root / htools / Ganeti / JSON.hs @ cefd4a4a
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{-# LANGUAGE TypeSynonymInstances, FlexibleInstances #-} |
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{-| JSON utility functions. -} |
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{- |
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|
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Copyright (C) 2009, 2010, 2011, 2012 Google Inc. |
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|
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This program is free software; you can redistribute it and/or modify |
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it under the terms of the GNU General Public License as published by |
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the Free Software Foundation; either version 2 of the License, or |
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(at your option) any later version. |
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|
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This program is distributed in the hope that it will be useful, but |
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WITHOUT ANY WARRANTY; without even the implied warranty of |
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
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General Public License for more details. |
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|
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You should have received a copy of the GNU General Public License |
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along with this program; if not, write to the Free Software |
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Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA |
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02110-1301, USA. |
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|
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-} |
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module Ganeti.JSON |
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( fromJResult |
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, readEitherString |
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, JSRecord |
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, loadJSArray |
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, fromObj |
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, maybeFromObj |
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, fromObjWithDefault |
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, fromKeyValue |
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, fromJVal |
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, jsonHead |
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, getMaybeJsonHead |
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, asJSObject |
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, asObjectList |
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, tryFromObj |
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, toArray |
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, optionalJSField |
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, optFieldsToObj |
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, HasStringRepr(..) |
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, GenericContainer(..) |
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, Container |
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) |
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where |
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|
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import Control.Monad (liftM) |
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import Data.Maybe (fromMaybe, catMaybes) |
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import qualified Data.Map as Map |
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import Text.Printf (printf) |
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|
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import qualified Text.JSON as J |
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import Text.JSON.Pretty (pp_value) |
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|
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-- Note: this module should not import any Ganeti-specific modules |
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-- beside BasicTypes, since it's used in THH which is used itself to |
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-- build many other modules. |
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|
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import Ganeti.BasicTypes |
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|
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-- * JSON-related functions |
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|
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-- | A type alias for a field of a JSRecord. |
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type JSField = (String, J.JSValue) |
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|
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-- | A type alias for the list-based representation of J.JSObject. |
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type JSRecord = [JSField] |
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|
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-- | Converts a JSON Result into a monadic value. |
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fromJResult :: Monad m => String -> J.Result a -> m a |
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fromJResult s (J.Error x) = fail (s ++ ": " ++ x) |
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fromJResult _ (J.Ok x) = return x |
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-- | Tries to read a string from a JSON value. |
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-- |
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-- In case the value was not a string, we fail the read (in the |
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-- context of the current monad. |
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readEitherString :: (Monad m) => J.JSValue -> m String |
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readEitherString v = |
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case v of |
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J.JSString s -> return $ J.fromJSString s |
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_ -> fail "Wrong JSON type" |
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|
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-- | Converts a JSON message into an array of JSON objects. |
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loadJSArray :: (Monad m) |
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=> String -- ^ Operation description (for error reporting) |
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-> String -- ^ Input message |
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-> m [J.JSObject J.JSValue] |
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loadJSArray s = fromJResult s . J.decodeStrict |
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-- | Reads the value of a key in a JSON object. |
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fromObj :: (J.JSON a, Monad m) => JSRecord -> String -> m a |
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fromObj o k = |
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case lookup k o of |
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Nothing -> fail $ printf "key '%s' not found, object contains only %s" |
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k (show (map fst o)) |
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Just val -> fromKeyValue k val |
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|
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-- | Reads the value of an optional key in a JSON object. Missing |
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-- keys, or keys that have a \'null\' value, will be returned as |
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-- 'Nothing', otherwise we attempt deserialisation and return a 'Just' |
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-- value. |
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maybeFromObj :: (J.JSON a, Monad m) => |
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JSRecord -> String -> m (Maybe a) |
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maybeFromObj o k = |
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case lookup k o of |
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Nothing -> return Nothing |
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-- a optional key with value JSNull is the same as missing, since |
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-- we can't convert it meaningfully anyway to a Haskell type, and |
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-- the Python code can emit 'null' for optional values (depending |
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-- on usage), and finally our encoding rules treat 'null' values |
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-- as 'missing' |
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Just J.JSNull -> return Nothing |
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Just val -> liftM Just (fromKeyValue k val) |
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|
