davidallsopp’s gists

davidallsopp / unfold.scala

Last active December 23, 2015 23:39

Converting integers into Roman numerals. An example of defining and using the "unfold" higher-order function in Scala (as an Eclipse Scala Worksheet).Adapted from http://daily-scala.blogspot.co.uk/2009/09/unfoldleft-and-right.html and http://www.scala-blogs.org/2008/01/roman-numerals-in-scala.html for the romanize example. See also Haskell versi…

	object unfold {

	def unfoldRight[A, B](seed: B)(f: B => Option[(A, B)]): List[A] = f(seed) match {
	case Some((a, b)) => a :: unfoldRight(b)(f)
	case None => Nil
	}

	def unfoldLeft[A, B](seed: B)(f: B => Option[(B, A)]) = {
	def loop(seed: B)(ls: List[A]): List[A] = f(seed) match {
	case Some((b, a)) => loop(b)(a :: ls)

davidallsopp / birthday.scala

Last active December 23, 2015 23:48

Happy Birthday in Scala - a couple of variations

	object birthday {

	1 to 4 map { i => "Happy Birthday " + (if (i == 3) "dear Scala" else "to you") } foreach println

	(1 to 4).foldLeft("")((r,c) => r + ("\nHappy Birthday " + (if (c == 3) "dear Scala" else "to you")))

	}

davidallsopp / findfiles.scala

Created September 26, 2013 09:53

Recursively find files from a starting directory

	def findFiles(d: File): Array[File] = {
	val (dirs, files) = d.listFiles.partition(_.isDirectory)
	files ++ dirs.flatMap(findFiles)
	}

davidallsopp / ProxyTester.scala

Last active December 24, 2015 01:59

Simple demo of checking HTTP proxy settings and/or downloading text or HTML pages in Scala, with equivalent code in Python for comparison

	import scala.sys.SystemProperties
	import scala.io.Source.fromURL

	object ProxyTester extends App {

	// Uncomment if you are behind a proxy, then edit the IP and port to match your proxy
	// val props = new SystemProperties
	//props("http.proxyHost") = "localhost"
	//props("http.proxyPort") = "8080"
	// Uncomment to see all the system properties including proxy settings

davidallsopp / phone_regex.scala

Last active April 27, 2018 13:44

Scala regex and pattern matching example - phone numbers

	import scala.util.matching.Regex

	object regex {

	val number = "07923 874123" //> number : String = 07923 874123
	val pattern = """(\d{5})[ -]?(\d{6})""" //> pattern : String = (\d{5})[ -]?(\d{6})

	number.matches(pattern) //> res0: Boolean = true

	// Compiled pattern

davidallsopp / letterfreq.scala

Created September 29, 2013 11:25

Letter frequency one-liner in Scala.

	"zoology".groupBy(identity).mapValues(_.length)

	// Map(y -> 1, g -> 1, l -> 1, o -> 3, z -> 1)
	// identity is equivalent to x => x

davidallsopp / retry.scala

Created October 12, 2013 20:57

Creating convenient syntax for retrying an unreliable operation, using Try, currying, call-by-name (and @tailrec, if you really need lots of retries!) This is a Scala Eclipse worksheet, so won't run standalone without a minor tweak.

	import scala.annotation.tailrec
	import scala.util.Random
	import scala.util.{ Try, Success, Failure }

	object retry {
	{ // extra block is a workaround for Scala worksheet bug with @tailrec

	@tailrec
	def withRetry[T](attempts: Int)(f: => T): T = {
	println("Attempts remaining: " + attempts)

davidallsopp / bitsAndBytes.scala

Last active December 25, 2015 23:09

Playing around with bytes and bits - based on a couple of forum threads on the Sept 2013 "Functional Programming Principles in Scala" MOOC on Coursera (progfun).

	val bytes = List[Byte](0, 1, -1, 127, -128)

	def toBits(b: Byte) = 0 to 7 map (b >> _ & 1)

	val bits = bytes flatMap toBits
	// Note: least-significant bit on the left!
	// List(0, 0, 0, 0, 0, 0, 0, 0,
	// 1, 0, 0, 0, 0, 0, 0, 0,
	// 1, 1, 1, 1, 1, 1, 1, 1,
	// 1, 1, 1, 1, 1, 1, 1, 0,

davidallsopp / SwingBuilder.scala

Last active December 26, 2015 09:09

When writing Swing GUI code (in particular) we often have many repetitive calls to the same object. When using Swing from Scala, it would be nice to remove that duplication:

	import javax.swing.JCheckBox

	object SwingBuilder {

	// Instead of this:
	val checkbox = new JCheckBox()
	checkbox.setText("Hello")
	checkbox.setAlignmentX(0.0f)
	checkbox.setLocation(0, 100)

davidallsopp / diamond.scala

Created November 5, 2013 16:41

Linearization of traits in Scala to handle the diamond inheritance problem. Adapted from http://jim-mcbeath.blogspot.co.uk/2009/08/scala-class-linearization.html It now seems OK to refer to indirect parent traits, i.e. super[A], which apparently wasn't possible at the time of the original article.

	// Adapted from http://jim-mcbeath.blogspot.co.uk/2009/08/scala-class-linearization.html

	object diamond {
	class A { def t = 1 }

	// super refers to the next trait in the linearization, not necessarily to A
	trait B extends A { override def t = super.t + 1 }
	trait C extends A { override def t = super.t * 2 }

	// Linearises to (D1, C, B, A)

David Allsopp davidallsopp