Fix multiple QueryParams

 * Add test API taking multiple `QueryParam`s
 * Add basic test using this API, generating an endpoint to ensure correct HTTP `one=foo&two=bar` query string generation is happening (that fails on `master`)
 * Fix (re)creation of query string to append `&` before the new parameter if there is already a built query string.

Fixes haskell-servant#23.

src/Servant/QuickCheck/Internal/HasGenRequest.hs

@@ -91,9 +91,9 @@ instance (KnownSymbol x, Arbitrary c, ToHttpApiData c, HasGenRequest b)
     genRequest _ = do
       new' <- new
       old' <- old
-      return $ \burl -> let r = old' burl in r {
-          queryString = queryString r
-                     <> param <> "=" <> cs (toQueryParam new') }
+      return $ \burl -> let r = old' burl
+                            qs = queryString r in r {
+          queryString = (if BS.null qs then "" else "&") <> qs <> param <> "=" <> cs (toQueryParam new') }
       where
         old = genRequest (Proxy :: Proxy b)
         param = cs $ symbolVal (Proxy :: Proxy x)

test/Servant/QuickCheck/InternalSpec.hs

@@ -4,12 +4,16 @@ module Servant.QuickCheck.InternalSpec (spec) where
 import           Control.Concurrent.MVar (newMVar, readMVar, swapMVar)
 import           Control.Monad.IO.Class  (liftIO)
 import qualified Data.ByteString         as BS
+import qualified Data.ByteString.Char8 as C
 import           Prelude.Compat
 import           Servant
 import           Test.Hspec              (Spec, context, describe, it, shouldBe,
                                           shouldContain)
 import           Test.Hspec.Core.Spec    (Arg, Example, Result (..),
                                           defaultParams, evaluateExample)
+import           Test.QuickCheck.Gen     (unGen)
+import           Test.QuickCheck.Random  (mkQCGen)
+import           Network.HTTP.Client     (queryString)
 
 #if MIN_VERSION_servant(0,8,0)
 import Servant.API.Internal.Test.ComprehensiveAPI (comprehensiveAPIWithoutRaw)
@@ -28,6 +32,7 @@ spec = do
   isComprehensiveSpec
   onlyJsonObjectSpec
   notLongerThanSpec
+  queryParamsSpec
 
 serversEqualSpec :: Spec
 serversEqualSpec = describe "serversEqual" $ do
@@ -107,6 +112,17 @@ isComprehensiveSpec = describe "HasGenRequest" $ do
     let _g = genRequest comprehensiveAPIWithoutRaw
     True `shouldBe` True -- This is a type-level check
 
+queryParamsSpec :: Spec
+queryParamsSpec = describe "QueryParams" $ do
+
+  it "reduce to an HTTP query string correctly" $ do
+    let rng = mkQCGen 0
+        burl = BaseUrl Http "localhost" 80 ""
+        gen = genRequest paramsAPI
+        req = (unGen gen rng 0) burl
+        qs = C.unpack $ queryString req
+    qs `shouldContain` ("one=")
+    qs `shouldContain` ("&two=")
 
 ------------------------------------------------------------------------------
 -- APIs
@@ -119,6 +135,11 @@ type API = ReqBody '[JSON] String :> Post '[JSON] String
 api :: Proxy API
 api = Proxy
 
+type ParamsAPI = QueryParam "one" String :> QueryParam "two" String :> Get '[JSON] String
+
+paramsAPI :: Proxy ParamsAPI
+paramsAPI = Proxy
+
 server :: IO (Server API)
 server = do
     mvar <- newMVar ""