Add Flag (#349)

* Update Parsers.hs

* Update simplexmq.cabal

simplexmq.cabal

@@ -27,6 +27,11 @@ extra-source-files:
     README.md
     CHANGELOG.md
 
+flag swiftJSON
+  Description: Enable swift JSON format
+  Default:     False
+  Manual:      True
+
 library
   exposed-modules:
       Simplex.Messaging.Agent
@@ -68,6 +73,10 @@ library
   hs-source-dirs:
       src
   ghc-options: -Wall -Wcompat -Werror=incomplete-patterns -Wredundant-constraints -Wincomplete-record-updates -Wincomplete-uni-patterns -Wunused-type-patterns
+
+  if flag(swiftJSON)
+      ghc-options: "-DswiftJSON"
+
   build-depends:
       QuickCheck ==2.14.*
     , aeson ==2.0.*

src/Simplex/Messaging/Parsers.hs

@@ -98,7 +98,7 @@ enumJSON tagModifier =
     }
 
 sumTypeJSON :: (String -> String) -> J.Options
-#if defined(darwin_HOST_OS)
+#if defined(darwin_HOST_OS) && defined(swiftJSON)
 sumTypeJSON = singleFieldJSON
 #else
 sumTypeJSON = taggedObjectJSON