diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,12 @@
 
 ## Unreleased changes
 
+## 0.2.0.0
+
+- Merged the `HasElmType` and `HasElmDefinition` classes
+- Merged the `HasElmDecoder` and `HasElmDecoderDefinition` classes
+- Merged the `HasElmEncoder` and `HasElmEncoderDefinition` classes
+
 ## 0.1.0.1
 
 - Tightened generics-sop bounds to < 0.5.0
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -67,27 +67,19 @@
 data User = User
   { name :: Text
   , age :: Int
-  } deriving
-    ( Generic
-    , Aeson.ToJSON
-    , SOP.Generic
-    , SOP.HasDatatypeInfo
-    , HasElmDecoder Aeson.Value
-    , HasElmEncoder Aeson.Value
-    , HasElmType
-    )
+  } deriving (Generic, Aeson.ToJSON, SOP.Generic, SOP.HasDatatypeInfo)
 
-instance HasElmDefinition User where
+instance HasElmType User where
   elmDefinition =
-    deriveElmTypeDefinition @User defaultOptions "Api.User.User"
+    Just $ deriveElmTypeDefinition @User defaultOptions "Api.User.User"
 
-instance HasElmDecoderDefinition Aeson.Value User where
+instance HasElmDecoder Aeson.Value User where
   elmDecoderDefinition =
-    deriveElmJSONDecoder @User defaultOptions Aeson.defaultOptions "Api.User.decoder"
+    Just $ deriveElmJSONDecoder @User defaultOptions Aeson.defaultOptions "Api.User.decoder"
 
