diff --git a/Symantic/HTTP/API.hs b/Symantic/HTTP/API.hs
--- a/Symantic/HTTP/API.hs
+++ b/Symantic/HTTP/API.hs
@@ -2,6 +2,7 @@
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE DeriveFunctor #-}
 {-# LANGUAGE StrictData #-}
+{-# LANGUAGE UndecidableInstances #-} -- for type instance defaults
 -- | Combinators to build a Web API.
 module Symantic.HTTP.API where
 
@@ -9,6 +10,7 @@
 import Data.Bool
 import Data.Either (Either(..))
 import Data.Eq (Eq(..))
+import Data.Function ((.))
 import Data.Functor (Functor)
 import Data.Kind (Constraint)
 import Data.Proxy (Proxy)
@@ -30,7 +32,13 @@
 -- with the notable exception of the server instance
 -- where some HTTP error codes must be prioritized.
 class Cat repr where
-	(<.>) :: repr a b -> repr b c -> repr a c; infixl 4 <.>
+	(<.>) :: repr a b -> repr b c -> repr a c; infixr 4 <.>
+	-- Trans defaults
+	default (<.>) ::
+	 Trans repr =>
+	 Cat (UnTrans repr) =>
+	 repr a b -> repr b c -> repr a c
+	x <.> y = noTrans (unTrans x <.> unTrans y)
 	-- (.>)  :: repr x y -> repr a c -> repr a c; infixl 4  .>
 
 -- * Class 'Alt'
@@ -42,45 +50,119 @@
 	type AltMerge repr :: * -> * -> *
 	(<!>) :: repr a b -> repr c d -> repr (a:!:c) (AltMerge repr b d); infixl 3 <!>
 	-}
-	(<!>) :: repr a k -> repr b k -> repr (a:!:b) k; infixl 3 <!>
+	(<!>) :: repr a k -> repr b k -> repr (a:!:b) k; infixr 3 <!>
+	-- Trans defaults
+	default (<!>) ::
+	 Trans repr =>
+	 Alt (UnTrans repr) =>
+	 repr a k -> repr b k -> repr (a:!:b) k
+	x <!> y = noTrans (unTrans x <!> unTrans y)
 	-- try :: repr k k -> repr k k
 	-- option :: k -> repr k k -> repr k k
 
 -- ** Type (':!:')
--- | Like @(,)@ but @infixl@.
+-- | Like @(,)@ but @infixr@.
 -- Used to get alternative commands from a 'Client'
 -- or to supply alternative handlers to a 'Server'.
 data (:!:) a b = a:!:b
-infixl 3 :!:
+infixr 3 :!:
 
