diff --git a/crates/aiken-lang/src/ast.rs b/crates/aiken-lang/src/ast.rs
index 750d7cc4..e2dec29f 100644
--- a/crates/aiken-lang/src/ast.rs
+++ b/crates/aiken-lang/src/ast.rs
@@ -1225,6 +1225,7 @@ impl TypedPattern {
     }
 
     pub fn tipo(&self, value: &TypedExpr) -> Option<Rc<Type>> {
+        // expect thing: Wow = thing
         match self {
             Pattern::Int { .. } => Some(builtins::int()),
             Pattern::Constructor { tipo, .. } => Some(tipo.clone()),
diff --git a/crates/aiken-lang/src/builtins.rs b/crates/aiken-lang/src/builtins.rs
index da98a870..962e4ca4 100644
--- a/crates/aiken-lang/src/builtins.rs
+++ b/crates/aiken-lang/src/builtins.rs
@@ -1267,6 +1267,7 @@ pub fn prelude_data_types(id_gen: &IdGenerator) -> IndexMap<DataTypeKey, TypedDa
 pub fn int() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         name: INT.to_string(),
         module: "".to_string(),
         args: vec![],
@@ -1277,6 +1278,7 @@ pub fn int() -> Rc<Type> {
 pub fn data() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         name: DATA.to_string(),
         module: "".to_string(),
         args: vec![],
@@ -1288,6 +1290,7 @@ pub fn byte_array() -> Rc<Type> {
     Rc::new(Type::App {
         args: vec![],
         public: true,
+        opaque: false,
         name: BYTE_ARRAY.to_string(),
         module: "".to_string(),
         alias: None,
@@ -1297,6 +1300,7 @@ pub fn byte_array() -> Rc<Type> {
 pub fn g1_element() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         module: "".to_string(),
         name: G1_ELEMENT.to_string(),
         args: vec![],
@@ -1307,6 +1311,7 @@ pub fn g1_element() -> Rc<Type> {
 pub fn g2_element() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         module: "".to_string(),
         name: G2_ELEMENT.to_string(),
         args: vec![],
@@ -1317,6 +1322,7 @@ pub fn g2_element() -> Rc<Type> {
 pub fn miller_loop_result() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         module: "".to_string(),
         name: MILLER_LOOP_RESULT.to_string(),
         args: vec![],
@@ -1332,6 +1338,7 @@ pub fn bool() -> Rc<Type> {
     Rc::new(Type::App {
         args: vec![],
         public: true,
+        opaque: false,
         name: BOOL.to_string(),
         module: "".to_string(),
         alias: None,
@@ -1342,6 +1349,7 @@ pub fn prng() -> Rc<Type> {
     Rc::new(Type::App {
         args: vec![],
         public: true,
+        opaque: false,
         name: PRNG.to_string(),
         module: "".to_string(),
         alias: None,
@@ -1355,6 +1363,7 @@ pub fn fuzzer(a: Rc<Type>) -> Rc<Type> {
         name: "PRNG".to_string(),
         arguments: vec![],
     };
+
     Rc::new(Type::Fn {
         args: vec![prng()],
         ret: option(tuple(vec![prng(), a])),
@@ -1391,6 +1400,7 @@ pub fn fuzzer(a: Rc<Type>) -> Rc<Type> {
 pub fn list(t: Rc<Type>) -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         name: LIST.to_string(),
         module: "".to_string(),
         args: vec![t],
@@ -1402,6 +1412,7 @@ pub fn string() -> Rc<Type> {
     Rc::new(Type::App {
         args: vec![],
         public: true,
+        opaque: false,
         name: STRING.to_string(),
         module: "".to_string(),
         alias: None,
@@ -1412,6 +1423,7 @@ pub fn void() -> Rc<Type> {
     Rc::new(Type::App {
         args: vec![],
         public: true,
+        opaque: false,
         name: VOID.to_string(),
         module: "".to_string(),
         alias: None,
@@ -1421,6 +1433,7 @@ pub fn void() -> Rc<Type> {
 pub fn option(a: Rc<Type>) -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         name: OPTION.to_string(),
         module: "".to_string(),
         args: vec![a],
@@ -1431,6 +1444,7 @@ pub fn option(a: Rc<Type>) -> Rc<Type> {
 pub fn ordering() -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         name: ORDERING.to_string(),
         module: "".to_string(),
         args: vec![],
@@ -1448,17 +1462,20 @@ pub fn function(args: Vec<Rc<Type>>, ret: Rc<Type>) -> Rc<Type> {
 
