djspiewak

Introduction to scalaz-stream

Every application ever written can be viewed as some sort of transformation on data. Data can come from different sources, such as a network or a file or user input or the Large Hadron Collider. It can come from many sources all at once to be merged and aggregated in interesting ways, and it can be produced into many different output sinks, such as a network or files or graphical user interfaces. You might produce your output all at once, as a big data dump at the end of the world (right before your program shuts down), or you might produce it more incrementally. Every application fits into this model.

The scalaz-stream project is an attempt to make it easy to construct, test and scale programs that fit within this model (which is to say, everything). It does this by providing an abstraction around a "stream" of data, which is really just this notion of some number of data being sequentially pulled out of some unspecified data source. On top of this abstraction, sca

milessabin

// Used in shapeless here: https://github.com/milessabin/shapeless/blob/master/core/src/main/scala/shapeless/syntax/singletons.scala#L42

scala> def narrow[T <: AnyRef](t: T): t.type = t
narrow: [T <: AnyRef](t: T)t.type

scala> val s1 = narrow("foo")  // Widened
s1: String = foo

scala> def narrow[T <: AnyRef](t: T): t.type {} = t  // Note empty refinement
narrow: [T <: AnyRef](t: T)t.type

tpolecat

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._

SimonRichardson

module Data.AddressBook where

import Data.Maybe

newtype Address = Address
  { street :: String
  , city   :: String
  , state  :: String
  }

nickbutcher

<?xml version="1.0" encoding="utf-8"?>
<!--
  Copyright (C) 2015 The Android Open Source Project

  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

johnynek

/**
 * To get started:
 * git clone https://github.com/twitter/algebird
 * cd algebird
 * ./sbt algebird-core/console
 */

/**
 * Let's get some data. Here is Alice in Wonderland, line by line
 */

johnynek

sealed trait Key {
    type Inner
}

trait IntKey extends Key {
    type Inner = Int
}
trait StringKey extends Key {
    type Inner = String
}

paf31

This blog post series has moved here.

You might also be interested in the 2016 version.

lsd

Upgrading to Lion or Yosemite and WebStorm 9, I noticed key repeat was
turned off for the IdeaVim plugin h j k l keys.

System-wide key repeat

defaults write -g ApplePressAndHoldEnabled -bool false in a terminal will enable
key repeat for every app. This can alternatively be found in the accessibility settings in OS X' preferences.

App specific key repeat

bvenners

scala> import org.scalactic._
import org.scalactic._

// A Validation is either a Pass or a Fail. A Validation is like an
// Option with the reverse attitude: Pass is like None, it contains no
// value and just means the value being validated is OK as is. Fail is
// like Some, it holds an error value describing the validation failure.
scala> def isPositive(i: Int): Validation[ErrorMessage] =
     |   if (i > 0) Pass else Fail(s"$i was not positive")
isPositive: (i: Int)org.scalactic.Validation[org.scalactic.ErrorMessage]