+-- * Class 'Trans'
+-- | A 'Trans'formation from one representation @('UnTrans' repr)@ to another one @(repr)@.
+--
+-- * 'noTrans' lifts to the identity 'Trans'formation
+--   (the one which does nothing wrt. the 'UnTrans'formed @(repr)@esentation).
+-- * 'unTrans' unlifts a 'Trans'formed value to its underlying @(repr)@esentation.
+--
+-- At its @class@ definition,
+-- a combinator should be defined with a default value using 'noTrans'.
+-- And at its @instance@ definition,
+-- a combinator can be overwritten to apply a specific 'Trans'formation for @(repr)@.
+--
+-- For an example, see the @('Trans' ('Router' repr))@ instance
+-- in <https://hackage.haskell.org/package/symantic-http-server symantic-http-server>.
+class Trans repr where
+	-- | The @(repr)@esentation that @(repr)@ 'Trans'forms.
+	type UnTrans repr :: * -> * -> *
+	-- | Lift the underlying @(repr)@esentation to @(repr)@.
+	-- Useful to define a combinator that does nothing in a 'Trans'formation.
+	noTrans :: UnTrans repr a b -> repr a b
+	-- | Unlift a @(repr)@esentation. Useful when a 'Trans'formation
+	-- combinator needs to access the 'UnTrans'formed @(repr)@esentation,
+	-- or at the end to get the underlying 'UnTrans'formed @(repr)@esentation
+	-- from the inferred @(repr)@ value (eg. in 'server').
+	unTrans :: repr a b -> UnTrans repr a b
+
 -- * Class 'Pro'
 -- | Mainly useful to write a combinator which is a specialization of another
 -- (eg. 'queryFlag' wrt. 'queryParams'),
 -- by calling it directly in the class declaration
 -- instead of rewriting its logic in the instance declaration.
 --
--- Because type @a@ is asked by a 'Client' but given to a 'Server',
--- both @a->b@ and @b->a@ are used. This is reminiscent of a 'Profunctor'.
+-- Because type @(a)@ is asked by a 'Client' but given to a 'Server',
+-- both @(a->b)@ and @(b->a)@ are used. This is reminiscent of a 'Profunctor'.
 -- Hence the names 'Pro' and 'dimap'.
 class Pro repr where
 	dimap :: (a -> b) -> (b -> a) -> repr (a -> k) k -> repr (b -> k) k
+	-- Trans defaults
+	default dimap ::
+	 Trans repr =>
+	 Pro (UnTrans repr) =>
+	 (a -> b) -> (b -> a) -> repr (a -> k) k -> repr (b -> k) k
+	dimap a2b b2a = noTrans . dimap a2b b2a . unTrans
 
+-- * Class 'HTTP_Raw'
+class HTTP_Raw repr where
+	type RawConstraint repr :: Constraint
+	type RawArgs repr :: *
+	type Raw repr :: *
+	raw ::
+	 RawConstraint repr =>
+	 repr (RawArgs repr) (Raw repr)
+	-- Trans defaults
+	type RawConstraint repr = RawConstraint (UnTrans repr)
+	type RawArgs repr = RawArgs (UnTrans repr)
+	type Raw repr = Raw (UnTrans repr)
+	default raw ::
+	 Trans repr =>
+	 HTTP_Raw (UnTrans repr) =>
+	 RawConstraint (UnTrans repr) =>
+	 RawArgs (UnTrans repr) ~ RawArgs repr =>
+	 Raw (UnTrans repr) ~ Raw repr =>
+	 repr (RawArgs repr) (Raw repr)
+	raw = noTrans raw
+
 -- * Class 'HTTP_Path'
 class HTTP_Path repr where
 	type PathConstraint repr a :: Constraint
-	type PathConstraint repr a = ()
-	segment :: Segment -> repr k k
-	capture' ::
-	 PathConstraint repr a =>
+	segment :: PathSegment -> repr k k
+	capture' :: PathConstraint repr a => Name -> repr (a -> k) k
+	captureAll :: repr ([PathSegment] -> k) k
+	-- Trans defaults
+	type PathConstraint repr a = PathConstraint (UnTrans repr) a
+	default segment ::
+	 Trans repr =>
+	 HTTP_Path (UnTrans repr) =>
+	 PathSegment -> repr k k
+	default capture' ::
+	 Trans repr =>
+	 HTTP_Path (UnTrans repr) =>
+	 PathConstraint (UnTrans repr) a =>
 	 Name -> repr (a -> k) k
-	captureAll :: repr ([Segment] -> k) k
+	default captureAll ::
+	 Trans repr =>
+	 HTTP_Path (UnTrans repr) =>
+	 repr ([PathSegment] -> k) k
+	segment    = noTrans . segment
+	capture'   = noTrans . capture'
+	captureAll = noTrans captureAll
 
 -- | Convenient wrapper of 'segment'.
-(</>) :: Cat repr => HTTP_Path repr => Segment -> repr a b -> repr a b
-(</>) n = (segment n <.>); infixr 5 </>
+(</>) :: Cat repr => HTTP_Path repr => PathSegment -> repr a b -> repr a b
+(</>) n = (segment n <.>); infixr 4 </>
 
