[Haskell-cafe] Aeson parsing of GeoJSON geometries

7 Sep 2016

      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?

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'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

[Haskell-cafe] Aeson parsing of GeoJSON geometries

mike thomas