {
-            Type::Pair(first, _) => Some(*first),
+            Type::Pair(first, rest) => Some((*first, *rest)),
             _ => None,
         }
     }
 
-    pub fn pair_rest(&self, type_id: TypeId) -> Option<TypeId> {
+    pub fn get_union(&self, type_id: TypeId) -> Option<&[TypeId]> {
         match self.get(type_id) {
-            Type::Pair(_, rest) => Some(*rest),
+            Type::Union(types) => Some(types),
             _ => None,
         }
     }
@@ -178,8 +178,8 @@ impl TypeSystem {
         check_type(self, lhs, rhs, &mut HashSet::new()).map(simplify_check)
     }
 
-    pub fn difference(&mut self, std: &StandardTypes, lhs: TypeId, rhs: TypeId) -> TypeId {
-        difference_type(self, std, lhs, rhs, &mut HashSet::new())
+    pub fn difference(&mut self, lhs: TypeId, rhs: TypeId) -> TypeId {
+        difference_type(self, lhs, rhs, &mut HashSet::new())
     }
 
     pub fn replace(