 pub fn generic_var(id: u64) -> Rc<Type> {
     let tipo = Rc::new(RefCell::new(TypeVar::Generic { id }));
+
     Rc::new(Type::Var { tipo, alias: None })
 }
 
 pub fn unbound_var(id: u64) -> Rc<Type> {
     let tipo = Rc::new(RefCell::new(TypeVar::Unbound { id }));
+
     Rc::new(Type::Var { tipo, alias: None })
 }
 
 pub fn wrapped_redeemer(redeemer: Rc<Type>) -> Rc<Type> {
     Rc::new(Type::App {
         public: true,
+        opaque: false,
         module: "".to_string(),
         name: REDEEMER_WRAPPER.to_string(),
         args: vec![redeemer],
diff --git a/crates/aiken-lang/src/tests/check.rs b/crates/aiken-lang/src/tests/check.rs
index 6aac1771..0bc67def 100644
--- a/crates/aiken-lang/src/tests/check.rs
+++ b/crates/aiken-lang/src/tests/check.rs
@@ -1793,3 +1793,41 @@ fn backpassing_type_annotation() {
 
     assert!(check(parse(source_code)).is_ok())
 }
+
+fn forbid_expect_into_opaque_type_from_data() {
+    let source_code = r#"
+        opaque type Thing { inner: Int }
+
+        fn bar(n: Data) {
+          expect a: Thing = n
+
+          a
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::ExpectOnOpaqueType { .. }))
+    ))
+}
+
+#[test]
+fn forbid_expect_into_opaque_type_constructor() {
+    let source_code = r#"
+        opaque type Thing {
+          Foo(Int)
+          Bar(Int)
+        }
+
+        fn bar(n: Thing) {
+          expect Foo(a) = n
+
+          a
+        }
+    "#;
+
+    assert!(matches!(
+        check(parse(source_code)),
+        Err((_, Error::ExpectOnOpaqueType { .. }))
+    ))
+}
diff --git a/crates/aiken-lang/src/tipo.rs b/crates/aiken-lang/src/tipo.rs
index 95442cae..9a113632 100644
--- a/crates/aiken-lang/src/tipo.rs
+++ b/crates/aiken-lang/src/tipo.rs
@@ -42,6 +42,7 @@ pub enum Type {
     ///
     App {
         public: bool,
+        opaque: bool,
         module: String,
         name: String,
         args: Vec<Rc<Type>>,
@@ -152,12 +153,14 @@ impl Type {
         Rc::new(match self {
             Type::App {
                 public,
+                opaque,
                 module,
                 name,
                 args,
                 ..
             } => Type::App {
                 public,
+                opaque,
                 module,
                 name,
                 args,
@@ -181,17 +184,13 @@ impl Type {
         }
     }
 
-    pub fn is_result_constructor(&self) -> bool {
+    pub fn is_opaque(&self) -> bool {
         match self {
-            Type::Fn { ret, .. } => ret.is_result(),
+            Type::App { opaque, .. } => *opaque,
             _ => false,
         }
     }
 