--- | Like 'capture'' but with the type variable 'a' first instead or 'repr'
--- so it can be passed using 'TypeApplications' without adding a @@_@ for 'repr'.
+-- | Like 'capture'' but with the type variable @(a)@ first instead or @(repr)@
+-- so it can be passed using 'TypeApplications' without adding a |\@_| for @(repr)@.
 capture ::
  forall a k repr.
  HTTP_Path repr =>
@@ -89,13 +171,19 @@
 capture = capture'
 {-# INLINE capture #-}
 
-type Segment = Text
-type Path    = [Segment]
-type Name    = String
+type PathSegment = Text
+type Path = [PathSegment]
+type Name = String
 
 -- * Class 'HTTP_Header'
 class HTTP_Header repr where
 	header :: HTTP.HeaderName -> repr (HeaderValue -> k) k
+	-- Trans defaults
+	default header ::
+	 Trans repr =>
+	 HTTP_Header (UnTrans repr) =>
+	 HTTP.HeaderName -> repr (HeaderValue -> k) k
+	header = noTrans . header
 
 type HeaderValue = BS.ByteString
 
@@ -103,14 +191,23 @@
 class HTTP_Body repr where
 	type BodyArg repr a (ts::[*]) :: *
 	type BodyConstraint repr a (ts::[*]) :: Constraint
-	type BodyConstraint repr a ts = ()
 	body' ::
-	 forall a (ts::[*]) k.
 	 BodyConstraint repr a ts =>
 	 repr (BodyArg repr a ts -> k) k
+	-- Trans defaults
+	type BodyArg repr a ts = BodyArg (UnTrans repr) a ts
+	type BodyConstraint repr a ts = BodyConstraint (UnTrans repr) a ts
+	default body' ::
+	 forall a (ts::[*]) k.
+	 Trans repr =>
+	 HTTP_Body (UnTrans repr) =>
+	 BodyConstraint (UnTrans repr) a ts =>
+	 BodyArg repr a ts ~ BodyArg (UnTrans repr) a ts =>
+	 repr (BodyArg repr a ts -> k) k
+	body' = noTrans (body' @_ @a @ts)
 
--- | Like 'body'' but with the type variables 'a' and 'ts' first instead or 'repr',
--- so it can be passed using 'TypeApplications' without adding a @@_@ for 'repr'.
+-- | Like 'body'' but with the type variables @(a)@ and @(ts)@ first instead or @(repr)@,
+-- so it can be passed using 'TypeApplications' without adding a |\@_| for @(repr)@.
 body ::
  forall a ts k repr.
  HTTP_Body repr =>
@@ -123,14 +220,24 @@
 class HTTP_BodyStream repr where
 	type BodyStreamArg repr as (ts::[*]) framing :: *
 	type BodyStreamConstraint repr as (ts::[*]) framing :: Constraint
-	type BodyStreamConstraint repr as ts framing = ()
 	bodyStream' ::
 	 BodyStreamConstraint repr as ts framing =>
 	 repr (BodyStreamArg repr as ts framing -> k) k
+	-- Trans defaults
+	type BodyStreamArg repr as ts framing = BodyStreamArg (UnTrans repr) as ts framing
+	type BodyStreamConstraint repr as ts framing = BodyStreamConstraint (UnTrans repr) as ts framing
+	default bodyStream' ::
+	 forall as ts framing k.
+	 Trans repr =>
+	 HTTP_BodyStream (UnTrans repr) =>
+	 BodyStreamConstraint (UnTrans repr) as ts framing =>
+	 BodyStreamArg repr as ts framing ~ BodyStreamArg (UnTrans repr) as ts framing =>
+	 repr (BodyStreamArg repr as ts framing -> k) k
+	bodyStream' = noTrans (bodyStream' @_ @as @ts @framing)
 
