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doobie experiment - postgres NOTIFY as scalaz-stream

One of the cool things PostgreSQL gives you is a simple notification system that you can use to let clients know that something interesting has happened. For instance you can set up rules that broadcast notifications when a table is updated, and client applications can update displays in response. The JDBC driver provides access to this API so I thought I would see what it would look like in doobie.

The program below constructs a Process[ConnectionIO, PGNotification] that registers for events, polls for them periodically (this is the best we can do with current driver support), and unregisters when the stream terminates for any reason. We use a Transactor to replace ConnectionIO with Task which gives us something we can actually run.

package doobie.example

import doobie.imports._

tpolecat

def mergeRanges(ns: List[Int]): List[Range] = 
  ns.foldRight(List.empty[Range]) { 
    case (n, r :: rs) if (n == r.min - 1) => (n to r.max) :: rs
    case (n, rs)                          => (n to n) :: rs
  }

tpolecat

import scalaz.{ Free, Coyoneda, Monad, ~>, State, NonEmptyList }
import scalaz.std.function._
import scalaz.syntax.monad._
import scalaz.effect.IO

import scala.util.Random

object freecoyo extends App {

  // An algebra of primitive operations in the context of a random number generator

tpolecat

Hi, looking for scalac flags?

This gist has been upgraded to a blog post here.

tpolecat

  implicit def treeEncode[A: EncodeJson]: EncodeJson[Tree[A]] =
    EncodeJson(t => (t.rootLabel, t.subForest).asJson)

  implicit def treeDecode[A: DecodeJson]: DecodeJson[Tree[A]] =
    DecodeJson(c =>
      for {
        rootLabel <- (c =\ 0).as[A]
        subForest <- (c =\ 1).as[Stream[Tree[A]]]
      } yield Tree.node(rootLabel, subForest)

tpolecat

> runMain doobie.hi.Test
[info] Compiling 37 Scala sources to /Users/rnorris/Scala/doobie/target/scala-2.10/classes...
[info] Running doobie.hi.Test 
The answer was: java.lang.RuntimeException: Bogus country: NCL

  nyQL log for hi.examples
  +- [ok] getConnection(jdbc:h2:mem:test;DB_CLOSE_DELAY=-1, sa, ***)      |  conn1: url=jdbc:h2:mem:test us   304.109 ms
  `- [ex] database session                                                |  Bogus country: NCL              1496.911 ms
     +- [ok] populate database from /Users/rnorris/Scala/doobie/world.sql |  false                           1378.663 ms
     |  +- [ok] prepareStatement(RUNSCRIPT FROM ? CHARSET 'UTF-8')        |  prep1: RUNSCRIPT FROM ? CHARSE     3.950 ms

tpolecat

scala> class Foo(val n: Int)
defined class Foo

scala> class Bar(n: Int) extends Foo(n)
defined class Bar

scala> List[Foo]().sorted
<console>:9: error: No implicit Ordering defined for Foo.
              List[Foo]().sorted
                          ^

tpolecat

package spire.examples

import spire.algebra._
import spire.math.{Natural, UInt}

import scala.collection.mutable

object SetUtil {
  def powerStream[A](members: Stream[A]): Stream[Set[A]] = {
    val done = Stream.empty[Set[A]]

tpolecat

import shapeless._

// typeclass for total number of fields (just an example)
class Width[A](val toInt: Int)

object Width {

  // Get an instance
  implicit def apply[A](implicit A: Width[A]) = A

tpolecat

scala> :pa
// Entering paste mode (ctrl-D to finish)

def subsets[A](as: List[A]): List[List[A]] = {
  def sub0(accum: List[List[A]], as: List[A]): List[List[A]] = 
    as match {
      case Nil => accum
      case a :: as => sub0(accum ++ accum.map(a :: _), as)
    }
  sub0(List(Nil), as)