pchiusano

/** GADTs in Scala and their limitations */

/** Background: what is an algebraic data type (ADT) ? 
  * ADT: (possibly) recursive datatype with sums and products
  * In scala - a trait with case classes (case class is product, subtyping is sum) 
  */

/** Motivation: untyped embedded DSL doesn't prevent nonsensical expressions */
sealed trait Expr { 
  def apply(other: Expr) = Ap(this, other)

rui314

int print_board(int board[][8]) {
  for (int i = 0; i < 8; i++) {
    for (int j = 0; j < 8; j++) {
      if (board[i][j])
        printf("Q ");
      else
        printf(". ");
    }
    printf("\n");
  }

gclaramunt

trait Entity
trait User extends Entity
trait Product extends Entity

case class Id[T<:Entity](id:String)


def buy(pId:Id[Product],uId:Id[User])="Bought product %s for user %s".format(pId.id,uId.id)

val pId=new Id[Product]("1")

tonymorris

trait Semigroup[A] {
  def op(a1: A, a2: A): A
}

trait Monoid[A] extends Semigroup[A] {
  def id: A
}

object Monoid {
  implicit def ListMonoid[A]: Monoid[List[A]] =

johnynek

// Taken from: http://jnordenberg.blogspot.com/2008/08/hlist-in-scala.html
object HListTest {
  sealed trait HList

  final class HNil extends HList {
    def ::[T](v : T) = HCons(v, this)
  }

  val HNil = new HNil()

johnynek

class WriteOnce[T] {
  private val ref = new java.util.concurrent.atomic.AtomicReference[Option[T]](None)
  def init(t: T) { if(!ref.compareAndSet(None, Some(t))) sys.error("Already written"); }
  def get: Option[T] = ref.get
}

scala> val w = new WriteOnce[Int]
w: WriteOnce[Int] = WriteOnce@215d55cb

scala> w.get

johnynek

import java.lang.ref.WeakReference

/** An option-like type that doesn't keep
 * the pointed item in scope
 */

trait WeakRef[+T] {
  // If the reference is still live, return it, else None
  def toOption: Option[T]
  // If either reference is dead, return Empty, else give the live ref

azymnis

import com.twitter.scalding._
import com.twitter.algebird.{ MinHasher, MinHasher32, MinHashSignature }

/**
 * Computes similar items (with a string itemId), based on approximate
 * Jaccard similarity, using LSH.
 *
 * Assumes an input data TSV file of the following format:
 *
 *    itemId   userId

tonymorris

// Questions about \/ and Validation
object scalaz {

/*
This explanation might help. This is a compilable source file with revisions 
available at https://gist.github.com/tonymorris/8263051

Fact: All monads are applicative functors. As has been seen we can witness the
`Applicative` that arises from the `Monad` primitives. Let's illustrate this:

johnynek

Unifying Sketch Monoids

As I discussed in Algebra for Analytics, many sketch monoids, such as Bloom filters, HyperLogLog, and Count-min sketch, can be described as a hashing (projection) of items into a sparse space, then using two different commutative monoids to read and write respectively. Finally, the read monoids always have the property that (a + b) <= a, b and the write monoids has the property that (a + b) >= a, b.

##Some questions:

1. Note how similar CMS and Bloom filters are. The difference: bloom hashes k times onto the same space, CMS hashes k times onto a k orthogonal subspaces. Why the difference? Imagine a fixed space bloom that hashes onto k orthogonal spaces, or an overlapping CMS that hashes onto k * m length space. How do the error asymptotics change?
2. CMS has many query modes (dot product, etc...) can those generalize to other sketchs (HLL, Bloom)?
3. What other sketch or non-sketch algorithms can be expressed in this dual mo