diff --git a/package.yaml b/package.yaml
index 06795ba..21401e8 100644
--- a/package.yaml
+++ b/package.yaml
@@ -19,14 +19,20 @@ extra-source-files:
 description:         Please see the README on GitHub at <https://github.com/githubuser/scalie#readme>
 
 default-extensions:
-  - Safe
+  - ImportQualifiedPost
   - NoImplicitPrelude
+  - StandaloneKindSignatures
+  - RoleAnnotations         
 
 dependencies:
-- base >= 4.7 && < 5
+  - base
+  - containers
+  - text
+  - vector
 
 ghc-options:
 - -Weverything
+- -Wno-unsafe
 
 library:
   source-dirs: src
diff --git a/scalie.cabal b/scalie.cabal
index 18491fb..0aa5f28 100644
--- a/scalie.cabal
+++ b/scalie.cabal
@@ -19,17 +19,30 @@ extra-source-files:
 
 library
   exposed-modules:
+      Control.Ord.LexicographicallySorted
+      Data.Map.Implicit
       Language.Scalie.Ast
+      Language.Scalie.Ast.Definition
+      Language.Scalie.Ast.Definition.Body
+      Language.Scalie.Ast.Expression
+      Language.Scalie.Ast.Module
+      Language.Scalie.Ast.Pattern
+      Language.Scalie.Domain.Type
   other-modules:
       Paths_scalie
   hs-source-dirs:
       src
   default-extensions:
-      Safe
+      ImportQualifiedPost
       NoImplicitPrelude
-  ghc-options: -Weverything
+      StandaloneKindSignatures
+      RoleAnnotations
+  ghc-options: -Weverything -Wno-unsafe
   build-depends:
-      base >=4.7 && <5
+      base
+    , containers
+    , text
+    , vector
   default-language: Haskell2010
 
 executable scalie-exe
@@ -39,12 +52,17 @@ executable scalie-exe
   hs-source-dirs:
       app
   default-extensions:
-      Safe
+      ImportQualifiedPost
       NoImplicitPrelude
-  ghc-options: -Weverything -threaded -rtsopts -with-rtsopts=-N
+      StandaloneKindSignatures
+      RoleAnnotations
+  ghc-options: -Weverything -Wno-unsafe -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      base
+    , containers
     , scalie
+    , text
+    , vector
   default-language: Haskell2010
 
