diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,16 @@
+2.5
+---
+
+Highlights:
+- Add instance for `TimeOfDay` (see [#196]( https://github.com/GetShopTV/swagger2/pull/196 ));
+- Add support for [`optics`](https://hackage.haskell.org/package/optics) (see [#200]( https://github.com/GetShopTV/swagger2/pull/200 ));
+- Slightly better validation errors (see [#195]( https://github.com/GetShopTV/swagger2/pull/195 ));
+
+Fixes and minor changes:
+- Minor cleanup (see [#194]( https://github.com/GetShopTV/swagger2/pull/194 ));
+- Allow base-4.13 and other GHC-8.8 related stuff (see [#198]( https://github.com/GetShopTV/swagger2/pull/198 ));
+- Fix `stack.yaml` (see [#201]( https://github.com/GetShopTV/swagger2/pull/201 ));
+
 2.4
 ---
 
diff --git a/src/Data/Swagger.hs b/src/Data/Swagger.hs
--- a/src/Data/Swagger.hs
+++ b/src/Data/Swagger.hs
@@ -31,6 +31,7 @@
 
   -- * Re-exports
   module Data.Swagger.Lens,
+  module Data.Swagger.Optics,
   module Data.Swagger.Operation,
   module Data.Swagger.ParamSchema,
   module Data.Swagger.Schema,
@@ -112,6 +113,7 @@
 ) where
 
 import Data.Swagger.Lens
+import Data.Swagger.Optics ()
 import Data.Swagger.Operation
 import Data.Swagger.ParamSchema
 import Data.Swagger.Schema
@@ -174,6 +176,8 @@
 -- @
 
 -- $lens
+--
+-- Note: if you're working with the <https://hackage.haskell.org/package/optics optics> library, take a look at "Data.Swagger.Optics".
 --
 -- Since @'Swagger'@ has a fairly complex structure, lenses and prisms are used
 -- to work comfortably with it. In combination with @'Monoid'@ instances, lenses
diff --git a/src/Data/Swagger/Internal/ParamSchema.hs b/src/Data/Swagger/Internal/ParamSchema.hs
--- a/src/Data/Swagger/Internal/ParamSchema.hs
+++ b/src/Data/Swagger/Internal/ParamSchema.hs
@@ -194,6 +194,12 @@
   toParamSchema _ = timeParamSchema "date"
 
 -- |
+-- >>> toParamSchema (Proxy :: Proxy TimeOfDay) ^. format
+-- Just "hh:MM:ss"
+instance ToParamSchema TimeOfDay where
+  toParamSchema _ = timeParamSchema "hh:MM:ss"
+
+-- |
 -- >>> toParamSchema (Proxy :: Proxy LocalTime) ^. format
 -- Just "yyyy-mm-ddThh:MM:ss"
 instance ToParamSchema LocalTime where
diff --git a/src/Data/Swagger/Internal/Schema.hs b/src/Data/Swagger/Internal/Schema.hs
--- a/src/Data/Swagger/Internal/Schema.hs
+++ b/src/Data/Swagger/Internal/Schema.hs
@@ -147,6 +147,10 @@
     Proxy a -> Declare (Definitions Schema) NamedSchema
   declareNamedSchema = genericDeclareNamedSchema defaultSchemaOptions
 
+instance ToSchema TimeOfDay where
+  declareNamedSchema _ = pure $ named "TimeOfDay" $ timeSchema "hh:MM:ss"
+    & example ?~ toJSON (TimeOfDay 12 33 15)
+
 -- | Convert a type into a schema and declare all used schema definitions.
 declareSchema :: ToSchema a => Proxy a -> Declare (Definitions Schema) Schema
 declareSchema = fmap _namedSchemaSchema . declareNamedSchema
diff --git a/src/Data/Swagger/Internal/Schema/Validation.hs b/src/Data/Swagger/Internal/Schema/Validation.hs
--- a/src/Data/Swagger/Internal/Schema/Validation.hs
+++ b/src/Data/Swagger/Internal/Schema/Validation.hs
@@ -22,23 +22,27 @@
 -- Validate JSON values with Swagger Schema.
 module Data.Swagger.Internal.Schema.Validation where
 
