Remove derive_json_via_th flag; always use TH.

This cuts down on code duplication and reduces the chance
for errors.  See #4083.

pandoc.cabal

@@ -359,11 +359,6 @@ flag embed_data_files
   Description:   Embed data files in binary for relocatable executable.
   Default:       False
 
-flag derive_json_via_th
-  Description:   Use Template Haskell instead of GHC Generics to derive ToJSON
-                 and FromJSON instances.
-  Default:       False
-
 flag trypandoc
   Description:   Build trypandoc cgi executable.
   Default:       False
@@ -441,9 +436,6 @@ library
      cpp-options:   -DEMBED_DATA_FILES
      build-depends: file-embed >= 0.0 && < 0.1
      other-modules: Text.Pandoc.Data
-  if flag(derive_json_via_th)
-     cpp-options:   -DDERIVE_JSON_VIA_TH
-     other-extensions: TemplateHaskell
   if os(windows)
     cpp-options:      -D_WINDOWS
   ghc-options:     -Wall -fno-warn-unused-do-bind
@@ -462,7 +454,7 @@ library
                      -fhide-source-paths
 
   default-language: Haskell2010
-  other-extensions: NoImplicitPrelude
+  other-extensions: NoImplicitPrelude, TemplateHaskell
   hs-source-dirs:  src
 
   exposed-modules: Text.Pandoc,

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -2,9 +2,6 @@
 {-# LANGUAGE CPP                 #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TupleSections       #-}
-#ifdef DERIVE_JSON_VIA_TH
-{-# LANGUAGE TemplateHaskell     #-}
-#endif
 {- |
    Module      : Text.Pandoc.App.CommandLineOptions
    Copyright   : Copyright (C) 2006-2019 John MacFarlane

src/Text/Pandoc/App/Opt.hs

@@ -2,9 +2,7 @@
 {-# LANGUAGE NoImplicitPrelude   #-}
 {-# LANGUAGE CPP                 #-}
 {-# LANGUAGE DeriveGeneric       #-}
-#ifdef DERIVE_JSON_VIA_TH
 {-# LANGUAGE TemplateHaskell     #-}
-#endif
 {- |
    Module      : Text.Pandoc.App.Opt
    Copyright   : Copyright (C) 2006-2019 John MacFarlane
@@ -36,11 +34,7 @@ import qualified Data.Text as T
 import Data.Aeson (defaultOptions, Options(..), FromJSON(..), ToJSON(..),
                   Value(..))
 import Data.Aeson.Types (typeMismatch)
-#ifdef DERIVE_JSON_VIA_TH
 import Data.Aeson.TH (deriveJSON)
-#else
-import Data.Aeson (genericToEncoding)
-#endif
 
 -- | The type of line-endings to be used when writing plain-text.
 data LineEnding = LF | CRLF | Native deriving (Show, Generic)
@@ -203,16 +197,8 @@ defaultOpts = Opt
     , optStripComments          = False
     }
 
-#ifdef DERIVE_JSON_VIA_TH
 -- see https://github.com/jgm/pandoc/pull/4083
 -- using generic deriving caused long compilation times
 $(deriveJSON
    defaultOptions{ fieldLabelModifier =
                       camelCaseToHyphenated . drop 3 } ''Opt)
-#else
-instance ToJSON Opt where
-  toEncoding = genericToEncoding
-               defaultOptions{ fieldLabelModifier =
-                                  camelCaseToHyphenated . drop 3 }
-instance FromJSON Opt
-#endif

src/Text/Pandoc/Extensions.hs

@@ -2,10 +2,8 @@
 {-# LANGUAGE NoImplicitPrelude          #-}
 {-# LANGUAGE DeriveDataTypeable         #-}
 {-# LANGUAGE DeriveGeneric              #-}
-#ifdef DERIVE_JSON_VIA_TH
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE TemplateHaskell            #-}
-#endif
 {- |
    Module      : Text.Pandoc.Extensions
    Copyright   : Copyright (C) 2012-2019 John MacFarlane
@@ -41,13 +39,7 @@ import Data.Typeable (Typeable)
 import GHC.Generics (Generic)
 import Safe (readMay)
 import Text.Parsec
-
-#ifdef DERIVE_JSON_VIA_TH
 import Data.Aeson.TH (deriveJSON, defaultOptions)
-#else
-import Data.Aeson (FromJSON (..), ToJSON (..),
-                   defaultOptions, genericToEncoding)
-#endif
 
 newtype Extensions = Extensions Integer
   deriving (Show, Read, Eq, Ord, Data, Typeable, Generic)
@@ -538,15 +530,5 @@ parseFormatSpec = parse formatSpec ""
                         '+' -> (ext : extsToEnable, extsToDisable)
                         _   -> (extsToEnable, ext : extsToDisable)
 
