{-# LANGUAGE OverloadedStrings, TemplateHaskell #-}
-------------------------------------------------------------------
-- |
-- Module : Data.Geospatial.CRS
-- Copyright : (C) 2014 Dom De Re
-- License : BSD-style (see the file etc/LICENSE.md)
-- Maintainer : Dom De Re
--
-- See Section 3 /Coordinate Reference System Objects/
-- in the GeoJSON Spec
--
-------------------------------------------------------------------
module Data.Geospatial.CRS (
-- * Types
CRSObject(..)
-- * Functions
, defaultCRS
-- * Prisms
, _NoCRS
, _NamedCRS
, _EPSG
, _LinkedCRS
) where
import Data.Geospatial.BasicTypes
import Data.Geospatial.Geometry
import Data.Geospatial.GeoPosition
import Control.Applicative ( (<$>), (<*>) )
import Control.Lens ( makePrisms )
import Control.Monad ( mzero )
import Data.Aeson
( FromJSON(..)
, ToJSON(..)
, Value(..)
, Object
, (.:)
, (.=)
, object
)
import Data.Aeson.Types ( Parser )
import Data.Text ( Text )
-- $setup
--
-- >>> import qualified Data.Aeson as A
-- >>> import qualified Data.ByteString.Lazy.Char8 as BS
--
-- Test CRS Data
-- >>> let testLinkCRSJSON = "{\"type\":\"link\",\"properties\":{\"href\":\"www.google.com.au\",\"type\":\"proj4\"}}"
-- >>> let testLinkCRS = LinkedCRS "www.google.com.au" "proj4"
-- >>> let testEPSGJSON = "{\"type\":\"epsg\",\"properties\":{\"code\":4326}}"
-- >>> let testEPSG = EPSG 4326
-- >>> let testNamedCRSJSON = "{\"type\":\"name\",\"properties\":{\"name\":\"urn:ogc:def:crs:OGC:1.3:CRS84\"}}"
-- >>> let testNamedCRS = NamedCRS "urn:ogc:def:crs:OGC:1.3:CRS84"
--
-- | 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
-- |
-- encode and decodes CRS Objects to and from GeoJSON
--
-- >>> (A.decode . BS.pack) testLinkCRSJSON == Just testLinkCRS
-- True
--
-- >>> (A.decode . BS.pack) testNamedCRSJSON == Just testNamedCRS
-- True
--
-- >>> (A.decode . BS.pack) testEPSGJSON == Just testEPSG
-- True
--
-- Aeson doesnt decode "null" to `Null` unfortunately
--
-- (A.decode . BS.pack) "null" == Just NoCRS
-- True
--
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
--
-- >>> (A.decode . A.encode) testLinkCRS == Just testLinkCRS
-- True
--
-- >>> (A.decode . A.encode) testNamedCRS == Just testNamedCRS
-- True
--
-- >>> (A.decode . A.encode) testEPSG == Just testEPSG
-- True
--
-- >>> A.encode NoCRS
-- "null"
--
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