+import           Prelude ()
+import           Prelude.Compat
+
 import           Control.Applicative
 import           Control.Lens
 import           Control.Monad                       (when)
 
 import           Data.Aeson                          hiding (Result)
+import           Data.Aeson.Encode.Pretty            (encodePretty)
 import           Data.Foldable                       (for_, sequenceA_,
                                                       traverse_)
 import           Data.HashMap.Strict                 (HashMap)
 import qualified Data.HashMap.Strict                 as HashMap
 import qualified Data.HashMap.Strict.InsOrd          as InsOrdHashMap
 import qualified "unordered-containers" Data.HashSet as HashSet
-import           Data.List                           (intercalate)
-import           Data.Monoid
 import           Data.Proxy
 import           Data.Scientific                     (Scientific, isInteger)
 import           Data.Text                           (Text)
 import qualified Data.Text                           as Text
+import qualified Data.Text.Lazy                      as TL
+import qualified Data.Text.Lazy.Encoding             as TL
 import           Data.Vector                         (Vector)
 import qualified Data.Vector                         as Vector
 
@@ -54,18 +58,101 @@
 --
 -- /NOTE:/ @'validateToJSON'@ does not perform string pattern validation.
 -- See @'validateToJSONWithPatternChecker'@.
+--
+-- See 'renderValidationErrors' on how the output is structured.
+validatePrettyToJSON :: forall a. (ToJSON a, ToSchema a) => a -> Maybe String
+validatePrettyToJSON = renderValidationErrors validateToJSON
+
+-- | Variant of 'validatePrettyToJSON' with typed output.
 validateToJSON :: forall a. (ToJSON a, ToSchema a) => a -> [ValidationError]
 validateToJSON = validateToJSONWithPatternChecker (\_pattern _str -> True)
 
 -- | Validate @'ToJSON'@ instance matches @'ToSchema'@ for a given value and pattern checker.
 -- This can be used with QuickCheck to ensure those instances are coherent.
 --
--- For validation without patterns see @'validateToJSON'@.
+-- For validation without patterns see @'validateToJSON'@.  See also:
+-- 'renderValidationErrors'.
 validateToJSONWithPatternChecker :: forall a. (ToJSON a, ToSchema a) => (Pattern -> Text -> Bool) -> a -> [ValidationError]
 validateToJSONWithPatternChecker checker = validateJSONWithPatternChecker checker defs sch . toJSON
   where
     (defs, sch) = runDeclare (declareSchema (Proxy :: Proxy a)) mempty
 
