Synnefo » snf-ganeti » ganeti-local

{-| JSON utility functions. -}

{-

Copyright (C) 2009, 2010, 2011, 2012 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.JSON

  ( fromJResult

  , readEitherString

  , JSRecord

  , loadJSArray

  , fromObj

  , maybeFromObj

  , fromObjWithDefault

  , fromKeyValue

  , fromJVal

  , jsonHead

  , getMaybeJsonHead

  , asJSObject

  , asObjectList

  , tryFromObj

  , toArray

  , Container(..)

  )

  where

import Control.Arrow (second)

import Control.Monad (liftM)

import Data.Maybe (fromMaybe)

import qualified Data.Map as Map

import Text.Printf (printf)

import qualified Text.JSON as J

import Text.JSON.Pretty (pp_value)

import Ganeti.BasicTypes

-- * 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. Missing

-- keys, or keys that have a \'null\' value, will be returned as

-- 'Nothing', otherwise we attempt deserialisation and return a 'Just'

-- value.

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

                JSRecord -> String -> m (Maybe a)

maybeFromObj o k =

  case lookup k o of

    Nothing -> return Nothing

    -- a optional key with value JSNull is the same as missing, since

    -- we can't convert it meaningfully anyway to a Haskell type, and

    -- the Python code can emit 'null' for optional values (depending

    -- on usage), and finally our encoding rules treat 'null' values

    -- as 'missing'

    Just J.JSNull -> return Nothing

    Just val -> liftM Just (fromKeyValue k val)

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

-- missing. Note that both missing keys and keys with value \'null\'

-- will case the default value to be returned.

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'" k) (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 (pp_value v) ++

                       "', error: " ++ s)

    J.Ok x -> return x

-- | Helper function that returns Null or first element of the list.

jsonHead :: (J.JSON b) => [a] -> (a -> b) -> J.JSValue

jsonHead [] _ = J.JSNull

jsonHead (x:_) f = J.showJSON $ f x

-- | Helper for extracting Maybe values from a possibly empty list.

getMaybeJsonHead :: (J.JSON b) => [a] -> (a -> Maybe b) -> J.JSValue

getMaybeJsonHead [] _ = J.JSNull

getMaybeJsonHead (x:_) f = maybe J.JSNull J.showJSON (f 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

-- | Try to extract a key from a object with better error reporting

-- than fromObj.

tryFromObj :: (J.JSON a) =>

              String     -- ^ Textual "owner" in error messages

           -> JSRecord   -- ^ The object array

           -> String     -- ^ The desired key from the object

           -> Result a

tryFromObj t o = annotateResult t . fromObj o

-- | Ensure a given JSValue is actually a JSArray.

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

toArray (J.JSArray arr) = return arr

toArray o =

  fail $ "Invalid input, expected array but got " ++ show (pp_value o)

-- * Container type (special type for JSON serialisation)

-- | The container type, a wrapper over Data.Map

newtype Container a = Container { fromContainer :: Map.Map String a }

  deriving (Show, Read, Eq)

-- | Container loader.

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

                 J.JSObject J.JSValue -> m (Container a)

readContainer obj = do

  let kjvlist = J.fromJSObject obj

  kalist <- mapM (\(k, v) -> fromKeyValue k v >>= \a -> return (k, a)) kjvlist

  return $ Container (Map.fromList kalist)

-- | Container dumper.

showContainer :: (J.JSON a) => Container a -> J.JSValue

showContainer =

  J.makeObj . map (second J.showJSON) . Map.toList . fromContainer

instance (J.JSON a) => J.JSON (Container a) where

  showJSON = showContainer

  readJSON (J.JSObject o) = readContainer o

  readJSON v = fail $ "Failed to load container, expected object but got "

               ++ show (pp_value v)