Sasha Krasnyanskiy

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

	package spire.algebra

	import spire.syntax.eq._

	trait MeetSemilattice[A] { self =>
	def meet(x: A, y: A): A

	def partialOrder: PartialOrder[A]

	def meetAlgebra: CSemigroup[A] =

	# Default system properties included when running spark-submit.
	# This is useful for setting default environmental settings.

	# Example:
	# spark.master spark://master:7077
	spark.eventLog.enabled true
	spark.eventLog.dir file:///home/ozawa/sparkeventlogs
	# spark.serializer org.apache.spark.serializer.KryoSerializer
	# spark.driver.memory 5g
	# spark.executor.extraJavaOptions -XX:+PrintGCDetails -Dkey=value -Dnumbers="one two three"

	ּ_בּ
	בּ_בּ
	טּ_טּ
	כּ‗כּ
	לּ_לּ
	מּ_מּ
	סּ_סּ
	תּ_תּ
	٩(×̯×)۶
	٩(̾●̮̮̃̾•̃̾)۶

	# Set variables in .bashrc file

	# don't forget to change your path correctly!

	export GOPATH=$HOME/golang
	export GOROOT=/usr/local/opt/go/libexec
	export PATH=$PATH:$GOPATH/bin
	export PATH=$PATH:$GOROOT/bin

	function whatDoesItDo(val){

	return val ? 1 : 2;

	}