+-- | Pretty print validation errors
+-- together with actual JSON and Swagger Schema
+-- (using 'encodePretty').
+--
+-- >>> import Data.Aeson as Aeson
+-- >>> import Data.Foldable (traverse_)
+-- >>> import GHC.Generics
+-- >>> data Phone = Phone { value :: String } deriving (Generic)
+-- >>> data Person = Person { name :: String, phone :: Phone } deriving (Generic)
+-- >>> instance ToJSON Person where toJSON p = object [ "name" Aeson..= name p ]
+-- >>> instance ToSchema Phone
+-- >>> instance ToSchema Person
+-- >>> let person = Person { name = "John", phone = Phone "123456" }
+-- >>> traverse_ putStrLn $ renderValidationErrors validateToJSON person
+-- Validation against the schema fails:
+--   * property "phone" is required, but not found in "{\"name\":\"John\"}"
+-- <BLANKLINE>
+-- JSON value:
+-- {
+--     "name": "John"
+-- }
+-- <BLANKLINE>
+-- Swagger Schema:
+-- {
+--     "required": [
+--         "name",
+--         "phone"
+--     ],
+--     "type": "object",
+--     "properties": {
+--         "phone": {
+--             "$ref": "#/definitions/Phone"
+--         },
+--         "name": {
+--             "type": "string"
+--         }
+--     }
+-- }
+-- <BLANKLINE>
+-- Swagger Description Context:
+-- {
+--     "Phone": {
+--         "required": [
+--             "value"
+--         ],
+--         "type": "object",
+--         "properties": {
+--             "value": {
+--                 "type": "string"
+--             }
+--         }
+--     }
+-- }
+-- <BLANKLINE>
+renderValidationErrors
+  :: forall a. (ToJSON a, ToSchema a)
+  => (a -> [ValidationError]) -> a -> Maybe String
+renderValidationErrors f x =
+  case f x of
+    []      -> Nothing
+    errors  -> Just $ unlines
+      [ "Validation against the schema fails:"
+      , unlines (map ("  * " ++) errors)
+      , "JSON value:"
+      , ppJSONString (toJSON x)
+      , ""
+      , "Swagger Schema:"
+      , ppJSONString (toJSON schema_)
+      , ""
+      , "Swagger Description Context:"
+      , ppJSONString (toJSON refs_)
+      ]
+  where
+    ppJSONString = TL.unpack . TL.decodeUtf8 . encodePretty
+    (refs_, schema_) = runDeclare (declareSchema (Proxy :: Proxy a)) mempty
+
 -- | Validate JSON @'Value'@ against Swagger @'Schema'@.
 --
 -- prop> validateJSON mempty (toSchema (Proxy :: Proxy Int)) (toJSON (x :: Int)) == []
@@ -397,7 +484,7 @@
     (Nothing, String s)               -> sub_ paramSchema (validateString s)
     (Nothing, Array xs)               -> sub_ paramSchema (validateArray xs)
     (Nothing, Object o)               -> validateObject o
-    param@(t, _) -> invalid $ "expected JSON value of type " ++ showType param
+    bad -> invalid $ "expected JSON value of type " ++ showType bad
 
 validateParamSchemaType :: Value -> Validation (ParamSchema t) ()
 validateParamSchemaType value = withSchema $ \sch ->
@@ -412,8 +499,7 @@
     (Nothing, Number n)               -> validateNumber n
     (Nothing, String s)               -> validateString s
     (Nothing, Array xs)               -> validateArray xs
-    (t, _) -> invalid $ "expected JSON value of type " ++ show t
-    param@(t, _) -> invalid $ "expected JSON value of type " ++ showType param
+    bad -> invalid $ "expected JSON value of type " ++ showType bad
 