--- | Like 'bodyStream'' but with the type variables 'as', 'ts' and 'framing'
--- first instead or 'repr', so it can be passed using 'TypeApplications'
--- without adding a @@_@ for 'repr'.
+-- | Like 'bodyStream'' but with the type variables @(as)@, @(ts)@ and @(framing)@
+-- first instead or @(repr)@, so it can be passed using 'TypeApplications'
+-- without adding a |\@_| for @(repr)@.
 bodyStream ::
  forall as ts framing k repr.
  HTTP_BodyStream repr =>
@@ -142,7 +249,6 @@
 -- * Class 'HTTP_Query'
 class HTTP_Query repr where
 	type QueryConstraint repr a :: Constraint
-	type QueryConstraint repr a = ()
 	queryParams' ::
 	 QueryConstraint repr a =>
 	 QueryName -> repr ([a] -> k) k
@@ -154,11 +260,19 @@
 	 QueryConstraint repr Bool =>
 	 QueryName -> repr (Bool -> k) k
 	queryFlag n = dimap and return (queryParams' n)
-type QueryName   = BS.ByteString
-type QueryValue  = BS.ByteString
+	-- Trans defaults
+	type QueryConstraint repr a = QueryConstraint (UnTrans repr) a
+	default queryParams' ::
+	 Trans repr =>
+	 HTTP_Query (UnTrans repr) =>
+	 QueryConstraint (UnTrans repr) a =>
+	 QueryName -> repr ([a] -> k) k
+	queryParams' = noTrans . queryParams'
+type QueryName  = BS.ByteString
+type QueryValue = BS.ByteString
 
--- | Like 'capture'' but with the type variable 'a' first instead or 'repr'
--- so it can be passed using 'TypeApplications' without adding a @@_@ for 'repr'.
+-- | Like 'capture'' but with the type variable @(a)@ first instead or @(repr)@
+-- so it can be passed using 'TypeApplications' without adding a |\@_| for @(repr)@.
 queryParams ::
  forall a k repr.
  HTTP_Query repr =>
@@ -171,14 +285,24 @@
 -- | <https://tools.ietf.org/html/rfc2617#section-2 Basic Access Authentication>
 class HTTP_BasicAuth repr where
 	type BasicAuthConstraint repr a :: Constraint
-	type BasicAuthConstraint repr a = ()
 	type BasicAuthArgs repr a k :: *
 	basicAuth' ::
 	 BasicAuthConstraint repr a =>
 	 BasicAuthRealm -> repr (BasicAuthArgs repr a k) k
+	-- Trans defaults
+	type BasicAuthConstraint repr a = BasicAuthConstraint (UnTrans repr) a
+	type BasicAuthArgs repr a k = BasicAuthArgs (UnTrans repr) a k
+	default basicAuth' ::
+	 forall a k.
+	 Trans repr =>
+	 HTTP_BasicAuth (UnTrans repr) =>
+	 BasicAuthConstraint (UnTrans repr) a =>
+	 BasicAuthArgs repr a k ~ BasicAuthArgs (UnTrans repr) a k =>
+	 BasicAuthRealm -> repr (BasicAuthArgs repr a k) k
+	basicAuth' = noTrans . basicAuth' @_ @a
 