-    pub fn is_result(&self) -> bool {
-        matches!(self, Self::App { name, module, .. } if "Result" == name && module.is_empty())
-    }
-
     pub fn is_unbound(&self) -> bool {
         matches!(self, Self::Var { tipo, .. } if tipo.borrow().is_unbound())
     }
@@ -459,6 +458,7 @@ impl Type {
     pub fn get_app_args(
         &self,
         public: bool,
+        opaque: bool,
         module: &str,
         name: &str,
         arity: usize,
@@ -481,7 +481,7 @@ impl Type {
             Self::Var { tipo, alias } => {
                 let args: Vec<_> = match tipo.borrow().deref() {
                     TypeVar::Link { tipo } => {
-                        return tipo.get_app_args(public, module, name, arity, environment);
+                        return tipo.get_app_args(public, opaque, module, name, arity, environment);
                     }
 
                     TypeVar::Unbound { .. } => {
@@ -496,6 +496,7 @@ impl Type {
                 *tipo.borrow_mut() = TypeVar::Link {
                     tipo: Rc::new(Self::App {
                         public,
+                        opaque,
                         name: name.to_string(),
                         module: module.to_owned(),
                         args: args.clone(),
@@ -650,6 +651,7 @@ pub fn convert_opaque_type(
         match t.as_ref() {
             Type::App {
                 public,
+                opaque,
                 module,
                 name,
                 args,
@@ -662,6 +664,7 @@ pub fn convert_opaque_type(
                 }
                 Type::App {
                     public: *public,
+                    opaque: *opaque,
                     module: module.clone(),
                     name: name.clone(),
                     args: new_args,
@@ -736,6 +739,7 @@ pub fn find_and_replace_generics(
             Type::App {
                 args,
                 public,
+                opaque,
                 module,
                 name,
                 alias,
@@ -748,6 +752,7 @@ pub fn find_and_replace_generics(
                 let t = Type::App {
                     args: new_args,
                     public: *public,
+                    opaque: *opaque,
                     module: module.clone(),
                     name: name.clone(),
                     alias: alias.clone(),
diff --git a/crates/aiken-lang/src/tipo/environment.rs b/crates/aiken-lang/src/tipo/environment.rs
index 1b240269..b83482a6 100644
--- a/crates/aiken-lang/src/tipo/environment.rs
+++ b/crates/aiken-lang/src/tipo/environment.rs
@@ -546,6 +546,7 @@ impl<'a> Environment<'a> {
         match t.deref() {
             Type::App {
                 public,
+                opaque,
                 name,
                 module,
                 args,
@@ -555,8 +556,10 @@ impl<'a> Environment<'a> {
                     .iter()
                     .map(|t| self.instantiate(t.clone(), ids, hydrator))
                     .collect();
+
                 Rc::new(Type::App {
                     public: *public,
+                    opaque: *opaque,
                     name: name.clone(),
                     module: module.clone(),
                     alias: alias.clone(),
@@ -983,6 +986,7 @@ impl<'a> Environment<'a> {
             Definition::DataType(DataType {
                 name,
                 public,
+                opaque,
                 parameters,
                 location,
                 constructors,
@@ -997,6 +1001,7 @@ impl<'a> Environment<'a> {
 
                 let tipo = Rc::new(Type::App {
                     public: *public,
+                    opaque: *opaque,
                     module: module.to_owned(),
                     name: name.clone(),
                     args: parameters.clone(),
@@ -1856,6 +1861,7 @@ pub(crate) fn generalise(t: Rc<Type>, ctx_level: usize) -> Rc<Type> {
 
         Type::App {
             public,
+            opaque,
             module,
             name,
             args,
@@ -1868,6 +1874,7 @@ pub(crate) fn generalise(t: Rc<Type>, ctx_level: usize) -> Rc<Type> {
 
             Rc::new(Type::App {
                 public: *public,
+                opaque: *opaque,
                 module: module.clone(),
                 name: name.clone(),
                 args,
diff --git a/crates/aiken-lang/src/tipo/error.rs b/crates/aiken-lang/src/tipo/error.rs
index ca1fbe7c..a8298cb0 100644
--- a/crates/aiken-lang/src/tipo/error.rs
+++ b/crates/aiken-lang/src/tipo/error.rs
@@ -259,6 +259,13 @@ You can use '{discard}' and numbers to distinguish between similar names.
         name: String,
     },
 
+    #[error("I found an expect assignment involving an opaque type. This is not allowed.\n")]
+    #[diagnostic(code("illegal::expect_on_opaque"))]
+    ExpectOnOpaqueType {
+        #[label]
+        location: Span,
+    },
+
     #[error("I found a type definition that has a function type in it. This is not allowed.\n")]
     #[diagnostic(code("illegal::function_in_type"))]
     #[diagnostic(help(
@@ -1045,6 +1052,7 @@ impl ExtraData for Error {
             | Error::IncorrectTestArity { .. }
             | Error::GenericLeftAtBoundary { .. }
             | Error::UnexpectedMultiPatternAssignment { .. }
+            | Error::ExpectOnOpaqueType { .. }
             | Error::ValidatorMustReturnBool { .. } => None,
 
             Error::UnknownType { name, .. }
diff --git a/crates/aiken-lang/src/tipo/expr.rs b/crates/aiken-lang/src/tipo/expr.rs
index b54e32c0..9a7e1abd 100644
--- a/crates/aiken-lang/src/tipo/expr.rs
+++ b/crates/aiken-lang/src/tipo/expr.rs
@@ -932,6 +932,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
         let mut value_typ = typed_value.tipo();
 
         let value_is_data = value_typ.is_data();
+        let value_is_opaque = value_typ.is_opaque();
 
         // Check that any type annotation is accurate.
         let pattern = if let Some(ann) = annotation {
@@ -939,6 +940,10 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                 .type_from_annotation(ann)
                 .map(|t| self.instantiate(t, &mut HashMap::new()))?;
 
+            if kind.is_expect() && (ann_typ.is_opaque() || value_is_opaque) {
+                return Err(Error::ExpectOnOpaqueType { location });
+            }
+
             self.unify(
                 ann_typ.clone(),
                 value_typ.clone(),
@@ -975,6 +980,10 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                 });
             }
 
