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Find-Nearby-Locations.md

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Find nearby locations using Postgres

This snipped is the direct translation of the concept and code presented here in google developer map resource. The only difference here is that we are using postgres database instead of mysql.

#!/usr/bin/env stack
{- stack
     --resolver lts-6.11
     --install-ghc
     runghc
     --package persistent
     --package persistent-postgresql
     --package persistent-template
     --package mtl
-}

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}

import Database.Persist
import Database.Persist.Postgresql
import Database.Persist.TH
import Control.Monad.IO.Class  (liftIO)
import Control.Monad.Logger    (runStderrLoggingT)
import Control.Monad.Reader

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Markers
    name String 
    address String
    lat Double
    lon Double
    deriving Show
|]

loadData :: MonadIO m => ReaderT SqlBackend m ()
loadData = do
  insert_ $ Markers "Frankie Johnnie & Luigo Too" "939 W El Camino Real, Mountain View, CA" 37.386339 (-122.085823)
  insert_ $ Markers "Amici's East Coast Pizzeria" "790 Castro St  Mountain View  CA" 37.38714 (-122.083235)
  insert_ $ Markers "Kapp's Pizza Bar & Grill" "191 Castro St  Mountain View  CA" 37.393885 (-122.078916)
  insert_ $ Markers "Round Table Pizza: Mountain View" "570 N Shoreline Blvd  Mountain View  CA" 37.402653 (-122.079354)
  insert_ $ Markers "Tony & Alba's Pizza & Pasta" "619 Escuela Ave  Mountain View  CA" 37.394011 (-122.095528)
  insert_ $Markers "Oregano's Wood-Fired Pizza" "4546 El Camino Real  Los Altos  CA" 37.401724 (-122.114646)

connStr = "host=localhost dbname=test1 user=postgres password=postgres port=5432"

data GeoLocation = GeoLocation
    { latitude :: Double,
      longitude :: Double 
    } deriving (Show, Eq, Ord)

-- Note that lat, lon inside the query refers to the column name
findNearbyMarkers :: (MonadIO m) => GeoLocation -> ReaderT SqlBackend m [(Single String, Single Double)]
findNearbyMarkers (GeoLocation lat lon) = rawSql "SELECT name, ( 3959 * acos( cos( radians(?) ) * cos( radians( lat ) ) * cos( radians( lon ) - radians(?) ) + sin( radians(?) ) * sin( radians( lat ) ) ) ) AS distance FROM markers" [PersistDouble lat, PersistDouble lon, PersistDouble lat]

main :: IO ()
main = runStderrLoggingT $ withPostgresqlPool connStr 10 $ liftSqlPersistMPool $ do
         runMigration migrateAll
         loadData
         let geo = GeoLocation 37.0 (-122.0)
         markers <- findNearbyMarkers geo
         liftIO $ print "Nearby markers"
         liftIO $ print markers

Execution:

sibi::jane { ~/scripts }-> ./geo.hs
Migrating: CREATe TABLE "markers"("id" SERIAL8  PRIMARY KEY UNIQUE,"name" VARCHAR NOT NULL,"address" VARCHAR NOT NULL,"lat" DOUBLE PRECISION NOT NULL,"lon" DOUBLE PRECISION NOT NULL)
[Debug#SQL] CREATe TABLE "markers"("id" SERIAL8  PRIMARY KEY UNIQUE,"name" VARCHAR NOT NULL,"address" VARCHAR NOT NULL,"lat" DOUBLE PRECISION NOT NULL,"lon" DOUBLE PRECISION NOT NULL); []
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Frankie Johnnie & Luigo Too",PersistText "939 W El Camino Real, Mountain View, CA",PersistDouble 37.386339,PersistDouble (-122.085823)]
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Amici's East Coast Pizzeria",PersistText "790 Castro St  Mountain View  CA",PersistDouble 37.38714,PersistDouble (-122.083235)]
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Kapp's Pizza Bar & Grill",PersistText "191 Castro St  Mountain View  CA",PersistDouble 37.393885,PersistDouble (-122.078916)]
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Round Table Pizza: Mountain View",PersistText "570 N Shoreline Blvd  Mountain View  CA",PersistDouble 37.402653,PersistDouble (-122.079354)]
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Tony & Alba's Pizza & Pasta",PersistText "619 Escuela Ave  Mountain View  CA",PersistDouble 37.394011,PersistDouble (-122.095528)]
[Debug#SQL] INSERT INTO "markers"("name","address","lat","lon") VALUES(?,?,?,?) RETURNING "id"; [PersistText "Oregano's Wood-Fired Pizza",PersistText "4546 El Camino Real  Los Altos  CA",PersistDouble 37.401724,PersistDouble (-122.114646)]
[Debug#SQL] SELECT name, ( 3959 * acos( cos( radians(37) ) * cos( radians( ? ) ) * cos( radians( ? ) - radians(-122) ) + sin( radians(37) ) * sin( radians( ? ) ) ) ) AS distance FROM markers; [PersistDouble 37.0,PersistDouble (-122.0),PersistDouble 37.0]
"Nearby cities"
[(Single {unSingle = "Frankie Johnnie & Luigo Too"},Single {unSingle = 0.0}),(Single {unSingle = "Amici's East Coast Pizzeria"},Single {unSingle = 0.0}),(Single {unSingle = "Kapp's Pizza Bar & Grill"},Single {unSingle = 0.0}),(Single {unSingle = "Round Table Pizza: Mountain View"},Single {unSingle = 0.0}),(Single {unSingle = "Tony & Alba's Pizza & Pasta"},Single {unSingle = 0.0}),(Single {unSingle = "Oregano's Wood-Fired Pizza"},Single {unSingle = 0.0})]