{-# LANGUAGE FlexibleInstances, FlexibleContexts #-}
{-
Copyright (c) 2013-2016, John MacFarlane

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modification, are permitted provided that the following conditions are met:

    * Redistributions of source code must retain the above copyright
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    * Redistributions in binary form must reproduce the above
      copyright notice, this list of conditions and the following
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      with the distribution.

    * Neither the name of John MacFarlane nor the names of other
      contributors may be used to endorse or promote products derived
      from this software without specific prior written permission.

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-}

{- |
   Module      : Text.Pandoc.JSON
   Copyright   : Copyright (C) 2013-2016 John MacFarlane
   License     : BSD3

   Maintainer  : John MacFarlane <jgm@berkeley.edu>
   Stability   : alpha
   Portability : portable

Functions for serializing the Pandoc AST to JSON and deserializing from JSON.

Example of use:  The following script (@capitalize.hs@) reads
reads a JSON representation of a Pandoc document from stdin,
and writes a JSON representation of a Pandoc document to stdout.
It changes all regular text in the document to uppercase, without
affecting URLs, code, tags, etc.  Run the script with

> pandoc -t json | runghc capitalize.hs | pandoc -f json

or (making capitalize.hs executable)

> pandoc --filter ./capitalize.hs

> #!/usr/bin/env runghc
> import Text.Pandoc.JSON
> import Data.Char (toUpper)
>
> main :: IO ()
> main = toJSONFilter capitalizeStrings
>
> capitalizeStrings :: Inline -> Inline
> capitalizeStrings (Str s) = Str $ map toUpper s
> capitalizeStrings x       = x

-}

module Text.Pandoc.JSON ( module Text.Pandoc.Definition
                        , ToJSONFilter(..)
                        )
where
import Text.Pandoc.Definition
import Text.Pandoc.Walk
import Data.Maybe (listToMaybe)
import qualified Data.ByteString.Lazy as BL
import Data.Aeson
import System.Environment (getArgs)

-- | 'toJSONFilter' convert a function into a filter that reads pandoc's
-- JSON serialized output from stdin, transforms it by walking the AST
-- and applying the specified function, and serializes the result as JSON
-- to stdout.
--
-- For a straight transformation, use a function of type @a -> a@ or
-- @a -> IO a@ where @a@ = 'Block', 'Inline','Pandoc', 'Meta', or 'MetaValue'.
--
-- If your transformation needs to be sensitive to the script's arguments,
-- use a function of type @[String] -> a -> a@ (with @a@ constrained as above).
-- The @[String]@ will be populated with the script's arguments.
--
-- An alternative is to use the type @Maybe Format -> a -> a@.
-- This is appropriate when the first argument of the script (if present)
-- will be the target format, and allows scripts to behave differently
-- depending on the target format.  The pandoc executable automatically
-- provides the target format as argument when scripts are called using
-- the `--filter` option.

class ToJSONFilter a where
  toJSONFilter :: a -> IO ()

instance (Walkable a Pandoc) => ToJSONFilter (a -> a) where
  toJSONFilter f = BL.getContents >>=
    BL.putStr . encode . (walk f :: Pandoc -> Pandoc) . either error id .
    eitherDecode'

instance (Walkable a Pandoc) => ToJSONFilter (a -> IO a) where
  toJSONFilter f = BL.getContents >>=
     (walkM f :: Pandoc -> IO Pandoc) . either error id . eitherDecode' >>=
     BL.putStr . encode

instance (Walkable [a] Pandoc) => ToJSONFilter (a -> [a]) where
  toJSONFilter f = BL.getContents >>=
    BL.putStr . encode . (walk (concatMap f) :: Pandoc -> Pandoc) .
    either error id . eitherDecode'

instance (Walkable [a] Pandoc) => ToJSONFilter (a -> IO [a]) where
  toJSONFilter f = BL.getContents >>=
     (walkM (fmap concat . mapM f) :: Pandoc -> IO Pandoc) .
     either error id . eitherDecode' >>=
     BL.putStr . encode

instance (ToJSONFilter a) => ToJSONFilter ([String] -> a) where
  toJSONFilter f = getArgs >>= toJSONFilter . f

instance (ToJSONFilter a) => ToJSONFilter (Maybe Format -> a) where
  toJSONFilter f = getArgs >>= toJSONFilter . f . fmap Format . listToMaybe
