{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}
-------------------------------------------------------------------
-- |
-- Module : Data.Geospatial.Internal.CRS
-- Copyright : (C) 2014-2018 HS-GeoJSON Project
-- License : BSD-style (see the file LICENSE.md)
-- Maintainer : Andrew Newman
--
-- See Section 3 /Coordinate Reference System Objects/
-- in the GeoJSON Spec
--
-------------------------------------------------------------------
module Data.Geospatial.Internal.CRS (
-- * Types
CRSObject(..)
-- * Functions
, defaultCRS
-- * Prisms
, _NoCRS
, _NamedCRS
, _EPSG
, _LinkedCRS
) where
import Data.Geospatial.Internal.BasicTypes
import Control.Applicative ((<$>), (<*>))
import Control.Lens (makePrisms)
import Control.Monad (mzero)
import Data.Aeson (FromJSON (..), Object,
ToJSON (..), Value (..),
object, (.:), (.=))
import Data.Aeson.Types (Parser)
import Data.Text (Text)
-- | See Section 3 /Coordinate Reference System Objects/ in the GeoJSON Spec
-- `NoCRS` is required because no 'crs' attribute in a GeoJSON feature is NOT the same thing as
-- a null 'crs' attribute. no 'crs' value implies the default CRS, while a null CRS means
-- you cannot assume a CRS, null will mapped to `NoCRS` while a non-existent attribute will
-- be mapped to a `Nothing` `Maybe` value
data CRSObject =
NoCRS
| NamedCRS !Name
| EPSG Code
| LinkedCRS !Href !FormatString deriving (Show, Eq)
makePrisms ''CRSObject
-- | The default CRS according to Section 3 /Coordinate Reference System Objects/ is WGS84 which I believe,
-- from which translates to this in JSON: )
-- is represented thus:
defaultCRS :: CRSObject
defaultCRS = EPSG 4326
-- instances
-- |
-- decode CRS Objects from GeoJSON
--
-- Aeson doesnt decode "null" to `Null` unfortunately
--
instance FromJSON CRSObject where
parseJSON Null = return NoCRS
parseJSON (Object obj) = do
crsType <- obj .: "type"
crsObjectFromAeson crsType obj
parseJSON _ = mzero
-- |
-- encode CRS Objects to GeoJSON
--
instance ToJSON CRSObject where
toJSON (NamedCRS name) = object ["type" .= ("name" :: Text), "properties" .= object ["name" .= name]]
toJSON (EPSG code) = object ["type" .= ("epsg" :: Text), "properties" .= object ["code" .= code]]
toJSON (LinkedCRS href format) = object ["type" .= ("link" :: Text), "properties" .= object ["href" .= href, "type" .= format]]
toJSON NoCRS = Null
-- helpers
crsPropertyFromAesonObj :: (FromJSON a) => Text -> Object -> Parser a
crsPropertyFromAesonObj name obj = do
props <- obj .: "properties"
props .: name
crsObjectFromAeson :: Text -> Object -> Parser CRSObject
crsObjectFromAeson "name" obj = NamedCRS <$> crsPropertyFromAesonObj "name" obj
crsObjectFromAeson "epsg" obj = EPSG <$> crsPropertyFromAesonObj "code" obj
crsObjectFromAeson "link" obj = LinkedCRS <$> crsPropertyFromAesonObj "href" obj <*> crsPropertyFromAesonObj "type" obj
crsObjectFromAeson _ _ = mzero