-instance HasElmEncoderDefinition Aeson.Value User where
+instance HasElmEncoder Aeson.Value User where
   elmEncoderDefinition =
-    deriveElmJSONEncoder @User defaultOptions Aeson.defaultOptions "Api.User.encoder"
+    Just $ deriveElmJSONEncoder @User defaultOptions Aeson.defaultOptions "Api.User.encoder"
 ```
 
 Then we can print the generated Elm code using the following code:
diff --git a/examples/User.hs b/examples/User.hs
--- a/examples/User.hs
+++ b/examples/User.hs
@@ -18,27 +18,19 @@
 data User = User
   { name :: Text
   , age :: Int
-  } deriving
-    ( Generic
-    , Aeson.ToJSON
-    , SOP.Generic
-    , SOP.HasDatatypeInfo
-    , HasElmDecoder Aeson.Value
-    , HasElmEncoder Aeson.Value
-    , HasElmType
-    )
+  } deriving (Generic, Aeson.ToJSON, SOP.Generic, SOP.HasDatatypeInfo)
 
-instance HasElmDefinition User where
+instance HasElmType User where
   elmDefinition =
-    deriveElmTypeDefinition @User defaultOptions "Api.User.User"
+    Just $ deriveElmTypeDefinition @User defaultOptions "Api.User.User"
 
-instance HasElmDecoderDefinition Aeson.Value User where
+instance HasElmDecoder Aeson.Value User where
   elmDecoderDefinition =
-    deriveElmJSONDecoder @User defaultOptions Aeson.defaultOptions "Api.User.decoder"
+    Just $ deriveElmJSONDecoder @User defaultOptions Aeson.defaultOptions "Api.User.decoder"
 
-instance HasElmEncoderDefinition Aeson.Value User where
+instance HasElmEncoder Aeson.Value User where
   elmEncoderDefinition =
-    deriveElmJSONEncoder @User defaultOptions Aeson.defaultOptions "Api.User.encoder"
+    Just $ deriveElmJSONEncoder @User defaultOptions Aeson.defaultOptions "Api.User.encoder"
 
 main :: IO ()
 main = do
diff --git a/haskell-to-elm.cabal b/haskell-to-elm.cabal
--- a/haskell-to-elm.cabal
+++ b/haskell-to-elm.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 54b62e8cc067711119bedcec60c48bdf2010ef77b1a99af49152a3e4b97d4cfe
+-- hash: f0ee8106a8cc61bd98e1211c9ed203b336606ef09376e707701d942998727a04
 
 name:           haskell-to-elm
-version:        0.1.0.1
+version:        0.2.0.0
 synopsis:       Generate Elm types and JSON encoders and decoders from Haskell types
 description:    Please see the README on GitHub at <https://github.com/folq/haskell-to-elm#readme>
 category:       Elm, Compiler, Language
diff --git a/src/Language/Haskell/To/Elm.hs b/src/Language/Haskell/To/Elm.hs
--- a/src/Language/Haskell/To/Elm.hs
+++ b/src/Language/Haskell/To/Elm.hs
@@ -40,31 +40,52 @@
 -- * Classes
 
 -- | Represents that the corresponding Elm type for the Haskell type @a@ is @'elmType' \@a@.
---
--- This class has a default instance for types that satisfy 'HasElmDefinition',
--- which refers to the name of that definition.
 class HasElmType a where
   elmType :: Type v
-  default elmType :: HasElmDefinition a => Type v
-  elmType = Type.Global $ Definition.name $ elmDefinition @a
+  default elmType :: Type v
+  elmType =
+    Type.Global $
+      maybe
+        (panic "default-implemented 'elmType' without a definition")
+        Definition.name $
+          elmDefinition @a
 
--- | Represents that we can generate the definition for the Elm type that
--- corresponds to @a@ using @'elmDefinition' \@a@.
---
--- See 'deriveElmTypeDefinition' for a way to automatically derive 'elmDefinition'.
-class HasElmDefinition a where
-  elmDefinition :: Definition
+  -- | When 'Just', this represents that we can generate the definition for the
+  -- Elm type that corresponds to @a@ using @'elmDefinition' \@a@.
+  --
+  -- See 'deriveElmTypeDefinition' for a way to automatically derive 'elmDefinition'.
+  --
+  -- When 'Nothing', it means that the type is an already existing Elm type
+  -- that does not need to be generated.
+  elmDefinition :: Maybe Definition
+  elmDefinition =
+    Nothing
 
+  {-# minimal elmType | elmDefinition #-}
+
 -- | Represents that the Elm type that corresponds to @a@ has a decoder from
 -- @value@, namely @'elmDecoder' \@value \@a@.
---
--- This class has a default instance for types that satisfy
--- 'HasElmDecoderDefinition', which refers to the name of that definition.
 class HasElmType a => HasElmDecoder value a where
   elmDecoder :: Expression v
-  default elmDecoder :: HasElmDecoderDefinition value a => Expression v
-  elmDecoder = Expression.Global $ Definition.name $ elmDecoderDefinition @value @a
+  default elmDecoder :: Expression v
+  elmDecoder =
+    Expression.Global $
+      maybe
+        (panic "default-implemented 'elmDecoder' without a definition")
+        Definition.name $
+          elmDecoderDefinition @value @a
 
+  -- | When 'Just', this represents that we can generate the Elm decoder definition
+  -- from @value@ for the Elm type that corresponds to @a@.
+  --
+  -- See 'deriveElmJSONDecoder' for a way to automatically derive
+  -- 'elmDecoderDefinition' when @value = 'Aeson.Value'@.
