-- | Utilities to extend Aeson's parsing functionality.
module Data.Aeson.ParseUtils
where

import Data.Aeson
import Data.Yaml as YAML
import Data.HashMap.Strict (HashMap)
import qualified Data.HashMap.Strict as HashMap
import Data.ByteString (ByteString)
import Data.Maybe (fromMaybe)

-- | Like 'fromJSON', but takes a default object and merges it with the parsed
-- JSON before decoding into a typed value.
fromJSONWithDefaults :: (ToJSON a, FromJSON a) => a -> Value -> Result a
fromJSONWithDefaults defaults =
    fromJSON . (mergeValues $ toJSON defaults)

-- | Helper function to perform a deep merge on two JSON 'Value's. The merging
-- semantics are defined as such:
-- 1. If both values are objects, they are merged; collisions are resolved
-- recursively.
-- 2. If either value is @null@, the other takes precedence
-- 3. Otherwise, the second value takes precedence
mergeValues :: Value -> Value -> Value
mergeValues (Object x) (Object y) =
    Object (HashMap.unionWith mergeValues x y)
mergeValues x Null = x
mergeValues _ x = x

-- | Like 'YAML.decode', but takes a defaul object and merges it with the
-- parsed JSON before decoding.
decodeWithDefaults :: (ToJSON a, FromJSON a) => a -> ByteString -> Maybe a
decodeWithDefaults defaults src =
    YAML.decode src >>= (resultToMaybe . fromJSONWithDefaults defaults)
    where
        resultToMaybe (Success x) = Just x
        resultToMaybe (Error e) = Nothing