gkossakowski

Scala's "type T in class C as seen from a prefix type S" with examples

Introduction

Recently, I found myself in need to precisely understand Scala's core typechecking rules. I was particulary interested in understanding rules responsible for typechecking signatures of members defined in classes (and all types derived from them). Scala Language Specification (SLS) contains definition of the rules but lacks any examples. The definition of the rules uses mutual recursion and nested switch-like constructs that make it hard to follow. I've written down examples together with explanation how specific set of rules (grouped thematically) is applied. These notes helped me gain confidence that I fully understand Scala's core typechecking algorithm.

As Seen From

Let's quote the Scala spec for As Seen From (ASF) rules numbered for an easier reference:

gigiigig

import shapeless._

import scalaz._
import Scalaz._

object console extends App {

  trait Foo[A] {
    type B
    def value: B

retronym

indylambda: Putting invokedynamic to work for Scala

Java 8 introduced lambdas to the Java language. While the design choices differ in many regards from Scala's functions, the underlying mechanics used to represent Java lambdas is flexible enough to be used as a target for the Scala compiler.

Lambdas in Java

Java does not have canonical heirarchy of generic function types (ala scala.FunctionN), but instead allows a lambda to be used as a shorthand for an anonymous implementation of an Functional Interface

Here's an example of creating a predicate that closes over one value:

pchiusano

trait ConsoleAlg[F[_]] {
  def readLine: F[Option[String]]
  def printLine(line: String): F[Unit]
}

trait Console[+A] {
  def run[F[+_]](F: ConsoleAlg[F]): F[A]
}

object Console {

pulse00

package com.example.actors.github

import spray.httpx.Json4sSupport
import org.json4s.{DefaultFormats, Formats}

/**
 * Marshalling/Unmarshalling for the github json format
 *
 * see http://developer.github.com/v3/oauth/#web-application-flow
 */

dherman

Original paper on A-normal form:

http://redlinernotes.com/docs/Classics/The%20Essence%20of%20Compiling%20with%20Continuations.pdf

A high-level intro to ANF:

http://matt.might.net/articles/a-normalization/

One of the earlier attempts to relate SSA and CPS: