diff --git a/scalie.cabal b/scalie.cabal
index 10b8b2a..3a56bee 100644
--- a/scalie.cabal
+++ b/scalie.cabal
@@ -26,6 +26,10 @@ library
       Language.Scalie.Compiler.Bytecode
       Language.Scalie.Core.Definition
       Language.Scalie.Core.Expression
+      Language.Scalie.Core.Expression.ConstructorIdentifier
+      Language.Scalie.Core.Expression.Pattern
+      Language.Scalie.Core.Expression.PatternMatchCase
+      Language.Scalie.Core.Expression.VariableIdentifier
       Language.Scalie.Core.Module
       Language.Scalie.Core.Pattern
       Language.Scalie.Core.Provenance
diff --git a/src/Language/Scalie/Compiler/Bytecode.hs b/src/Language/Scalie/Compiler/Bytecode.hs
index f81cc07..1e067d1 100644
--- a/src/Language/Scalie/Compiler/Bytecode.hs
+++ b/src/Language/Scalie/Compiler/Bytecode.hs
@@ -17,6 +17,7 @@ import Data.Maybe (Maybe(Nothing, Just))
 import Language.Scalie.Core.Definition (Definition(body, name))
 import Language.Scalie.Core.Expression (Expression(RawInt))
 import Language.Scalie.Bytecode.Instruction qualified as Instruction
+import Prelude (undefined)
 
 -- | The name of the entrypoint function
 
@@ -27,7 +28,8 @@ type CompilationError :: Type
 newtype CompilationError = NameNotFound Text
 
 compile :: Module (Provenance SourceLocation) -> EntryPoint -> Either CompilationError Bytecode.Object
-compile mod entry = let
+compile mod entry = undefined
+{- let
   topLevelDefinitions = mod.definitions.value
   in Bytecode.Object . Vector.fromList <$> case ImplicitMap.lookup entry topLevelDefinitions of
     Nothing -> Left $ NameNotFound entry
@@ -35,3 +37,4 @@ compile mod entry = let
       [ (\(RawInt i) -> Instruction.PushInteger i) <$> def.body
       , Provenance def.name.source (Instruction.ReturnValues 1)
       ]
+-}
diff --git a/src/Language/Scalie/Core/Definition.hs b/src/Language/Scalie/Core/Definition.hs
index 8330ef5..a71a040 100644
--- a/src/Language/Scalie/Core/Definition.hs
+++ b/src/Language/Scalie/Core/Definition.hs
@@ -52,13 +52,13 @@ data Definition f = Definition
   -- ^ What is the type
   , name        :: f Text
   -- ^ Which name can be used to refer to this definition
-  , body        :: f Expression
+  , body        :: f (Expression f)
   -- ^ What needs to be evaluated to get the value
   }
 
-deriving stock instance (Show (f Expression), Show (f Scalie.Domain.Type), Show (f Text)) => Show (Definition f)
-deriving stock instance (Read (f Expression), Read (f Scalie.Domain.Type), Read (f Text)) => Read (Definition f)
-deriving stock instance (Eq (f Expression)  , Eq (f Scalie.Domain.Type)  , Eq (f Text))   => Eq (Definition f)
+deriving stock instance (Show (f (Expression f)), Show (f Scalie.Domain.Type), Show (f Text)) => Show (Definition f)
+deriving stock instance (Read (f (Expression f)), Read (f Scalie.Domain.Type), Read (f Text)) => Read (Definition f)
+deriving stock instance (Eq   (f (Expression f)), Eq (f Scalie.Domain.Type)  , Eq (f Text))   => Eq (Definition f)
 
 instance ImplicitKeyOf (Definition (Provenance a)) where
   type KeyType (Definition (Provenance a)) = Text
@@ -75,7 +75,7 @@ instance ImplicitKeyOf (Definition Maybe) where
   keyOf :: Definition Maybe -> KeyType (Definition Maybe)
   keyOf = name
 
-instance (Functor f, Arbitrary (f UnicodeString), Arbitrary (f Scalie.Domain.Type), Arbitrary (f Expression)) => Arbitrary (Definition f) where
+instance (Functor f, Arbitrary (f UnicodeString), Arbitrary (f Scalie.Domain.Type), Arbitrary (f (Expression f))) => Arbitrary (Definition f) where
   arbitrary :: Gen (Definition f)
   arbitrary = Definition
     <$> arbitrary
diff --git a/src/Language/Scalie/Core/Expression.hs b/src/Language/Scalie/Core/Expression.hs
index 1d7a05e..c8d986b 100644
--- a/src/Language/Scalie/Core/Expression.hs
+++ b/src/Language/Scalie/Core/Expression.hs
@@ -1,23 +1,54 @@
 {-# LANGUAGE Safe #-}
+{-# LANGUAGE StandaloneDeriving #-}   -- to specify instance contexts
+{-# LANGUAGE UndecidableInstances #-} -- for the instance constraints containing the functor
 {-# LANGUAGE InstanceSigs #-}
 module Language.Scalie.Core.Expression (Expression(..)) where
 
-import Prelude (Integer)
+import Prelude (Integer, Integral (div))
 import Data.Kind (Type)
 import Text.Show (Show)
 import Text.Read (Read)
 import Data.Eq (Eq)
-import Test.QuickCheck (Arbitrary (arbitrary), Gen, oneof)
-import Data.Functor ((<$>))
+import Data.Ratio (Rational)
+import Data.Text (Text)
+import Data.List.NonEmpty (NonEmpty)
+import Language.Scalie.Core.Expression.PatternMatchCase (PatternMatchCase)
+import Test.QuickCheck (Arbitrary (arbitrary), oneof, UnicodeString (getUnicodeString), NonEmptyList (getNonEmpty))
+import Test.QuickCheck.Gen (Gen)
+import Data.Functor ((<$>), Functor)
+import Control.Category ((.))
+import Data.Text qualified as Text
+import Language.Scalie.Core.Expression.VariableIdentifier (VariableIdentifier)
+import Control.Applicative ((<*>))
+import Data.List.NonEmpty qualified as NonEmpty
+import Test.QuickCheck qualified as Gen
 
-type Expression :: Type
-data Expression
-  = RawInt Integer
-  deriving stock (Show, Read, Eq)
+type Expression :: (Type -> Type) -> Type
+type role Expression nominal
+data Expression f
+  = RawInt              (f Integer)
+  | RawRational         (f Rational)
+  | RawString           (f Text)
+  | PatternMatch        (f (NonEmpty (PatternMatchCase f)))
+  | Lambda              (f VariableIdentifier) (f (Expression f))
 
-instance Arbitrary Expression where
-  arbitrary :: Gen Expression
+-- to me, this looks incredibly dangerous, because of Show (f (Expression f)) => Show (Expression f)
+-- let's hope UndecidableInstances won't have the compiler diverge
+deriving stock instance (Show (f Integer), Show (f Rational), Show (f Text), Show (f (NonEmpty (PatternMatchCase f))), Show (f VariableIdentifier), Show (f (Expression f))) => Show (Expression f)
+deriving stock instance (Read (f Integer), Read (f Rational), Read (f Text), Read (f (NonEmpty (PatternMatchCase f))), Read (f VariableIdentifier), Read (f (Expression f))) => Read (Expression f)
+deriving stock instance (Eq   (f Integer), Eq   (f Rational), Eq   (f Text), Eq   (f (NonEmpty (PatternMatchCase f))), Eq   (f VariableIdentifier), Eq (f (Expression f))) => Eq   (Expression f)
+
+instance (Arbitrary (f Integer), Arbitrary (f Rational), Arbitrary (f UnicodeString), Functor f, Arbitrary (f (Expression f)), Arbitrary (f VariableIdentifier), Arbitrary (f (NonEmptyList (PatternMatchCase f)))               ) => Arbitrary (Expression f) where
+  arbitrary :: Gen (Expression f)
   arbitrary = oneof
     [ RawInt <$> arbitrary
+    , RawRational <$> arbitrary
+    , RawString . (Text.pack . getUnicodeString <$>) <$> arbitrary
+    , PatternMatch . (NonEmpty.fromList . getNonEmpty <$>) <$> Gen.scale (`div` 2) arbitrary -- scale the recursive case down
+    , Lambda <$> arbitrary <*> Gen.scale (`div` 2) arbitrary -- scale the recursive case down
     ]
 
+-- >>> import Test.QuickCheck
+-- >>> import Data.Functor.Identity
+-- >>> sample (arbitrary :: Gen (Expression Identity))
+
diff --git a/src/Language/Scalie/Core/Expression.hs-boot b/src/Language/Scalie/Core/Expression.hs-boot
new file mode 100644
index 0000000..57e84b3
--- /dev/null
+++ b/src/Language/Scalie/Core/Expression.hs-boot
@@ -0,0 +1,8 @@
+{-# LANGUAGE Safe #-}
+module Language.Scalie.Core.Expression (Expression) where
+import Data.Kind (Type)
+
+type Expression :: (Type -> Type) -> Type
+type role Expression nominal
+data Expression f
+
diff --git a/src/Language/Scalie/Core/Expression/ConstructorIdentifier.hs b/src/Language/Scalie/Core/Expression/ConstructorIdentifier.hs
new file mode 100644
index 0000000..0728fc6
--- /dev/null
+++ b/src/Language/Scalie/Core/Expression/ConstructorIdentifier.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE InstanceSigs #-}
+
+-- | Newtype synonym for type safety.
+
+module Language.Scalie.Core.Expression.ConstructorIdentifier (ConstructorIdentifier(..)) where
+
+import Data.Kind (Type)
+import Data.Text (Text)
+import Text.Show (Show)
+import Text.Read (Read)
+import Data.Eq (Eq)
+import Test.QuickCheck (Arbitrary (arbitrary), Gen, getUnicodeString)
+import Control.Category ((.))
+import Data.Text qualified as Text
+import Data.Functor ((<$>))
+
+-- | Encodes the knowledge that the contained text is always a Constructor name.
+
+type ConstructorIdentifier :: Type
+newtype ConstructorIdentifier = ConstructorIdentifier { get :: Text }
+  deriving stock (Show, Read, Eq)
+
+
+instance Arbitrary ConstructorIdentifier where
+  arbitrary :: Gen ConstructorIdentifier
+  arbitrary = ConstructorIdentifier . Text.pack . getUnicodeString <$> arbitrary
diff --git a/src/Language/Scalie/Core/Expression/Pattern.hs b/src/Language/Scalie/Core/Expression/Pattern.hs
new file mode 100644
index 0000000..1d45d36
--- /dev/null
+++ b/src/Language/Scalie/Core/Expression/Pattern.hs
@@ -0,0 +1,43 @@
+{-# LANGUAGE Trustworthy #-} -- this module uses vectors
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE InstanceSigs #-}
+
+-- | Pattern types for the core language. 'Pattern' enumerates all the possible ways to match on a value.
+
+module Language.Scalie.Core.Expression.Pattern (Pattern(..)) where
+import Data.Kind (Type)
+import Language.Scalie.Core.Expression.VariableIdentifier (VariableIdentifier)
+import Language.Scalie.Core.Expression.ConstructorIdentifier (ConstructorIdentifier)
+import Data.Vector (Vector)
+import Text.Show (Show)
+import Text.Read (Read)
+import Data.Eq (Eq)
+import Test.QuickCheck (Arbitrary (arbitrary), Gen, oneof)
+import Data.Functor ((<$>), Functor)
+import Control.Applicative ((<*>))
+import Data.Vector qualified as Vector
+import Test.QuickCheck.Gen qualified as Gen
+import Prelude (div) -- seems to be the safe way to import div
+
+-- | This is a single pattern. It may be used for a single argument.
+
+type Pattern :: (Type -> Type) -> Type
+type role Pattern nominal
+data Pattern f
+  = Variable (f VariableIdentifier) -- this is a catch-all, it matches anything
+  | Record (f ConstructorIdentifier) (f (Vector (Pattern f))) -- match the constructor with the sub-patterns for the arguments
+
+-- Remember to add cases to the arbitrary instance when adding data constructors
+
+deriving stock instance (Show (f (Vector (Pattern f))), Show (f VariableIdentifier), Show (f ConstructorIdentifier)) => Show (Pattern f)