+            if kind.is_expect() && value_is_opaque {
+                return Err(Error::ExpectOnOpaqueType { location });
+            }
+
             // Ensure the pattern matches the type of the value
             PatternTyper::new(self.environment, &self.hydrator).unify(
                 untyped_pattern.clone(),
diff --git a/crates/aiken-lang/src/tipo/pattern.rs b/crates/aiken-lang/src/tipo/pattern.rs
index cf437ec7..3fb595c3 100644
--- a/crates/aiken-lang/src/tipo/pattern.rs
+++ b/crates/aiken-lang/src/tipo/pattern.rs
@@ -202,7 +202,7 @@ impl<'a, 'b> PatternTyper<'a, 'b> {
                 location,
                 elements,
                 tail,
-            } => match tipo.get_app_args(true, "", "List", 1, self.environment) {
+            } => match tipo.get_app_args(true, false, "", "List", 1, self.environment) {
                 Some(args) => {
                     let tipo = args
                         .first()
diff --git a/crates/aiken-lang/src/tipo/pretty.rs b/crates/aiken-lang/src/tipo/pretty.rs
index 27da2f6e..3a6d7af5 100644
--- a/crates/aiken-lang/src/tipo/pretty.rs
+++ b/crates/aiken-lang/src/tipo/pretty.rs
@@ -368,6 +368,7 @@ mod tests {
                 module: "whatever".to_string(),
                 name: "Int".to_string(),
                 public: true,
+                opaque: false,
                 args: vec![],
                 alias: None
             },
@@ -378,12 +379,14 @@ mod tests {
                 module: "".to_string(),
                 name: "Pair".to_string(),
                 public: true,
+                opaque: false,
                 alias: None,
                 args: vec![
                     Rc::new(Type::App {
                         module: "whatever".to_string(),
                         name: "Int".to_string(),
                         public: true,
+                        opaque: false,
                         args: vec![],
                         alias: None
                     }),
@@ -391,6 +394,7 @@ mod tests {
                         module: "whatever".to_string(),
                         name: "Bool".to_string(),
                         public: true,
+                        opaque: false,
                         args: vec![],
                         alias: None
                     }),
@@ -406,6 +410,7 @@ mod tests {
                         module: "whatever".to_string(),
                         name: "Int".to_string(),
                         public: true,
+                        opaque: false,
                         alias: None,
                     }),
                     Rc::new(Type::App {
@@ -413,6 +418,7 @@ mod tests {
                         module: "whatever".to_string(),
                         name: "Bool".to_string(),
                         public: true,
+                        opaque: false,
                         alias: None,
                     }),
                 ],
@@ -421,6 +427,7 @@ mod tests {
                     module: "whatever".to_string(),
                     name: "Bool".to_string(),
                     public: true,
+                    opaque: false,
                     alias: None,
                 }),
                 alias: None,
@@ -437,6 +444,7 @@ mod tests {
                         module: "whatever".to_string(),
                         name: "Int".to_string(),
                         public: true,
+                        opaque: false,
                     }),
                 })),
             },
@@ -479,6 +487,7 @@ mod tests {
             Type::Fn {
                 args: vec![Rc::new(Type::App {
                     public: true,
+                    opaque: false,
                     module: "".to_string(),
                     name: "PRNG".to_string(),
                     args: vec![],
@@ -486,12 +495,14 @@ mod tests {
                 })],
                 ret: Rc::new(Type::App {
                     public: true,
+                    opaque: false,
                     module: "".to_string(),
                     name: "Option".to_string(),
                     args: vec![Rc::new(Type::Tuple {
                         elems: vec![
                             Rc::new(Type::App {
                                 public: true,
+                                opaque: false,
                                 module: "".to_string(),
                                 name: "PRNG".to_string(),
                                 args: vec![],
@@ -499,6 +510,7 @@ mod tests {
                             }),
                             Rc::new(Type::App {
                                 public: true,
+                                opaque: false,
                                 module: "".to_string(),
                                 name: "Bool".to_string(),
                                 args: vec![],
@@ -549,6 +561,7 @@ mod tests {
             Type::Fn {
                 args: vec![Rc::new(Type::App {
                     public: true,
+                    opaque: false,
                     module: "".to_string(),
                     name: "PRNG".to_string(),
                     args: vec![],
@@ -556,12 +569,14 @@ mod tests {
                 })],
                 ret: Rc::new(Type::App {
                     public: true,
+                    opaque: false,
                     module: "".to_string(),
                     name: "Option".to_string(),
                     args: vec![Rc::new(Type::Tuple {
                         elems: vec![
                             Rc::new(Type::App {
                                 public: true,
+                                opaque: false,
                                 module: "".to_string(),
                                 name: "PRNG".to_string(),
                                 args: vec![],
diff --git a/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_multi_variants.snap b/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_multi_variants.snap
index 333f1d8b..1b65548e 100644
--- a/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_multi_variants.snap
+++ b/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_multi_variants.snap
@@ -8,6 +8,7 @@ Schema {
         breadcrumbs: [
             App {
                 public: true,
+                opaque: true,
                 module: "test_module",
                 name: "Rational",
                 args: [],
diff --git a/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_variants.snap b/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_variants.snap
index f5d9e83c..bcfa9f67 100644
--- a/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_variants.snap
+++ b/crates/aiken-project/src/blueprint/snapshots/aiken_project__blueprint__validator__tests__opaque_singleton_variants.snap
@@ -8,6 +8,7 @@ Schema {
         breadcrumbs: [
             App {
                 public: true,
+                opaque: true,
                 module: "test_module",
                 name: "Dict",
                 args: [
@@ -16,6 +17,7 @@ Schema {
                             value: Link {
                                 tipo: App {
                                     public: false,
+                                    opaque: false,
                                     module: "test_module",
                                     name: "UUID",
                                     args: [],
@@ -30,6 +32,7 @@ Schema {
                             value: Link {
                                 tipo: App {
                                     public: true,
+                                    opaque: false,
                                     module: "",
                                     name: "Int",
                                     args: [],