--- | Like 'basicAuth'' but with the type variable 'a' first instead or 'repr'
--- so it can be passed using 'TypeApplications' without adding a @@_@ for 'repr'.
+-- | Like 'basicAuth'' but with the type variable @(a)@ first instead or @(repr)@
+-- so it can be passed using 'TypeApplications' without adding a |\@_| for @(repr)@.
 basicAuth ::
  forall a k repr.
  HTTP_BasicAuth repr =>
@@ -231,7 +355,6 @@
 -- * Class 'HTTP_Response'
 class HTTP_Response repr where
 	type ResponseConstraint repr a (ts::[*]) :: Constraint
-	type ResponseConstraint repr a ts = ()
 	type ResponseArgs repr a (ts::[*]) :: *
 	type Response repr :: *
 	response ::
@@ -239,11 +362,26 @@
 	 HTTP.Method ->
 	 repr (ResponseArgs repr a ts)
 	      (Response repr)
+	-- Trans defaults
+	type ResponseConstraint repr a ts = ResponseConstraint (UnTrans repr) a ts
+	type ResponseArgs repr a ts = ResponseArgs (UnTrans repr) a ts
+	type Response repr = Response (UnTrans repr)
+	default response ::
+	 forall a ts.
+	 Trans repr =>
+	 HTTP_Response (UnTrans repr) =>
+	 ResponseConstraint (UnTrans repr) a ts =>
+	 ResponseArgs repr a ts ~ ResponseArgs (UnTrans repr) a ts =>
+	 Response repr ~ Response (UnTrans repr) =>
+	 HTTP.Method ->
+	 repr (ResponseArgs repr a ts)
+	      (Response repr)
+	response = noTrans . response @_ @a @ts
 
 -- | Wrap 'response' by giving it the corresponding 'HTTP.Method',
--- and put the type variables 'a' then 'ts' first instead or 'repr'
+-- and put the type variables @(a)@ then @(ts)@ first instead or @(repr)@
 -- so they can be passed using 'TypeApplications'
--- without adding a |@_| for 'repr'.
+-- without adding a |@_| for @(repr)@.
 get,head,put,post,delete,trace,connect,options,patch ::
  forall a ts repr.
  HTTP_Response repr =>
@@ -272,7 +410,6 @@
 -- * Class 'HTTP_ResponseStream'
 class HTTP_ResponseStream repr where
 	type ResponseStreamConstraint repr as (ts::[*]) framing :: Constraint
-	type ResponseStreamConstraint repr as ts framing = ()
 	type ResponseStreamArgs repr as (ts::[*]) framing :: *
 	type ResponseStream repr :: *
 	responseStream ::
@@ -280,6 +417,21 @@
 	 HTTP.Method ->
 	 repr (ResponseStreamArgs repr as ts framing)
 	      (ResponseStream repr)
+	-- Trans defaults
+	type ResponseStreamConstraint repr as ts framing = ResponseStreamConstraint (UnTrans repr) as ts framing
+	type ResponseStreamArgs repr as ts framing = ResponseStreamArgs (UnTrans repr) as ts framing
+	type ResponseStream repr = ResponseStream (UnTrans repr)
+	default responseStream ::
+	 forall as ts framing.
+	 Trans repr =>
+	 HTTP_ResponseStream (UnTrans repr) =>
+	 ResponseStreamConstraint (UnTrans repr) as ts framing =>
+	 ResponseStreamArgs repr as ts framing ~ ResponseStreamArgs (UnTrans repr) as ts framing =>
+	 ResponseStream repr ~ ResponseStream (UnTrans repr) =>
+	 HTTP.Method ->
+	 repr (ResponseStreamArgs repr as ts framing)
+	      (ResponseStream repr)
+	responseStream = noTrans . responseStream @_ @as @ts @framing
 
 getStream,headStream,putStream,postStream,deleteStream,traceStream,connectStream,optionsStream,patchStream ::
  forall as ts framing repr.
diff --git a/symantic-http.cabal b/symantic-http.cabal
--- a/symantic-http.cabal
+++ b/symantic-http.cabal
@@ -2,7 +2,7 @@
 -- PVP:  +-+------- breaking API changes
 --       | | +----- non-breaking API additions
 --       | | | +--- code changes with no API change
-version: 0.0.0.20190324
+version: 0.1.1.20190410
 category: Protocol
 synopsis: Symantic combinators for deriving clients or a server from an HTTP API
 description:
@@ -118,5 +118,4 @@
     , network-uri >= 2.6
     , stm >= 2.4.5
     , text >= 1.2
-    , time >= 1.8
     , transformers >= 0.5