Add cabal flag derive_json_via_th

Disabling the flag will cause derivation of ToJSON and FromJSON
instances via GHC Generics instead of Template Haskell. The flag is
enabled by default, as deriving via Generics can be slow (see #4083).

pandoc.cabal

@@ -337,6 +337,11 @@ 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:       True
+
 flag trypandoc
   Description:   Build trypandoc cgi executable.
   Default:       False
@@ -413,6 +418,8 @@ 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
   if os(windows)
     cpp-options:      -D_WINDOWS
   ghc-options:     -Wall -fno-warn-unused-do-bind

src/Text/Pandoc/App/CommandLineOptions.hs

@@ -2,8 +2,10 @@
 {-# LANGUAGE CPP                 #-}
 {-# LANGUAGE DeriveGeneric       #-}
 {-# LANGUAGE ScopedTypeVariables #-}
-{-# LANGUAGE TemplateHaskell     #-}
 {-# LANGUAGE TupleSections       #-}
+#ifdef DERIVE_JSON_VIA_TH
+{-# LANGUAGE TemplateHaskell     #-}
+#endif
 {-
 Copyright (C) 2006-2018 John MacFarlane <jgm@berkeley.edu>
 
@@ -46,7 +48,6 @@ import Control.Monad
 import Control.Monad.Trans
 import Data.Aeson.Encode.Pretty (encodePretty', Config(..), keyOrder,
          defConfig, Indent(..), NumberFormat(..))
-import Data.Aeson.TH (deriveJSON, defaultOptions)
 import Data.Char (toLower, toUpper)
 import Data.List (intercalate, sort)
 import Data.Maybe (fromMaybe)
@@ -65,6 +66,13 @@ import Text.Pandoc.Writers.Math (defaultMathJaxURL, defaultKaTeXURL)
 import Text.Pandoc.Shared (ordNub, safeRead)
 import Text.Printf
 
+#ifdef DERIVE_JSON_VIA_TH
+import Data.Aeson.TH (deriveJSON, defaultOptions)
+#else
+import Data.Aeson (FromJSON (..), ToJSON (..),
+                   defaultOptions, genericToEncoding)
+#endif
+
 #ifdef EMBED_DATA_FILES
 import Text.Pandoc.Data (dataFiles)
 import System.Directory (getAppUserDataDirectory)
@@ -1086,5 +1094,15 @@ deprecatedOption o msg =
 
 -- see https://github.com/jgm/pandoc/pull/4083
 -- using generic deriving caused long compilation times
+#ifdef DERIVE_JSON_VIA_TH
 $(deriveJSON defaultOptions ''LineEnding)
 $(deriveJSON defaultOptions ''Opt)
+#else
+instance ToJSON LineEnding where
+  toEncoding = genericToEncoding defaultOptions
+instance FromJSON LineEnding
+
+instance ToJSON Opt where
+  toEncoding = genericToEncoding defaultOptions
+instance FromJSON Opt
+#endif

src/Text/Pandoc/Extensions.hs

@@ -1,9 +1,11 @@
-{-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE CPP                        #-}
+{-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE DeriveDataTypeable         #-}
 {-# LANGUAGE DeriveGeneric              #-}
+#ifndef AVOID_TEMPLATE_HASKELL
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 {-# LANGUAGE TemplateHaskell            #-}
+#endif
 {-
 Copyright (C) 2012-2018 John MacFarlane <jgm@berkeley.edu>
 
@@ -50,8 +52,6 @@ module Text.Pandoc.Extensions ( Extension(..)
                               , multimarkdownExtensions )
 where
 import Prelude
-import Data.Aeson (FromJSON (..), ToJSON (..), defaultOptions)
-import Data.Aeson.TH (deriveJSON)
 import Data.Bits (clearBit, setBit, testBit, (.|.))
 import Data.Data (Data)
 import Data.Typeable (Typeable)
@@ -59,8 +59,15 @@ import GHC.Generics (Generic)
 import Text.Pandoc.Shared (safeRead)
 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, ToJSON, FromJSON)
+  deriving (Show, Read, Eq, Ord, Data, Typeable, Generic)
 
 instance Semigroup Extensions where
   (Extensions a) <> (Extensions b) = Extensions (a .|. b)
@@ -387,4 +394,15 @@ parseFormatSpec = parse formatSpec ""
                         '-' -> disableExtension ext
                         _   -> enableExtension ext
 