 test-suite scalie-test
@@ -55,10 +73,15 @@ test-suite scalie-test
   hs-source-dirs:
       test
   default-extensions:
-      Safe
+      ImportQualifiedPost
       NoImplicitPrelude
-  ghc-options: -Weverything -threaded -rtsopts -with-rtsopts=-N
+      StandaloneKindSignatures
+      RoleAnnotations
+  ghc-options: -Weverything -Wno-unsafe -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      base
+    , containers
     , scalie
+    , text
+    , vector
   default-language: Haskell2010
diff --git a/src/Data/Map/Implicit.hs b/src/Data/Map/Implicit.hs
new file mode 100644
index 0000000..f983432
--- /dev/null
+++ b/src/Data/Map/Implicit.hs
@@ -0,0 +1,22 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE TypeFamilies #-}
+--
+-- | A Map that derives the keys for the mapping from the items.
+
+module Data.Map.Implicit (ImplicitMap(), get, ImplicitKeyOf(..), empty) where
+import Data.Kind (Type, Constraint)
+import Data.Map (Map)
+import Data.Map qualified as Map
+
+type ImplicitMap :: Type -> Type
+type role ImplicitMap nominal
+newtype ImplicitMap v = ImplicitMap { get :: Map (KeyType v) v }
+
+type ImplicitKeyOf :: Type -> Constraint
+
+class ImplicitKeyOf v where
+  type KeyType v :: Type
+  keyOf :: v -> KeyType v
+
+empty :: ImplicitMap v
+empty = ImplicitMap Map.empty
diff --git a/src/Language/Scalie/Ast.hs b/src/Language/Scalie/Ast.hs
deleted file mode 100644
index 3991c50..0000000
--- a/src/Language/Scalie/Ast.hs
+++ /dev/null
@@ -1 +0,0 @@
-module Language.Scalie.Ast () where
diff --git a/src/Language/Scalie/Ast/Definition.hs b/src/Language/Scalie/Ast/Definition.hs
new file mode 100644
index 0000000..f7f092d
--- /dev/null
+++ b/src/Language/Scalie/Ast/Definition.hs
@@ -0,0 +1,28 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE InstanceSigs #-}
+{-# LANGUAGE TypeFamilies #-}
+module Language.Scalie.Ast.Definition (Definition(..)) where
+
+import Data.Kind (Type)
+import Data.Text (Text)
+
+import Language.Scalie.Domain.Type qualified as Scalie.Domain
+import Language.Scalie.Ast.Definition.Body (DefinitionBody)
+import Data.Map.Implicit (ImplicitKeyOf (KeyType, keyOf))
+
+type Definition :: (Type -> Type) -> Type
+type role Definition nominal
+data Definition f = Definition
+  { signature   :: f Scalie.Domain.Type
+  -- ^ What is the type
+  , name        :: f Text 
+  -- ^ Which name will be used to refer to this definition
+  , body        :: f (DefinitionBody f)
+  -- ^ What needs to be evaluated to get the value
+  }
+
+instance ImplicitKeyOf (Definition f) where
+  type KeyType (Definition f) = f Text
+  keyOf :: Definition f -> KeyType (Definition f)
+  keyOf = name
+
diff --git a/src/Language/Scalie/Ast/Definition/Body.hs b/src/Language/Scalie/Ast/Definition/Body.hs
new file mode 100644
index 0000000..796c022
--- /dev/null
+++ b/src/Language/Scalie/Ast/Definition/Body.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE Trustworthy #-} -- I declare trustworthiness because I only use Vector, I do not call any unsafe functions.
+module Language.Scalie.Ast.Definition.Body (DefinitionBody(..)) where
+
+import Data.Vector (Vector)
+
+import Language.Scalie.Ast.Expression (Expression)
+import Language.Scalie.Ast.Pattern (Pattern)
+import Data.Kind (Type)
+
+type DefinitionBody :: (Type -> Type) -> Type
+type role DefinitionBody representational
+data DefinitionBody f = DefinitionBody
+  { arguments :: f (Vector Pattern)
+  , rhs       :: f Expression
+  }
diff --git a/src/Language/Scalie/Ast/Expression.hs b/src/Language/Scalie/Ast/Expression.hs
new file mode 100644
index 0000000..061c94e
--- /dev/null
+++ b/src/Language/Scalie/Ast/Expression.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE Safe #-}
+module Language.Scalie.Ast.Expression (Expression(..)) where
+
+import Prelude (Integer)
+import Data.Kind (Type)
+
+type Expression :: Type
+data Expression
+  = RawInt Integer
diff --git a/src/Language/Scalie/Ast/Module.hs b/src/Language/Scalie/Ast/Module.hs
new file mode 100644
index 0000000..ca7587f
--- /dev/null
+++ b/src/Language/Scalie/Ast/Module.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE Safe #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+module Language.Scalie.Ast.Module (Module(..)) where
+
+import Data.Kind (Type)
+import Language.Scalie.Ast.Definition (Definition)
+import Data.Map.Implicit (ImplicitMap)
+
+type Module :: (Type -> Type) -> Type
+type role Module nominal
+data Module f = Module
+  { definitions :: ImplicitMap (Definition f)
+  }
diff --git a/src/Language/Scalie/Ast/Pattern.hs b/src/Language/Scalie/Ast/Pattern.hs
new file mode 100644
index 0000000..57e9ee6
--- /dev/null
+++ b/src/Language/Scalie/Ast/Pattern.hs
@@ -0,0 +1,6 @@
+{-# LANGUAGE Safe #-}
+module Language.Scalie.Ast.Pattern (Pattern(..)) where
+import Prelude (Integer)
+
+data Pattern
+  = RawInt Integer
diff --git a/src/Language/Scalie/Domain/Type.hs b/src/Language/Scalie/Domain/Type.hs
new file mode 100644
index 0000000..757d4d2
--- /dev/null
+++ b/src/Language/Scalie/Domain/Type.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE Safe #-}
+module Language.Scalie.Domain.Type (Type(..)) where
+
+import Data.Kind qualified
+
+type Type :: Data.Kind.Type
+data Type 
+  = RawInt
diff --git a/stack.yaml b/stack.yaml
index 885f3d1..d2efd23 100644
--- a/stack.yaml
+++ b/stack.yaml
@@ -19,6 +19,7 @@
 # snapshot: https://example.com/snapshots/2024-01-01.yaml
 snapshot:
   url: https://raw.githubusercontent.com/commercialhaskell/stackage-snapshots/master/lts/24/3.yaml
+compiler: ghc-9.12.1
 
 # User packages to be built.
 # Various formats can be used as shown in the example below.