Add noHeader function.

Which allows not adding a header where a signature declares one, making
    response headers optional.

servant/src/Servant/API.hs

@@ -75,7 +75,7 @@ import           Servant.API.QueryParam      (QueryFlag, QueryParam,
 import           Servant.API.Raw             (Raw)
 import           Servant.API.RemoteHost      (RemoteHost)
 import           Servant.API.ReqBody         (ReqBody)
-import           Servant.API.ResponseHeaders (AddHeader (addHeader),
+import           Servant.API.ResponseHeaders (AddHeader, addHeader, noHeader,
                                               BuildHeadersTo (buildHeadersTo),
                                               GetHeaders (getHeaders),
                                               HList (..), Headers (..),

servant/src/Servant/API/ResponseHeaders.hs

@@ -23,7 +23,9 @@
 -- example above).
 module Servant.API.ResponseHeaders
     ( Headers(..)
-    , AddHeader(addHeader)
+    , AddHeader
+    , addHeader
+    , noHeader
     , BuildHeadersTo(buildHeadersTo)
     , GetHeaders(getHeaders)
     , HeaderValMap
@@ -108,17 +110,48 @@ instance OVERLAPPABLE_ ( KnownSymbol h, GetHeaders (HList rest), ToHttpApiData v
 -- We need all these fundeps to save type inference
 class AddHeader h v orig new
     | h v orig -> new, new -> h, new -> v, new -> orig where
-  addHeader :: v -> orig -> new  -- ^ N.B.: The same header can't be added multiple times
+  addHeader' :: Header h v -> orig -> new  -- ^ N.B.: The same header can't be added multiple times
 
 
 instance OVERLAPPING_ ( KnownSymbol h, ToHttpApiData v )
          => AddHeader h v (Headers (fst ': rest)  a) (Headers (Header h v  ': fst ': rest) a) where
-    addHeader a (Headers resp heads) = Headers resp (HCons (Header a) heads)
+    addHeader' hdr (Headers resp heads) = Headers resp (HCons hdr heads)
 
 instance OVERLAPPABLE_ ( KnownSymbol h, ToHttpApiData v
                        , new ~ (Headers '[Header h v] a) )
          => AddHeader h v a new where
-    addHeader a resp = Headers resp (HCons (Header a) HNil)
+    addHeader' hdr resp = Headers resp (HCons hdr HNil)
+
+-- | @addHeader@ adds a header to a response. Note that it changes the type of
+-- the value in the following ways:
+--
+--   1. A simple value is wrapped in "Headers [<hdr>]":
+--
+-- >>> let example1 = addHeader 5 "hi" :: Headers '[Header "someheader" Int] String;
+-- >>> getHeaders example1
+-- [("someheader","5")]
+--
+--   2. A value that already has a header has its new header *prepended* to the
+--   existing list:
+--
+-- >>> let example1 = addHeader 5 "hi" :: Headers '[Header "someheader" Int] String;
+-- >>> let example2 = addHeader True example1 :: Headers '[Header "1st" Bool, Header "someheader" Int] String
+-- >>> getHeaders example2
+-- [("1st","true"),("someheader","5")]
+--
+-- Note that while in your handlers type annotations are not required, since
+-- the type can be inferred from the API type, in other cases you may find
+-- yourself needing to add annotations.
+addHeader :: AddHeader h v orig new => v -> orig -> new
+addHeader = addHeader' . Header
+
+-- | Deliberately do not add a header to a value.
+--
+-- >>> let example1 = noHeader "hi" :: Headers '[Header "someheader" Int] String
+-- >>> getHeaders example1
+-- []
+noHeader :: AddHeader h v orig new => orig -> new
+noHeader = addHeader' MissingHeader
 
 -- $setup
 -- >>> import Servant.API

servant/test/Servant/API/ResponseHeadersSpec.hs

@@ -22,3 +22,9 @@ spec = describe "Servant.API.ResponseHeaders" $ do
     it "adds headers to the front of the list" $ do
       let val = addHeader 10 $ addHeader "b" 5 :: Headers '[Header "first" Int, Header "second" String] Int
       getHeaders val `shouldBe` [("first", "10"), ("second", "b")]
+
+  describe "noHeader" $ do
+
+    it "does not add a header" $ do
+      let val = noHeader 5 :: Headers '[Header "test" Int] Int
+      getHeaders val `shouldBe` []