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	htools/Ganeti/HTools/JSON.hs \

{-| JSON utility functions. -}

{-

Copyright (C) 2009, 2010, 2011 Google Inc.

This program is free software; you can redistribute it and/or modify

it under the terms of the GNU General Public License as published by

the Free Software Foundation; either version 2 of the License, or

(at your option) any later version.

This program is distributed in the hope that it will be useful, but

WITHOUT ANY WARRANTY; without even the implied warranty of

MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

General Public License for more details.

You should have received a copy of the GNU General Public License

along with this program; if not, write to the Free Software

Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA

02110-1301, USA.

-}

module Ganeti.HTools.JSON

    ( fromJResult

    , readEitherString

    , JSRecord

    , loadJSArray

    , fromObj

    , maybeFromObj

    , fromObjWithDefault

    , fromJVal

    , asJSObject

    , asObjectList

    )

    where

import Control.Monad (liftM)

import Data.Maybe (fromMaybe)

import Text.Printf (printf)

import qualified Text.JSON as J

-- * JSON-related functions

-- | A type alias for the list-based representation of J.JSObject.

type JSRecord = [(String, J.JSValue)]

-- | Converts a JSON Result into a monadic value.

fromJResult :: Monad m => String -> J.Result a -> m a

fromJResult s (J.Error x) = fail (s ++ ": " ++ x)

fromJResult _ (J.Ok x) = return x

-- | Tries to read a string from a JSON value.

--

-- In case the value was not a string, we fail the read (in the

-- context of the current monad.

readEitherString :: (Monad m) => J.JSValue -> m String

readEitherString v =

    case v of

      J.JSString s -> return $ J.fromJSString s

      _ -> fail "Wrong JSON type"

-- | Converts a JSON message into an array of JSON objects.

loadJSArray :: (Monad m)

               => String -- ^ Operation description (for error reporting)

               -> String -- ^ Input message

               -> m [J.JSObject J.JSValue]

loadJSArray s = fromJResult s . J.decodeStrict

-- | Reads the value of a key in a JSON object.

fromObj :: (J.JSON a, Monad m) => JSRecord -> String -> m a

fromObj o k =

    case lookup k o of

      Nothing -> fail $ printf "key '%s' not found, object contains only %s"

                 k (show (map fst o))

      Just val -> fromKeyValue k val

-- | Reads the value of an optional key in a JSON object.

maybeFromObj :: (J.JSON a, Monad m) =>

                JSRecord -> String -> m (Maybe a)

maybeFromObj o k =

    case lookup k o of

      Nothing -> return Nothing

      Just val -> liftM Just (fromKeyValue k val)

-- | Reads the value of a key in a JSON object with a default if missing.

fromObjWithDefault :: (J.JSON a, Monad m) =>

                      JSRecord -> String -> a -> m a

fromObjWithDefault o k d = liftM (fromMaybe d) $ maybeFromObj o k

-- | Reads a JValue, that originated from an object key.

fromKeyValue :: (J.JSON a, Monad m)

              => String     -- ^ The key name

              -> J.JSValue  -- ^ The value to read

              -> m a

fromKeyValue k val =

  fromJResult (printf "key '%s', value '%s'" k (show val)) (J.readJSON val)

-- | Small wrapper over readJSON.

fromJVal :: (Monad m, J.JSON a) => J.JSValue -> m a

fromJVal v =

    case J.readJSON v of

      J.Error s -> fail ("Cannot convert value '" ++ show v ++

                         "', error: " ++ s)

      J.Ok x -> return x

-- | Converts a JSON value into a JSON object.

asJSObject :: (Monad m) => J.JSValue -> m (J.JSObject J.JSValue)

asJSObject (J.JSObject a) = return a

asJSObject _ = fail "not an object"

-- | Coneverts a list of JSON values into a list of JSON objects.

asObjectList :: (Monad m) => [J.JSValue] -> m [J.JSObject J.JSValue]

asObjectList = mapM asJSObject

-- we will re-export these for our existing users

import Ganeti.HTools.JSON