+#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

@@ -1,5 +1,9 @@
+{-# LANGUAGE CPP               #-}
+{-# LANGUAGE DeriveGeneric     #-}
 {-# LANGUAGE NoImplicitPrelude #-}
+#ifndef AVOID_TEMPLATE_HASKELL
 {-# LANGUAGE TemplateHaskell   #-}
+#endif
 {-
 Copyright (C) 2006-2018 John MacFarlane <jgm@berkeley.edu>
 
@@ -34,9 +38,14 @@ module Text.Pandoc.Filter
   ) where
 
 import Prelude
-import Data.Aeson (defaultOptions)
-import Data.Aeson.TH (deriveJSON)
+#ifdef AVOID_TEMPLATE_HASKELL
+import Data.Aeson (FromJSON (..), ToJSON (..),
+                   defaultOptions, genericToEncoding)
+#else
+import Data.Aeson.TH (deriveJSON, defaultOptions)
+#endif
 import Data.Foldable (foldrM)
+import GHC.Generics (Generic)
 import Text.Pandoc.Class (PandocIO)
 import Text.Pandoc.Definition (Pandoc)
 import Text.Pandoc.Options (ReaderOptions)
@@ -47,7 +56,7 @@ import qualified Text.Pandoc.Filter.Path as Path
 -- | Type of filter and path to filter file.
 data Filter = LuaFilter FilePath
             | JSONFilter FilePath
-            deriving (Show)
+            deriving (Show, Generic)
 
 -- | Modify the given document using a filter.
 applyFilters :: ReaderOptions
@@ -67,4 +76,10 @@ expandFilterPath :: Filter -> PandocIO Filter
 expandFilterPath (LuaFilter fp) = LuaFilter <$> Path.expandFilterPath fp
 expandFilterPath (JSONFilter fp) = JSONFilter <$> Path.expandFilterPath fp
 
+#ifdef AVOID_TEMPLATE_HASKELL
+instance ToJSON Filter where
+  toEncoding = genericToEncoding defaultOptions
+instance FromJSON Filter
+#else
 $(deriveJSON defaultOptions ''Filter)
+#endif

src/Text/Pandoc/Options.hs

@@ -1,7 +1,11 @@
-{-# LANGUAGE NoImplicitPrelude #-}
+{-# LANGUAGE CPP                #-}
 {-# LANGUAGE DeriveDataTypeable #-}
 {-# LANGUAGE DeriveGeneric      #-}
+{-# LANGUAGE NoImplicitPrelude  #-}
+#ifndef AVOID_TEMPLATE_HASKELL
 {-# LANGUAGE TemplateHaskell    #-}
+#endif
+
 {-
 Copyright (C) 2012-2018 John MacFarlane <jgm@berkeley.edu>
 
@@ -48,8 +52,6 @@ module Text.Pandoc.Options ( module Text.Pandoc.Extensions
                            , isEnabled
                            ) where
 import Prelude
-import Data.Aeson (defaultOptions)
-import Data.Aeson.TH (deriveJSON)
 import Data.Data (Data)
 import Data.Default
 import qualified Data.Set as Set
@@ -59,6 +61,13 @@ import Skylighting (SyntaxMap, defaultSyntaxMap)
 import Text.Pandoc.Extensions
 import Text.Pandoc.Highlighting (Style, pygments)
 
+#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
 
@@ -239,6 +248,7 @@ 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)
@@ -248,3 +258,40 @@ $(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