+deriving stock instance (Read (f (Vector (Pattern f))), Read (f VariableIdentifier), Read (f ConstructorIdentifier)) => Read (Pattern f)
+deriving stock instance (Eq   (f (Vector (Pattern f))), Eq   (f VariableIdentifier), Eq   (f ConstructorIdentifier)) => Eq   (Pattern f)
+
+instance (Arbitrary (f VariableIdentifier), Arbitrary (f ConstructorIdentifier), Arbitrary (f [Pattern f]), Functor f) => Arbitrary (Pattern f) where
+  arbitrary :: Gen (Pattern f)
+  arbitrary = oneof
+    [ Variable <$> arbitrary
+    , Record <$> arbitrary <*> ((Vector.fromList <$>) <$> Gen.scale (`div` 2) arbitrary)
+    ]
+
diff --git a/src/Language/Scalie/Core/Expression/PatternMatchCase.hs b/src/Language/Scalie/Core/Expression/PatternMatchCase.hs
new file mode 100644
index 0000000..6935a45
--- /dev/null
+++ b/src/Language/Scalie/Core/Expression/PatternMatchCase.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# LANGUAGE InstanceSigs #-}
+module Language.Scalie.Core.Expression.PatternMatchCase (PatternMatchCase(..)) where
+import Data.Kind (Type)
+import {-# SOURCE #-} Language.Scalie.Core.Expression (Expression)
+import Language.Scalie.Core.Expression.Pattern (Pattern)
+import Text.Show (Show)
+import Text.Read (Read)
+import Data.Eq (Eq)
+import Test.QuickCheck (Arbitrary (arbitrary), Gen)
+import Data.Functor ((<$>))
+import Control.Applicative ((<*>))
+import Test.QuickCheck.Gen qualified as Gen
+import Prelude (div)
+
+type PatternMatchCase :: (Type -> Type) -> Type
+type role PatternMatchCase nominal
+data PatternMatchCase f = PatternMatchCase
+  { pattern :: f (Pattern f)
+  , body    :: f (Expression f)
+  }
+
+deriving stock instance (Show (f (Pattern f)), Show (f (Expression f))) => Show (PatternMatchCase f)
+deriving stock instance (Read (f (Pattern f)), Read (f (Expression f))) => Read (PatternMatchCase f)
+deriving stock instance (Eq   (f (Pattern f)), Eq   (f (Expression f))) => Eq   (PatternMatchCase f)
+
+instance (Arbitrary (f (Pattern f)), Arbitrary (f (Expression f))) => Arbitrary (PatternMatchCase f) where
+  arbitrary :: Gen (PatternMatchCase f)
+  arbitrary = PatternMatchCase
+    <$> arbitrary
+    <*> Gen.scale (`div` 2) arbitrary
+
diff --git a/src/Language/Scalie/Core/Expression/VariableIdentifier.hs b/src/Language/Scalie/Core/Expression/VariableIdentifier.hs
new file mode 100644
index 0000000..54c6687
--- /dev/null
+++ b/src/Language/Scalie/Core/Expression/VariableIdentifier.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE InstanceSigs #-}
+
+-- | Newtype synonym for type safety.
+
+module Language.Scalie.Core.Expression.VariableIdentifier (VariableIdentifier(..)) where
+
+import Data.Kind (Type)
+import Data.Text (Text)
+import Text.Show (Show)
+import Text.Read (Read)
+import Data.Eq (Eq)
+import Test.QuickCheck (Arbitrary (arbitrary), Gen, UnicodeString (getUnicodeString))
+import Control.Category ((.))
+import Data.Text qualified as Text
+import Data.Functor ((<$>))
+
+-- | Encodes the knowledge that the contained text is always a variable name.
+
+type VariableIdentifier :: Type
+newtype VariableIdentifier = VariableIdentifier { get :: Text }
+  deriving stock (Show, Read, Eq)
+
+instance Arbitrary VariableIdentifier where
+  arbitrary :: Gen VariableIdentifier
+  arbitrary = VariableIdentifier . Text.pack . getUnicodeString <$> arbitrary
+