T.P.Options: add type HighlightMethod and patterns [API Change]

The Text.Pandoc.Options module exports the `HighlightMethod` data type
and it's constructors. Furthermore, three pattern synonyms are exported,
`NoHighlightingString`, `DefaultHighlightingString`, and
`IdiomaticHighlightingString`, which hold the string value that's used
for decoding and encoding of HighlightMethod values.

src/Text/Pandoc/Options.hs

@@ -3,6 +3,7 @@
 {-# LANGUAGE DeriveGeneric      #-}
 {-# LANGUAGE LambdaCase         #-}
 {-# LANGUAGE OverloadedStrings  #-}
+{-# LANGUAGE PatternSynonyms    #-}
 {-# LANGUAGE TemplateHaskell    #-}
 {- |
    Module      : Text.Pandoc.Options
@@ -21,6 +22,10 @@ module Text.Pandoc.Options ( module Text.Pandoc.Extensions
                            , HTMLMathMethod (..)
                            , CiteMethod (..)
                            , ObfuscationMethod (..)
+                           , HighlightMethod (..)
+                           , pattern NoHighlightingString
+                           , pattern DefaultHighlightingString
+                           , pattern IdiomaticHighlightingString
                            , HTMLSlideVariant (..)
                            , EPUBVersion (..)
                            , WrapOption (..)
@@ -42,13 +47,14 @@ import Data.Default
 import Data.Char (toLower)
 import Data.Text (Text)
 import qualified Data.Set as Set
+import qualified Data.Text as T
 import Data.Typeable (Typeable)
 import GHC.Generics (Generic)
 import Skylighting (SyntaxMap, defaultSyntaxMap)
 import Text.DocTemplates (Context(..), Template)
 import Text.Pandoc.Extensions
 import Text.Pandoc.Chunks (PathTemplate)
-import Text.Pandoc.Highlighting (Style, defaultStyle)
+import Text.Pandoc.Highlighting (Style)
 import Text.Pandoc.UTF8 (toStringLazy)
 import Data.Aeson.TH (deriveJSON)
 import Data.Aeson
@@ -186,6 +192,43 @@ instance ToJSON ObfuscationMethod where
    toJSON ReferenceObfuscation = String "references"
    toJSON JavascriptObfuscation = String "javascript"
 
+-- | Method to provide code highlighting.
+data HighlightMethod =
+    Skylighting Style
+  | IdiomaticHighlighting
+  | DefaultHighlighting
+  | NoHighlighting
+  deriving (Show, Read, Eq, Data, Typeable, Generic)
+
+-- | String representation of the idiomatic highlighting option.
+pattern IdiomaticHighlightingString :: Text
+pattern IdiomaticHighlightingString = "idiomatic"
+
+-- | String representation of the default highlighting option.
+pattern DefaultHighlightingString :: Text
+pattern DefaultHighlightingString = "default"
+
+-- | String representation of the no highlighting option
+pattern NoHighlightingString :: Text
+pattern NoHighlightingString = "none"
+
+instance ToJSON HighlightMethod where
+  toJSON NoHighlighting        = String NoHighlightingString
+  toJSON IdiomaticHighlighting = String IdiomaticHighlightingString
+  toJSON DefaultHighlighting   = String DefaultHighlightingString
+  toJSON (Skylighting style)   = toJSON style
+
+instance FromJSON HighlightMethod where
+  parseJSON = \case
+    String NoHighlightingString        -> pure NoHighlighting
+    String IdiomaticHighlightingString -> pure IdiomaticHighlighting
+    String DefaultHighlightingString   -> pure DefaultHighlighting
+    String x           -> fail $ "Unknown highlighting method " <> T.unpack x
+    Bool True          -> pure DefaultHighlighting
+    Bool False         -> pure NoHighlighting
+    Null               -> pure NoHighlighting
+    v                  -> Skylighting <$> parseJSON v
+
 -- | Varieties of HTML slide shows.
 data HTMLSlideVariant = S5Slides
                       | SlidySlides
@@ -330,9 +373,7 @@ data WriterOptions = WriterOptions
   , writerHtmlQTags         :: Bool       -- ^ Use @<q>@ tags for quotes in HTML
   , writerSlideLevel        :: Maybe Int  -- ^ Force header level of slides
   , writerTopLevelDivision  :: TopLevelDivision -- ^ Type of top-level divisions
-  , writerListings          :: Bool       -- ^ Use listings package for code
-  , writerHighlightStyle    :: Maybe Style  -- ^ Style to use for highlighting
-                                           -- (Nothing = no highlighting)
+  , writerHighlightMethod   :: HighlightMethod  -- ^ Style to use for highlighting
   , writerSetextHeaders     :: Bool       -- ^ Use setext headers for levels 1-2 in markdown
   , writerListTables        :: Bool       -- ^ Use list tables for RST tables
   , writerEpubSubdirectory  :: Text       -- ^ Subdir for epub in OCF
@@ -374,8 +415,7 @@ instance Default WriterOptions where
                       , writerHtmlQTags        = False
                       , writerSlideLevel       = Nothing
                       , writerTopLevelDivision = TopLevelDefault
-                      , writerListings         = False
-                      , writerHighlightStyle   = Just defaultStyle
+                      , writerHighlightMethod  = DefaultHighlighting
                       , writerSetextHeaders    = False
                       , writerListTables       = False
                       , writerEpubSubdirectory = "EPUB"