Write new Format and Formats types, some helper functions

A new Format type enumerates exactly what formats Pandoc can recognize
in some way (input, output, raw content).

A new Formats type and related functions can be used in future
IfFormatBlock and IfFormatInline blocks, and are used to implement the
castsTo function that writers can use to determine when they can
include raw content in their output.

Author: cjjdespres

pandoc-types.cabal

@@ -47,6 +47,7 @@ Library
                      Text.Pandoc.Builder
                      Text.Pandoc.JSON
                      Text.Pandoc.Arbitrary
+                     Text.Pandoc.Format
   Other-modules:     Paths_pandoc_types
   Autogen-modules:   Paths_pandoc_types
   Build-depends:     base >= 4.5 && < 5,

src/Text/Pandoc/Arbitrary.hs

@@ -6,10 +6,12 @@ where
 import Test.QuickCheck
 import Control.Applicative (Applicative ((<*>), pure), (<$>))
 import Control.Monad (forM)
+import qualified Data.Set as Set
 import Data.Text (Text)
 import qualified Data.Text as T
 import Text.Pandoc.Definition
 import Text.Pandoc.Builder
+import qualified Text.Pandoc.Format as F
 
 realString :: Gen Text
 realString = fmap T.pack $ resize 8 $ listOf $ frequency [ (9, elements [' '..'\127'])
@@ -398,3 +400,10 @@ instance Arbitrary ListNumberDelim where
                    2 -> return OneParen
                    3 -> return TwoParens
                    _ -> error "FATAL ERROR: Arbitrary instance, logic bug"
+
+instance Arbitrary F.Format where
+        arbitrary = arbitraryBoundedEnum
+
+instance Arbitrary F.Formats where
+        arbitrary = F.exactly <$> arbitrary
+        shrink (F.Formats s) = F.exactly <$> shrink (Set.toList s)