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-- | Reads the value of a key in a JSON object with a default if |
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-- missing. Note that both missing keys and keys with value \'null\' |
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-- will case the default value to be returned. |
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fromObjWithDefault :: (J.JSON a, Monad m) => |
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JSRecord -> String -> a -> m a |
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fromObjWithDefault o k d = liftM (fromMaybe d) $ maybeFromObj o k |
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-- | Reads a JValue, that originated from an object key. |
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fromKeyValue :: (J.JSON a, Monad m) |
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=> String -- ^ The key name |
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-> J.JSValue -- ^ The value to read |
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-> m a |
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fromKeyValue k val = |
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fromJResult (printf "key '%s'" k) (J.readJSON val) |
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-- | Small wrapper over readJSON. |
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fromJVal :: (Monad m, J.JSON a) => J.JSValue -> m a |
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fromJVal v = |
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case J.readJSON v of |
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J.Error s -> fail ("Cannot convert value '" ++ show (pp_value v) ++ |
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"', error: " ++ s) |
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J.Ok x -> return x |
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-- | Helper function that returns Null or first element of the list. |
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jsonHead :: (J.JSON b) => [a] -> (a -> b) -> J.JSValue |
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jsonHead [] _ = J.JSNull |
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jsonHead (x:_) f = J.showJSON $ f x |
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-- | Helper for extracting Maybe values from a possibly empty list. |
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getMaybeJsonHead :: (J.JSON b) => [a] -> (a -> Maybe b) -> J.JSValue |
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getMaybeJsonHead [] _ = J.JSNull |
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getMaybeJsonHead (x:_) f = maybe J.JSNull J.showJSON (f x) |
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-- | Converts a JSON value into a JSON object. |
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asJSObject :: (Monad m) => J.JSValue -> m (J.JSObject J.JSValue) |
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asJSObject (J.JSObject a) = return a |
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asJSObject _ = fail "not an object" |
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-- | Coneverts a list of JSON values into a list of JSON objects. |
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asObjectList :: (Monad m) => [J.JSValue] -> m [J.JSObject J.JSValue] |
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asObjectList = mapM asJSObject |
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-- | Try to extract a key from a object with better error reporting |
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-- than fromObj. |
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tryFromObj :: (J.JSON a) => |
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String -- ^ Textual "owner" in error messages |
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-> JSRecord -- ^ The object array |
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-> String -- ^ The desired key from the object |
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-> Result a |
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tryFromObj t o = annotateResult t . fromObj o |
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-- | Ensure a given JSValue is actually a JSArray. |
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toArray :: (Monad m) => J.JSValue -> m [J.JSValue] |
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toArray (J.JSArray arr) = return arr |
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toArray o = |
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fail $ "Invalid input, expected array but got " ++ show (pp_value o) |
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-- | Creates a Maybe JSField. If the value string is Nothing, the JSField |
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-- will be Nothing as well. |
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optionalJSField :: (J.JSON a) => String -> Maybe a -> Maybe JSField |
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optionalJSField name (Just value) = Just (name, J.showJSON value) |
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optionalJSField _ Nothing = Nothing |
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-- | Creates an object with all the non-Nothing fields of the given list. |
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optFieldsToObj :: [Maybe JSField] -> J.JSValue |
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optFieldsToObj = J.makeObj . catMaybes |
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-- * Container type (special type for JSON serialisation) |
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|
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-- | Class of types that can be converted from Strings. This is |
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-- similar to the 'Read' class, but it's using a different |
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-- serialisation format, so we have to define a separate class. Mostly |
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-- useful for custom key types in JSON dictionaries, which have to be |
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-- backed by strings. |
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class HasStringRepr a where |
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fromStringRepr :: (Monad m) => String -> m a |
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toStringRepr :: a -> String |
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-- | Trivial instance 'HasStringRepr' for 'String'. |
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instance HasStringRepr String where |
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fromStringRepr = return |
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toStringRepr = id |
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-- | The container type, a wrapper over Data.Map |
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newtype GenericContainer a b = |
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GenericContainer { fromContainer :: Map.Map a b } |
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deriving (Show, Eq) |
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-- | Type alias for string keys. |
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type Container = GenericContainer String |
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-- | Container loader. |
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readContainer :: (Monad m, HasStringRepr a, Ord a, J.JSON b) => |
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J.JSObject J.JSValue -> m (GenericContainer a b) |
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readContainer obj = do |
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let kjvlist = J.fromJSObject obj |
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kalist <- mapM (\(k, v) -> do |
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k' <- fromStringRepr k |
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v' <- fromKeyValue k v |
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return (k', v')) kjvlist |
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return $ GenericContainer (Map.fromList kalist) |
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{-# ANN showContainer "HLint: ignore Use ***" #-} |
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-- | Container dumper. |
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showContainer :: (HasStringRepr a, J.JSON b) => |
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GenericContainer a b -> J.JSValue |
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showContainer = |
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J.makeObj . map (\(k, v) -> (toStringRepr k, J.showJSON v)) . |
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Map.toList . fromContainer |
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instance (HasStringRepr a, Ord a, J.JSON b) => |
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J.JSON (GenericContainer a b) where |
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showJSON = showContainer |
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readJSON (J.JSObject o) = readContainer o |
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readJSON v = fail $ "Failed to load container, expected object but got " |
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++ show (pp_value v) |