-#ifdef DERIVE_JSON_VIA_TH
 $(deriveJSON defaultOptions ''Extension)
 $(deriveJSON defaultOptions ''Extensions)
-#else
-instance ToJSON Extension where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON Extension
-
-instance ToJSON Extensions where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON Extensions
-#endif

src/Text/Pandoc/Filter.hs

@@ -1,9 +1,7 @@
 {-# LANGUAGE CPP               #-}
 {-# LANGUAGE DeriveGeneric     #-}
 {-# LANGUAGE NoImplicitPrelude #-}
-#ifdef DERIVE_JSON_VIA_TH
 {-# LANGUAGE TemplateHaskell   #-}
-#endif
 {- |
    Module      : Text.Pandoc.Filter
    Copyright   : Copyright (C) 2006-2019 John MacFarlane
@@ -21,12 +19,7 @@ module Text.Pandoc.Filter
   ) where
 
 import Prelude
-#ifdef DERIVE_JSON_VIA_TH
 import Data.Aeson.TH (deriveJSON, defaultOptions)
-#else
-import Data.Aeson (FromJSON (..), ToJSON (..),
-                   defaultOptions, genericToEncoding)
-#endif
 import Data.Foldable (foldrM)
 import GHC.Generics (Generic)
 import Text.Pandoc.Class (PandocIO)
@@ -59,10 +52,4 @@ expandFilterPath :: Filter -> PandocIO Filter
 expandFilterPath (LuaFilter fp) = LuaFilter <$> Path.expandFilterPath fp
 expandFilterPath (JSONFilter fp) = JSONFilter <$> Path.expandFilterPath fp
 
-#ifdef DERIVE_JSON_VIA_TH
 $(deriveJSON defaultOptions ''Filter)
-#else
-instance ToJSON Filter where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON Filter
-#endif

src/Text/Pandoc/Options.hs

@@ -2,9 +2,7 @@
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveGeneric      #-}
 {-# LANGUAGE NoImplicitPrelude  #-}
-#ifdef DERIVE_JSON_VIA_TH
 {-# LANGUAGE TemplateHaskell    #-}
-#endif
 {- |
    Module      : Text.Pandoc.Options
    Copyright   : Copyright (C) 2012-2019 John MacFarlane
@@ -42,13 +40,7 @@ import Skylighting (SyntaxMap, defaultSyntaxMap)
 import Text.Pandoc.Extensions
 import Text.Pandoc.Highlighting (Style, pygments)
 import Text.DocTemplates (Template)
-
-#ifdef DERIVE_JSON_VIA_TH
 import Data.Aeson.TH (deriveJSON, defaultOptions)
-#else
-import Data.Aeson (FromJSON (..), ToJSON (..),
-                   defaultOptions, genericToEncoding)
-#endif
 
 class HasSyntaxExtensions a where
   getExtensions :: a -> Extensions
@@ -230,7 +222,6 @@ instance HasSyntaxExtensions WriterOptions where
 isEnabled :: HasSyntaxExtensions a => Extension -> a -> Bool
 isEnabled ext opts = ext `extensionEnabled` getExtensions opts
 
-#ifdef DERIVE_JSON_VIA_TH
 $(deriveJSON defaultOptions ''ReaderOptions)
 $(deriveJSON defaultOptions ''HTMLMathMethod)
 $(deriveJSON defaultOptions ''CiteMethod)
@@ -240,40 +231,3 @@ $(deriveJSON defaultOptions ''TrackChanges)
 $(deriveJSON defaultOptions ''WrapOption)
 $(deriveJSON defaultOptions ''TopLevelDivision)
 $(deriveJSON defaultOptions ''ReferenceLocation)
-#else
-instance ToJSON CiteMethod where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON CiteMethod
-
-instance ToJSON ReaderOptions where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON ReaderOptions
-
-instance ToJSON ObfuscationMethod where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON ObfuscationMethod
-
-instance ToJSON WrapOption where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON WrapOption
-
-instance ToJSON HTMLMathMethod where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON HTMLMathMethod
-
-instance ToJSON HTMLSlideVariant where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON HTMLSlideVariant
-
-instance ToJSON TopLevelDivision where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON TopLevelDivision
-
-instance ToJSON ReferenceLocation where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON ReferenceLocation
-
-instance ToJSON TrackChanges where
-  toEncoding = genericToEncoding defaultOptions
-instance FromJSON TrackChanges
-#endif

stack.yaml

@@ -2,7 +2,6 @@ flags:
   pandoc:
     trypandoc: false
     embed_data_files: true
-    derive_json_via_th: false
     static: false
   pandoc-citeproc:
     bibutils: true