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sourcecode:
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module JSON.Data
( JValue(..)
, JObject, fromJObject, toJObject, lookupName, insertField
) where
import Data.List ( nubBy, partition )
--- A JSON value.
---
--- @cons JBool - a Boolean value (`true` or `false` in JSON)
--- @cons JNull - null, i.e. a missing value
--- @cons JString - a JSON string
--- @cons JInt - a JSON number without decimal point and exponent
--- @cons JNumber - a JSON number (numbers are always floats in JSON)
--- @cons JArray - a JSON array, represented by a list of JValues
--- @cons JObject - a JSON object, represented by a map from Strings to JValues
data JValue = JBool Bool
| JNull
| JString String
| JInt Int
| JNumber Float
| JArray [JValue]
| JObject JObject
deriving (Eq, Show)
--- A JSON object is just some representation of a mapping from names (strings)
--- to JSON values.
--- It is an abstract type (rather than an explicit list) to ensure
--- that there are no duplicate entries.
newtype JObject = JSONObject [(String, JValue)]
deriving Show
-- JSON objects are equivalent if they contain the same name/value pairs
-- possibly in a different order.
instance Eq JObject where
JSONObject jo1 == JSONObject jo2 = eqJObject jo1 jo2
where
eqJObject [] [] = True
eqJObject [] (_:_) = False
eqJObject ((k,v):kvs) jo =
let (eqk,neqk) = partition ((==k) . fst) jo
in case eqk of [(_,v1)] -> v==v1 && eqJObject kvs neqk
_ -> False
--- Extracts the list of name / JSON value pairs from a JSON object.
fromJObject :: JObject -> [(String, JValue)]
fromJObject (JSONObject jo) = jo
--- Transforms a list of name / JSON value pairs into a JSON object.
--- Pairs with duplicated names are deleted to ensure that the JSON object
--- is a map from names to values.
toJObject :: [(String,JValue)] -> JObject
toJObject = JSONObject . nubBy (\(k1, _) (k2, _) -> k1 == k2)
--- Retrieves the JSON value with a given name from a JSON object, if it exists.
lookupName :: String -> JObject -> Maybe JValue
lookupName name (JSONObject jo) = lookup name jo
--- Inserts a name / JSON value pair in a JSON object.
--- If the name already exists, the existing value is overwritten.
insertField :: String -> JValue -> JObject -> JObject
insertField name val (JSONObject jo) = JSONObject (insert jo)
where
insert [] = [(name,val)]
insert ((k,v):fs) | k == name = (name,val) : fs
| otherwise = (k,v) : insert fs
------------------------------------------------------------------------------
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