 showType :: (Maybe (SwaggerType t), Value) -> String
 showType (Just type_, _)     = show type_
diff --git a/src/Data/Swagger/Optics.hs b/src/Data/Swagger/Optics.hs
new file mode 100644
--- /dev/null
+++ b/src/Data/Swagger/Optics.hs
@@ -0,0 +1,597 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE OverloadedLabels #-}
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE UndecidableInstances #-}
+{-# OPTIONS_GHC -fno-warn-orphans #-}
+-- |
+-- Module:      Data.Swagger.Optics
+-- Maintainer:  Andrzej Rybczak <andrzej@rybczak.net>
+-- Stability:   experimental
+--
+-- Lenses and prisms for the <https://hackage.haskell.org/package/optics optics>
+-- library.
+--
+-- >>> import Data.Aeson
+-- >>> import Optics.Core
+-- >>> :set -XOverloadedLabels
+--
+-- Example from the "Data.Swagger" module using @optics@:
+--
+-- >>> :{
+-- encode $ (mempty :: Swagger)
+--   & #definitions .~ [ ("User", mempty & #type ?~ SwaggerString) ]
+--   & #paths .~
+--     [ ("/user", mempty & #get ?~ (mempty
+--         & #produces ?~ MimeList ["application/json"]
+--         & at 200 ?~ ("OK" & #_Inline % #schema ?~ Ref (Reference "User"))
+--         & at 404 ?~ "User info not found")) ]
+-- :}
+-- "{\"swagger\":\"2.0\",\"info\":{\"version\":\"\",\"title\":\"\"},\"paths\":{\"/user\":{\"get\":{\"produces\":[\"application/json\"],\"responses\":{\"404\":{\"description\":\"User info not found\"},\"200\":{\"schema\":{\"$ref\":\"#/definitions/User\"},\"description\":\"OK\"}}}}},\"definitions\":{\"User\":{\"type\":\"string\"}}}"
+--
+-- For convenience optics are defined as /labels/. It means that field accessor
+-- names can be overloaded for different types. One such common field is
+-- @#description@. Many components of a Swagger specification can have
+-- descriptions, and you can use the same name for them:
+--
+-- >>> encode $ (mempty :: Response) & #description .~ "No content"
+-- "{\"description\":\"No content\"}"
+-- >>> :{
+-- encode $ (mempty :: Schema)
+--   & #type        ?~ SwaggerBoolean
+--   & #description ?~ "To be or not to be"
+-- :}
+-- "{\"description\":\"To be or not to be\",\"type\":\"boolean\"}"
+--
+-- @'ParamSchema'@ is basically the /base schema specification/ and many types
+-- contain it. So for convenience, all @'ParamSchema'@ fields are transitively
+-- made fields of the type that has it. For example, you can use @#type@ to
+-- access @'SwaggerType'@ of @'Header'@ schema without having to use
+-- @#paramSchema@:
+--
+-- >>> encode $ (mempty :: Header) & #type ?~ SwaggerNumber
+-- "{\"type\":\"number\"}"
+--
+-- Additionally, to simplify working with @'Response'@, both @'Operation'@ and
+-- @'Responses'@ have direct access to it via @'Optics.Core.At.at'@. Example:
+--
+-- >>> :{
+-- encode $ (mempty :: Operation)
+--   & at 404 ?~ "Not found"
+-- :}
+-- "{\"responses\":{\"404\":{\"description\":\"Not found\"}}}"
+--
+module Data.Swagger.Optics () where
+
+import Data.Aeson (Value)
+import Data.Scientific (Scientific)
+import Data.Swagger.Internal
+import Data.Text (Text)
+import Optics.Core
+import Optics.TH
+
+-- Lenses
+
+makeFieldLabels ''Swagger
+makeFieldLabels ''Host
+makeFieldLabels ''Info
+makeFieldLabels ''Contact
+makeFieldLabels ''License
+makeFieldLabels ''PathItem
+makeFieldLabels ''Tag
+makeFieldLabels ''Operation
+makeFieldLabels ''Param
+makeFieldLabels ''ParamOtherSchema
+makeFieldLabels ''Header
+makeFieldLabels ''Schema
+makeFieldLabels ''NamedSchema
+makeFieldLabels ''ParamSchema
+makeFieldLabels ''Xml
+makeFieldLabels ''Responses
+makeFieldLabels ''Response
+makeFieldLabels ''SecurityScheme
+makeFieldLabels ''ApiKeyParams
+makeFieldLabels ''OAuth2Params
+makeFieldLabels ''ExternalDocs
+
+-- Prisms
+
+makePrismLabels ''ParamAnySchema
+makePrismLabels ''SecuritySchemeType
+makePrismLabels ''Referenced
+
+-- SwaggerItems prisms
+
+instance
+  ( a ~ [Referenced Schema]
+  , b ~ [Referenced Schema]
+  ) => LabelOptic "_SwaggerItemsArray"
+         A_Review
+         (SwaggerItems 'SwaggerKindSchema)
+         (SwaggerItems 'SwaggerKindSchema)
+         a
+         b where
+  labelOptic = unto (\x -> SwaggerItemsArray x)
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Referenced Schema
+  , b ~ Referenced Schema
+  ) => LabelOptic "_SwaggerItemsObject"
+         A_Review
+         (SwaggerItems 'SwaggerKindSchema)
+         (SwaggerItems 'SwaggerKindSchema)
+         a
+         b where
+  labelOptic = unto (\x -> SwaggerItemsObject x)
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ (Maybe (CollectionFormat t), ParamSchema t)
+  , b ~ (Maybe (CollectionFormat t), ParamSchema t)
+  ) => LabelOptic "_SwaggerItemsPrimitive"
+         A_Review
+         (SwaggerItems t)
+         (SwaggerItems t)
+         a
+         b where
+  labelOptic = unto (\(c, p) -> SwaggerItemsPrimitive c p)
+  {-# INLINE labelOptic #-}
+
+-- =============================================================
+-- More helpful instances for easier access to schema properties
+
+type instance Index Responses = HttpStatusCode
+type instance Index Operation = HttpStatusCode
+
+type instance IxValue Responses = Referenced Response
+type instance IxValue Operation = Referenced Response
+
+instance Ixed Responses where
+  ix n = #responses % ix n
+  {-# INLINE ix #-}
+instance At   Responses where
+  at n = #responses % at n
+  {-# INLINE at #-}
+
+instance Ixed Operation where
+  ix n = #responses % ix n
