djspiewak

Quick Tips for Fast Code on the JVM

I was talking to a coworker recently about general techniques that almost always form the core of any effort to write very fast, down-to-the-metal hot path code on the JVM, and they pointed out that there really isn't a particularly good place to go for this information. It occurred to me that, really, I had more or less picked up all of it by word of mouth and experience, and there just aren't any good reference sources on the topic. So… here's my word of mouth.

This is by no means a comprehensive gist. It's also important to understand that the techniques that I outline in here are not 100% absolute either. Performance on the JVM is an incredibly complicated subject, and while there are rules that almost always hold true, the "almost" remains very salient. Also, for many or even most applications, there will be other techniques that I'm not mentioning which will have a greater impact. JMH, Java Flight Recorder, and a good profiler are your very best friend! Mea

OlivierBlanvillain

Revisiting Tagless Final Interpreters

Tageless Final interpreters are an alternative to the traditional Algebraic Data Type (and generalized ADT) based implementation of the interpreter pattern. This document presents the Tageless Final approach with Scala, and shows how Dotty with it's recently added implicits functions makes the approach even more appealing. All examples are direct translations of their Haskell version presented in the Typed Tagless Final Interpreters: Lecture Notes (section 2).

The interpreter pattern has recently received a lot of attention in the Scala community. A lot of efforts have been invested in trying to address the biggest shortcomings of ADT/GADT based solutions: extensibility. One can first look at cats' Inject typeclass for an implementation of [Data Type à la Carte](http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesA

tmspzz

#!/bin/sh

# Update: As of March 24th 2017 this script won't work. 
# The script at https://get.haskellstack.org/ is broken 
# because the binary of Stack 1.4 for ARM is missing from https://www.stackage.org/stack/ .
# You can still get the binary for Stack 1.3.2 from
# https://github.com/commercialhaskell/stack/releases/download/v1.3.2/stack-1.3.2-linux-arm.tar.gz
# and place it at $USR_LOCAL_BIN/stack in your system.
# Once Stack is installed you can proceed with the Install LLVM step

djspiewak

Getting Started in Scala

This is my attempt to give Scala newcomers a quick-and-easy rundown to the prerequisite steps they need to a) try Scala, and b) get a standard project up and running on their machine. I'm not going to talk about the language at all; there are plenty of better resources a google search away. This is just focused on the prerequisite tooling and machine setup. I will not be assuming you have any background in JVM languages. So if you're coming from Python, Ruby, JavaScript, Haskell, or anywhere…  I hope to present the information you need without assuming anything.

Disclaimer It has been over a decade since I was new to Scala, and when I was new to Scala, I was coming from a Java and Ruby background. This has probably caused me to unknowingly make some assumptions. Please feel free to call me out in comments/tweets!

One assumption I'm knowingly making is that you're on a Unix-like platform. Sorry, Windows users.

Getting the JVM

smarter

Generalized Algebraic Data Types in Scala

Basic GADTs

Here's an ADT which is not a GADT, in Haskell:

data Expr = IntExpr Int | BoolExpr Bool

cprima

Scraping custom statistics from a Devolo Powerline adapter

THAT moment when you realize that the webinterface of the access point in a Devolo dLAN® 1200+ Powerline gets updated via jQuery from a json file!

curl 'http://10.xxx.yyy.zzz/cgi-bin/htmlmgr?_file=getjson&service=hpdevices'
[{"loc":"local","type":"hp200","name":"dLAN 1200+ WiFi ac","mac":"F4:06:ab:ab:ab:ab","tx":"","rx":"","ustr":"teh remote"},{"loc":"remote","type":"hp200","name":"dLAN 1200+","mac":"F4:06:aa:bb:cc:dd","tx":"85.3125","rx":"254.625","ustr":"teh master"},{}]

milessabin

scala> class Foo { self: Singleton => }
defined class Foo

scala> class Bar extends Foo
<console>:12: error: illegal inheritance;
 self-type Bar does not conform to Foo's selftype Foo with Singleton
       class Bar extends Foo
                         ^

scala> object Bar extends Foo

djspiewak

Explaining Miles's Magic

Miles Sabin recently opened a pull request fixing the infamous SI-2712. First off, this is remarkable and, if merged, will make everyone's life enormously easier. This is a bug that a lot of people hit often without even realizing it, and they just assume that either they did something wrong or the compiler is broken in some weird way. It is especially common for users of scalaz or cats.

But that's not what I wanted to write about. What I want to write about is the exact semantics of Miles's fix, because it does impose some very specific assumptions about the way that type constructors work, and understanding those assumptions is the key to getting the most of it his fix.

For starters, here is the sort of thing that SI-2712 affects:

def foo[F[_], A](fa: F[A]): String = fa.toString

ocharles

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Monad.IO.Class
import Control.Monad.Trans.Class
import Prelude hiding (log)

--------------------------------------------------------------------------------
-- The API for cloud files.
class Monad m => MonadCloud m where
  saveFile :: Path -> Bytes -> m ()

milessabin

import shapeless._

object CaseClassMergeDemo extends App {
  import mergeSyntax._

  case class Foo(i: Int, s: String, b: Boolean)
  case class Bar(b: Boolean, s: String)

  val foo = Foo(23, "foo", true)
  val bar = Bar(false, "bar")