+  elmDecoderDefinition :: Maybe Definition
+  elmDecoderDefinition =
+    Nothing
+
+  {-# minimal elmDecoder | elmDecoderDefinition #-}
+
 -- | Represents that the Elm type that corresponds to @a@ has an encoder into
 -- @value@, namely @'elmEncoder' \@value \@a@.
 --
@@ -72,24 +93,24 @@
 -- 'HasElmEncoderDefinition', which refers to the name of that definition.
 class HasElmType a => HasElmEncoder value a where
   elmEncoder :: Expression v
-  default elmEncoder :: HasElmEncoderDefinition value a => Expression v
-  elmEncoder = Expression.Global $ Definition.name $ elmEncoderDefinition @value @a
+  default elmEncoder :: Expression v
+  elmEncoder =
+    Expression.Global $
+      maybe
+        (panic "default-implemented 'elmEncoder' without a definition")
+        Definition.name $
+          elmEncoderDefinition @value @a
 
--- | Represents that we can generate the Elm decoder definition from @value@
--- for the Elm type that corresponds to @a@.
---
--- See 'deriveElmJSONDecoder' for a way to automatically derive
--- 'elmDecoderDefinition' when @value = 'Aeson.Value'@.
-class HasElmDecoderDefinition value a where
-  elmDecoderDefinition :: Definition
+  -- | When 'Just', this represents that we can generate the Elm encoder
+  -- definition into @value@ for the Elm type that corresponds to @a@.
+  --
+  -- See 'deriveElmJSONEncoder' for a way to automatically derive
+  -- 'elmEncoderDefinition' when @value = 'Aeson.Value'@.
+  elmEncoderDefinition :: Maybe Definition
+  elmEncoderDefinition =
+    Nothing
 
--- | Represents that we can generate the Elm encoder definition into @value@
--- for the Elm type that corresponds to @a@.
---
--- See 'deriveElmJSONEncoder' for a way to automatically derive
--- 'elmEncoderDefinition' when @value = 'Aeson.Value'@.
-class HasElmEncoderDefinition value a where
-  elmEncoderDefinition :: Definition
+  {-# minimal elmEncoder | elmEncoderDefinition #-}
 
 -------------------------------------------------------------------------------
 -- * Derivers
@@ -107,12 +128,12 @@
 
 -- | Automatically create an Elm definition given a Haskell type.
 --
--- This is suitable for use as a 'HasElmDefinition' instance:
+-- This is suitable for use as the 'elmDefinition' in a 'HasElmType' instance:
 --
 -- @
--- instance 'HasElmDefinition' MyType where
+-- instance 'HasElmType' MyType where
 --   'elmDefinition' =
---     'deriveElmTypeDefinition' \@MyType 'defaultOptions' \"Api.MyType.MyType\"
+--     'Just' $ 'deriveElmTypeDefinition' \@MyType 'defaultOptions' \"Api.MyType.MyType\"
 -- @
 deriveElmTypeDefinition :: forall a. (HasDatatypeInfo a, All2 HasElmType (Code a)) => Options -> Name.Qualified -> Definition
 deriveElmTypeDefinition options name =
@@ -155,12 +176,13 @@
 
 -- | Automatically create an Elm JSON decoder definition given a Haskell type.
 --
--- This is suitable for use as a @'HasElmDecoderDefinition' 'Aeson.Value'@ instance:
+-- This is suitable for use as the 'elmDecoderDefinition' in a
+-- @'HasElmDecoder' 'Aeson.Value'@ instance:
 --
 -- @
--- instance 'HasElmDecoderDefinition' 'Aeson.Value' MyType where
+-- instance 'HasElmDecoder' 'Aeson.Value' MyType where
 --   'elmDecoderDefinition' =
---     'deriveElmJSONDecoder' \@MyType 'defaultOptions' 'Aeson.defaultOptions' "Api.MyType.decoder"
+--     Just $ 'deriveElmJSONDecoder' \@MyType 'defaultOptions' 'Aeson.defaultOptions' "Api.MyType.decoder"
 -- @
 --
 -- Uses the given 'Aeson.Options' to match the JSON format of derived
@@ -422,12 +444,12 @@
 
 -- | Automatically create an Elm JSON encoder definition given a Haskell type.
 --
--- This is suitable for use as a @'HasElmEncoderDefinition' 'Aeson.Value'@ instance:
+-- This is suitable for use as the 'elmEncoderDefinition' in a @'HasElmEncoder 'Aeson.Value'@ instance:
 --
 -- @
--- instance 'HasElmEncoderDefinition' 'Aeson.Value' MyType where
+-- instance 'HasElmEncoder' 'Aeson.Value' MyType where
 --   'elmEncoderDefinition' =
---     'deriveElmJSONEncoder' \@MyType 'defaultOptions' 'Aeson.defaultOptions' "Api.MyType.encoder"
+--     'Just' $ 'deriveElmJSONEncoder' \@MyType 'defaultOptions' 'Aeson.defaultOptions' "Api.MyType.encoder"
 -- @
 --
 -- Uses the given 'Aeson.Options' to match the JSON format of derived
@@ -811,8 +833,9 @@
 -- @'jsonDefinitions' \@MyType@ is a shorthand for creating a list of its
 -- 'elmDefinition', @'elmEncoderDefinition' \@'Aeson.Value'@, and
 -- @'elmDecoderDefinition' \@'Aeson.Value'@.
-jsonDefinitions :: forall t. (HasElmDefinition t, HasElmEncoderDefinition Aeson.Value t, HasElmDecoderDefinition Aeson.Value t) => [Definition]
+jsonDefinitions :: forall t. (HasElmEncoder Aeson.Value t, HasElmDecoder Aeson.Value t) => [Definition]
 jsonDefinitions =
+  catMaybes
   [ elmDefinition @t
   , elmEncoderDefinition @Aeson.Value @t
   , elmDecoderDefinition @Aeson.Value @t