+  {-# INLINE ix #-}
+instance At   Operation where
+  at n = #responses % at n
+  {-# INLINE at #-}
+
+-- #paramSchema
+
+instance
+  ( a ~ ParamSchema 'SwaggerKindSchema
+  , b ~ ParamSchema 'SwaggerKindSchema
+  ) => LabelOptic "paramSchema" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #schema % #paramSchema
+  {-# INLINE labelOptic #-}
+
+-- #type
+
+instance
+  ( a ~ Maybe (SwaggerType ('SwaggerKindNormal Header))
+  , b ~ Maybe (SwaggerType ('SwaggerKindNormal Header))
+  ) => LabelOptic "type" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #type
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerType 'SwaggerKindSchema)
+  , b ~ Maybe (SwaggerType 'SwaggerKindSchema)
+  ) => LabelOptic "type" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #type
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerType 'SwaggerKindSchema)
+  , b ~ Maybe (SwaggerType 'SwaggerKindSchema)
+  ) => LabelOptic "type" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #type
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerType 'SwaggerKindParamOtherSchema)
+  , b ~ Maybe (SwaggerType 'SwaggerKindParamOtherSchema)
+  ) => LabelOptic "type" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #type
+  {-# INLINE labelOptic #-}
+
+-- #default
+
+instance
+  ( a ~ Maybe Value, b ~ Maybe Value
+  ) => LabelOptic "default" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #default
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Value, b ~ Maybe Value
+  ) => LabelOptic "default" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #default
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Value, b ~ Maybe Value
+  ) => LabelOptic "default" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #default
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Value, b ~ Maybe Value
+  ) => LabelOptic "default" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #default
+  {-# INLINE labelOptic #-}
+
+-- #format
+
+instance
+  ( a ~ Maybe Format, b ~ Maybe Format
+  ) => LabelOptic "format" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #format
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Format, b ~ Maybe Format
+  ) => LabelOptic "format" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #format
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Format, b ~ Maybe Format
+  ) => LabelOptic "format" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #format
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Format, b ~ Maybe Format
+  ) => LabelOptic "format" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #format
+  {-# INLINE labelOptic #-}
+
+-- #items
+
+instance
+  ( a ~ Maybe (SwaggerItems ('SwaggerKindNormal Header))
+  , b ~ Maybe (SwaggerItems ('SwaggerKindNormal Header))
+  ) => LabelOptic "items" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #items
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerItems 'SwaggerKindSchema)
+  , b ~ Maybe (SwaggerItems 'SwaggerKindSchema)
+  ) => LabelOptic "items" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #items
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerItems 'SwaggerKindSchema)
+  , b ~ Maybe (SwaggerItems 'SwaggerKindSchema)
+  ) => LabelOptic "items" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #items
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe (SwaggerItems 'SwaggerKindParamOtherSchema)
+  , b ~ Maybe (SwaggerItems 'SwaggerKindParamOtherSchema)
+  ) => LabelOptic "items" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #items
+  {-# INLINE labelOptic #-}
+
+-- #maximum
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "maximum" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #maximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "maximum" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #maximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "maximum" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #maximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "maximum" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #maximum
+  {-# INLINE labelOptic #-}
+
+-- #exclusiveMaximum
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMaximum" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #exclusiveMaximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMaximum" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #exclusiveMaximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMaximum" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #exclusiveMaximum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMaximum" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #exclusiveMaximum
+  {-# INLINE labelOptic #-}
+
+-- #minimum
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "minimum" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #minimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "minimum" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #minimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "minimum" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #minimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "minimum" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #minimum
+  {-# INLINE labelOptic #-}
+
+-- #exclusiveMinimum
