Re: [Haskell-cafe] Aeson parsing of GeoJSON geometries

9 Sep 2016

      On Thu, Sep 8, 2016 at 1:06 AM, mike thomas  wrote:
...
Hi all.
Having returned to commercial software development after a 10 year break
in environmental regulation and travel, it was not long before I was using
my favorite language Haskell, fortunately, as part of my job this time.
Haskell has changed in many ways since last I used it, and I am having an
internal (lack of) knowledge collision with the problem below:
As set out in the example below, given a polygon as GeoJSON from a Postgis
enabled Postgres datasbase, I want to get to a list of Double tuples:
[(Double,Double)], using Aeson, to draw some polygons.  I am stopped at the
point of dismembering the nested coordinate arrays.
With Stackage nightly-2016-09-01 ghci, and the series of functions defined
at the end of this message, I get a runtime type error regarding Vectors,
which baffles me.
Question 1. Why does Aeson nest the arrays using list syntax around the
Array type constructor arguments?
This is just the Show instance for Values, you can `encode' to print the
actual json representation.
...
Question 2. How do I best convert this part of the AST:
"(Array [Array [Array [Number 4.5305923601,Number 50.6585120423],Array
[Number 4.5307511543,Number 50.657719833],Array [Number 4.5310580333,Number
50.657539732],Array [Number 4.5309023972,Number 50.6584422261],Array
[Number 4.5308797482,Number 50.6586166629],Array [Number
4.5305923601,Number 50.6585120423]]])"
to a list of Double tuples?
I usually try to avoid doing custom parsing, it's much easier to use an
existing instance when you can.

Try this:

#!/usr/bin/env stack
-- stack --resolver nightly-2016-09-01 --install-ghc runghc --package aeson
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
module Main (main) where

import GHC.Generics
import Data.Aeson

data Geo = Geo { coordinates :: [[(Double, Double)]] } deriving (Generic,
Show)
instance ToJSON Geo
instance FromJSON Geo

main :: IO ()
main = do
  let ex1 = eitherDecode "[[1,2]]" :: Either String Value
  print ex1
  print $ fmap encode ex1
  let ex2 =
"{\"type\":\"Polygon\",\"coordinates\":[[[4.5305923601,50.6585120423],[4.5307511543,50.657719833],[4.5310580333,50.657539732],[4.5309023972,50.6584422261],[4.5308797482,50.6586166629],[4.5305923601,50.6585120423]]]}"
  print (eitherDecode ex2 :: Either String Geo)
...
I've tried several variations of the function dropOuterArray to try and
resolve this run-time error, with no success.
All help welcome, and thanks for your time
Mike.
========= GHCI Session With Error ======================
p = parseGeoJSONGeometry "{\"type\":\"Polygon\",\"
coordinates\":[[[4.5305923601,50.6585120423],[4.5307511543,
50.657719833],[4.5310580333,50.657539732],[4.5309023972,
50.6584422261],[4.5308797482,50.6586166629],[4.5305923601,
50.6585120423]]]}"
p (Just (String "Polygon"),Just (Array [Array [Array [Number
4.5305923601,Number 50.6585120423],Array [Number 4.5307511543,Number
50.657719833],Array [Number 4.5310580333,Number 50.657539732],Array [Number
4.5309023972,Number 50.6584422261],Array [Number 4.5308797482,Number
50.6586166629],Array [Number 4.5305923601,Number 50.6585120423]]]))
p1 = dropOuterArrayJ (snd p)
p1 [Array [Array [Number 4.5305923601,Number 50.6585120423],Array [Number
4.5307511543,Number 50.657719833],Array [Number 4.5310580333,Number
50.657539732],Array [Number 4.5309023972,Number 50.6584422261],Array
[Number 4.5308797482,Number 50.6586166629],Array [Number
4.5305923601,Number 50.6585120423]]]
p2 = dropOuterArray p1
:151:21: error: • Couldn't match type ‘Data.Vector.Vector Value’ with
‘Value’ Expected type: Value Actual type: Array • In the first argument of
‘dropOuterArray’, namely ‘p1’ In the expression: dropOuterArray p1 In an
equation for ‘p2’: p2 = dropOuterArray p1
============ Code ====================
{-# LANGUAGE OverloadedStrings, DeriveGeneric, DeriveAnyClass,
ScopedTypeVariables, LambdaCase, FlexibleContexts #-}
module GeoJSONGeometry (
    parseGeoJSONGeometry
    ,Coordinates
    ,dropOuterArrayJ
    ,dropOuterArray
    ,Pt
    ,gTypeString
) where
import Data.Aeson
import Data.Aeson.Types --(parse, parseMaybe, parseEither, Value(..))
import GHC.Generics
import Data.ByteString.Lazy.Char8
import Data.HashMap.Strict
import qualified Data.Vector as V
import Data.Either.Extra (fromRight)
import Data.Maybe (fromJust)
import Control.Applicative
import Codec.Picture
import Graphics.Rasterific
import Graphics.Rasterific.Texture
import GHC.Generics
parseGeoJSONGeometry gjg =
 let
    eresult = (eitherDecode (pack gjg)) :: Either String Object -- Value
-- Object
    result = (fromRight eresult)
    gType = Data.HashMap.Strict.lookup "type" result
    gCoords = Data.HashMap.Strict.lookup "coordinates" result
 in
    (gType, gCoords)
gTypeString (Just (Data.Aeson.Types.String s)) = s
dropOuterArrayJ (Just (Array u)) = u
dropOuterArray (Array u) = u
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Re: [Haskell-cafe] Aeson parsing of GeoJSON geometries

Adam Bergmark