diff --git a/src/Language/Scalie/Core/Definition.hs b/src/Language/Scalie/Core/Definition.hs
index c1f0f60..60e2cb8 100644
--- a/src/Language/Scalie/Core/Definition.hs
+++ b/src/Language/Scalie/Core/Definition.hs
@@ -60,6 +60,8 @@ deriving stock instance (Show (f (Expression f)), Show (f Scalie.Domain.Type), S
 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)
 
+-- | Using custom instances allows extracting just the text whereever possible.
+
 instance ImplicitKeyOf (Definition (Provenance a)) where
   type KeyType (Definition (Provenance a)) = Text
   keyOf :: Definition (Provenance a) -> KeyType (Definition (Provenance a))
diff --git a/src/Language/Scalie/Core/Expression.hs b/src/Language/Scalie/Core/Expression.hs
index 3c84c15..6511fb7 100644
--- a/src/Language/Scalie/Core/Expression.hs
+++ b/src/Language/Scalie/Core/Expression.hs
@@ -24,6 +24,8 @@ import Test.QuickCheck.Gen (Gen)
 import Test.QuickCheck qualified as Gen
 
 
+-- | Desugared expression.
+
 type Expression :: (Type -> Type) -> Type
 type role Expression nominal
 data Expression f
diff --git a/src/Language/Scalie/Core/Module.hs b/src/Language/Scalie/Core/Module.hs
index 0a0ce69..b344fa7 100644
--- a/src/Language/Scalie/Core/Module.hs
+++ b/src/Language/Scalie/Core/Module.hs
@@ -9,7 +9,7 @@ import Data.Kind (Type)
 import Language.Scalie.Core.Definition (Definition)
 import Data.Map.Implicit (ImplicitMap)
 
--- | A module groups multiple related definitions.
+-- | A module groups multiple related variable definitions.
 --
 -- >>> import Data.Functor.Identity (Identity(..))
 -- >>> import Data.Map.Implicit qualified as ImplicitMap
diff --git a/src/Language/Scalie/Core/Provenance.hs b/src/Language/Scalie/Core/Provenance.hs
index d067c69..50714c7 100644
--- a/src/Language/Scalie/Core/Provenance.hs
+++ b/src/Language/Scalie/Core/Provenance.hs
@@ -11,6 +11,8 @@ import Data.Bifunctor (Bifunctor (bimap))
 import Data.Bifoldable (Bifoldable (bifoldMap))
 import Data.Bitraversable (Bitraversable (bitraverse))
 
+-- | Provenance records where something comes from. E.g. file name, file line, other sources.
+
 type Provenance :: Type -> Type -> Type
 type role Provenance representational representational 
 data Provenance source value = Provenance
diff --git a/src/Language/Scalie/Core/Provenance/SourceLocation.hs b/src/Language/Scalie/Core/Provenance/SourceLocation.hs
index 301b1e2..3ecf436 100644
--- a/src/Language/Scalie/Core/Provenance/SourceLocation.hs
+++ b/src/Language/Scalie/Core/Provenance/SourceLocation.hs
@@ -2,6 +2,10 @@
 module Language.Scalie.Core.Provenance.SourceLocation (SourceLocation(..)) where
 import Data.Kind (Type)
 
+-- | Location of anything in a user-provided source file.
+--
+-- TODO: Add more constructors
+
 type SourceLocation :: Type
 data SourceLocation
   = Synthesized
diff --git a/src/Language/Scalie/Domain/Type.hs b/src/Language/Scalie/Domain/Type.hs
index 106c061..97354de 100644
--- a/src/Language/Scalie/Domain/Type.hs
+++ b/src/Language/Scalie/Domain/Type.hs
@@ -12,6 +12,8 @@ data Type
   = RawInt
   deriving stock (Show, Read, Eq)
 
+  -- add to the arbitrary instance when defining constructors
+
 instance Arbitrary Type where
   arbitrary :: Gen Type
   arbitrary = oneof [ pure RawInt ]