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMinimum" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #exclusiveMinimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMinimum" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #exclusiveMinimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMinimum" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #exclusiveMinimum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "exclusiveMinimum" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #exclusiveMinimum
+  {-# INLINE labelOptic #-}
+
+-- #maxLength
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxLength" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #maxLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxLength" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #maxLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxLength" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #maxLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxLength" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #maxLength
+  {-# INLINE labelOptic #-}
+
+-- #minLength
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minLength" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #minLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minLength" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #minLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minLength" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #minLength
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minLength" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #minLength
+  {-# INLINE labelOptic #-}
+
+-- #pattern
+
+instance
+  ( a ~ Maybe Text, b ~ Maybe Text
+  ) => LabelOptic "pattern" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #pattern
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Text, b ~ Maybe Text
+  ) => LabelOptic "pattern" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #pattern
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Text, b ~ Maybe Text
+  ) => LabelOptic "pattern" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #pattern
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Text, b ~ Maybe Text
+  ) => LabelOptic "pattern" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #pattern
+  {-# INLINE labelOptic #-}
+
+-- #maxItems
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxItems" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #maxItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxItems" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #maxItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxItems" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #maxItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "maxItems" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #maxItems
+  {-# INLINE labelOptic #-}
+
+-- #minItems
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minItems" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #minItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minItems" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #minItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minItems" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #minItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Integer, b ~ Maybe Integer
+  ) => LabelOptic "minItems" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #minItems
+  {-# INLINE labelOptic #-}
+
+-- #uniqueItems
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "uniqueItems" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #uniqueItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "uniqueItems" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #uniqueItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "uniqueItems" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #uniqueItems
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Bool, b ~ Maybe Bool
+  ) => LabelOptic "uniqueItems" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #uniqueItems
+  {-# INLINE labelOptic #-}
+
+-- #enum
+
+instance
+  ( a ~ Maybe [Value], b ~ Maybe [Value]
+  ) => LabelOptic "enum" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #enum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe [Value], b ~ Maybe [Value]
+  ) => LabelOptic "enum" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #enum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe [Value], b ~ Maybe [Value]
+  ) => LabelOptic "enum" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #enum
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe [Value], b ~ Maybe [Value]
+  ) => LabelOptic "enum" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #enum
+  {-# INLINE labelOptic #-}
+
+-- #multipleOf
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "multipleOf" A_Lens Header Header a b where
+  labelOptic = #paramSchema % #multipleOf
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "multipleOf" A_Lens Schema Schema a b where
+  labelOptic = #paramSchema % #multipleOf
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "multipleOf" A_Lens NamedSchema NamedSchema a b where
+  labelOptic = #paramSchema % #multipleOf
+  {-# INLINE labelOptic #-}
+
+instance
+  ( a ~ Maybe Scientific, b ~ Maybe Scientific
+  ) => LabelOptic "multipleOf" A_Lens ParamOtherSchema ParamOtherSchema a b where
+  labelOptic = #paramSchema % #multipleOf
+  {-# INLINE labelOptic #-}
diff --git a/src/Data/Swagger/Schema/Validation.hs b/src/Data/Swagger/Schema/Validation.hs
--- a/src/Data/Swagger/Schema/Validation.hs
+++ b/src/Data/Swagger/Schema/Validation.hs
@@ -19,8 +19,10 @@
   ValidationError,
 
   -- ** Using 'ToJSON' and 'ToSchema'
+  validatePrettyToJSON,
   validateToJSON,
   validateToJSONWithPatternChecker,
+  renderValidationErrors,
 
   -- ** Using 'Value' and 'Schema'
   validateJSON,
diff --git a/swagger2.cabal b/swagger2.cabal
--- a/swagger2.cabal
+++ b/swagger2.cabal
@@ -1,6 +1,6 @@
 cabal-version:       >=1.10
 name:                swagger2
-version:             2.4
+version:             2.5
 
 synopsis:            Swagger 2.0 data model
 category:            Web, Swagger
@@ -24,9 +24,7 @@
   , examples/*.hs
   , include/overlapping-compat.h
 tested-with:
-  GHC ==7.8.4
-   || ==7.10.3
-   || ==8.0.2
+  GHC ==8.0.2
    || ==8.2.2
    || ==8.4.4
    || ==8.6.5
@@ -44,6 +42,7 @@
     Data.Swagger.Declare
     Data.Swagger.Lens
     Data.Swagger.Operation
+    Data.Swagger.Optics
     Data.Swagger.ParamSchema
     Data.Swagger.Schema
     Data.Swagger.Schema.Generator
@@ -61,10 +60,11 @@
 
   -- GHC boot libraries
   build-depends:
-      base             >=4.7      && <4.13
+      base             >=4.9      && <4.14
+    , aeson-pretty     >=0.8.7    && <0.9
     , bytestring       >=0.10.4.0 && <0.11
     , containers       >=0.5.5.1  && <0.7
-    , template-haskell >=2.9.0.0  && <2.15
+    , template-haskell >=2.9.0.0  && <2.16
     , time             >=1.4.2    && <1.10
     , transformers     >=0.3.0.0  && <0.6
 
@@ -74,16 +74,18 @@
 
   -- other dependencies
   build-depends:
-      base-compat-batteries     >=0.10.4   && <0.11
+      base-compat-batteries     >=0.10.4   && <0.12
     , aeson                     >=1.4.2.0  && <1.5
     -- cookie 0.4.3 is needed by GHC 7.8 due to time>=1.4 constraint
     , cookie                    >=0.4.3    && <0.5
     , generics-sop              >=0.3.2.0  && <0.6
     , hashable                  >=1.2.7.0  && <1.4
     , http-media                >=0.7.1.2  && <0.9
-    , insert-ordered-containers >=0.2.2    && <0.3
-    , lens                      >=4.16.1   && <4.18
+    , insert-ordered-containers >=0.2.3    && <0.3
+    , lens                      >=4.16.1   && <4.19
     , network                   >=2.6.3.5  && <3.2
+    , optics-core               >=0.2      && <0.3
+    , optics-th                 >=0.2      && <0.3
     , scientific                >=0.3.6.2  && <0.4
     , transformers-compat       >=0.3      && <0.7
     , unordered-containers      >=0.2.9.0  && <0.3
diff --git a/test/Data/Swagger/CommonTestTypes.hs b/test/Data/Swagger/CommonTestTypes.hs
--- a/test/Data/Swagger/CommonTestTypes.hs
+++ b/test/Data/Swagger/CommonTestTypes.hs
@@ -18,8 +18,6 @@
 import           GHC.Generics
 
 import           Data.Swagger
-import           Data.Swagger.Declare
-import           Data.Swagger.Internal (SwaggerKind (..))
 
 -- ========================================================================
 -- Unit type
@@ -648,5 +646,34 @@
     {
       "singleMaybeField": { "type": "string" }
     }
+}
+|]
+
+
+-- ========================================================================
+-- TimeOfDay
+-- ========================================================================
+data TimeOfDay
+  = Int
+  | Pico
+  deriving (Generic)
+instance ToSchema TimeOfDay
+instance ToParamSchema TimeOfDay
+
+
+timeOfDaySchemaJSON :: Value
+timeOfDaySchemaJSON = [aesonQQ|
+{
+  "example": "12:33:15",
+  "type": "string",
+  "format": "hh:MM:ss"
+}
+|]
+
+timeOfDayParamSchemaJSON :: Value
+timeOfDayParamSchemaJSON = [aesonQQ|
+{
+  "type": "string",
+  "format": "hh:MM:ss"
 }
 |]
diff --git a/test/Data/Swagger/ParamSchemaSpec.hs b/test/Data/Swagger/ParamSchemaSpec.hs
--- a/test/Data/Swagger/ParamSchemaSpec.hs
+++ b/test/Data/Swagger/ParamSchemaSpec.hs
@@ -3,9 +3,10 @@
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE QuasiQuotes #-}
+{-# LANGUAGE OverloadedStrings #-}
 module Data.Swagger.ParamSchemaSpec where
 
-import Data.Aeson (Value)
+import Data.Aeson
 import Data.Aeson.QQ.Simple
 import Data.Char
 import Data.Proxy
@@ -17,7 +18,10 @@
 import Data.Swagger.CommonTestTypes
 import SpecCommon
 import Test.Hspec
+import Data.Time.LocalTime
 
+import qualified Data.HashMap.Strict as HM
+
 checkToParamSchema :: ToParamSchema a => Proxy a -> Value -> Spec
 checkToParamSchema proxy js = (toParamSchema proxy :: ParamSchema ('SwaggerKindNormal Param)) <=> js
 
@@ -30,6 +34,7 @@
     context "Unary records" $ do
       context "Email (unary record)"  $ checkToParamSchema (Proxy :: Proxy Email)  emailSchemaJSON
       context "UserId (non-record newtype)" $ checkToParamSchema (Proxy :: Proxy UserId) userIdSchemaJSON
+    context "TimeOfDay" $ checkToParamSchema (Proxy :: Proxy Data.Time.LocalTime.TimeOfDay) timeOfDayParamSchemaJSON
 
 main :: IO ()
 main = hspec spec
diff --git a/test/Data/Swagger/SchemaSpec.hs b/test/Data/Swagger/SchemaSpec.hs
--- a/test/Data/Swagger/SchemaSpec.hs
+++ b/test/Data/Swagger/SchemaSpec.hs
@@ -8,16 +8,11 @@
 import Prelude.Compat
 
 import Control.Lens ((^.))
-import Data.Aeson (Value, ToJSON(..), ToJSONKey(..))
-import Data.Aeson.Types (toJSONKeyText)
-import Data.Aeson.QQ.Simple
-import Data.Char
+import Data.Aeson (Value)
 import qualified Data.HashMap.Strict.InsOrd as InsOrdHashMap
 import Data.Proxy
 import Data.Set (Set)
-import Data.Map (Map)
 import qualified Data.Text as Text
-import GHC.Generics
 
 import Data.Swagger
 import Data.Swagger.Declare
@@ -26,6 +21,9 @@
 import SpecCommon
 import Test.Hspec
 
+import qualified Data.HashMap.Strict as HM
+import Data.Time.LocalTime
+
 checkToSchema :: ToSchema a => Proxy a -> Value -> Spec
 checkToSchema proxy js = toSchema proxy <=> js
 
@@ -110,6 +108,7 @@
     context "MyRoseTree' (inlineNonRecursiveSchemas)" $ checkInlinedRecSchema (Proxy :: Proxy MyRoseTree') myRoseTreeSchemaJSON'
   describe "Bounded Enum key mapping" $ do
     context "ButtonImages" $ checkToSchema (Proxy :: Proxy ButtonImages) buttonImagesSchemaJSON
+    context "TimeOfDay" $ checkToSchema (Proxy :: Proxy Data.Time.LocalTime.TimeOfDay) timeOfDaySchemaJSON
 
 main :: IO ()
 main = hspec spec