Dávid Szakállas dszakallas

We all know about the Fibonacci sequence. Some of us also know a song that uses it to achieve a spiraling feeling (yes, Tool fans!).

In Haskell:

fib 1 = 0
fib 2 = 1
fib x = fib (x - 1) (x - 2)

As a rookie Haskeller coming from imperative languages I am still often shocked how elegantly mathematical problems can be expressed in this great language. Last time a wrote a blogpost about recursion. As an example, I presented the Relation type and implemented an operation to it I called join. If you are familiar with relational algebra you might have recognized that this is a specific case of equijoin, where you join pairs so that the first pair's second element matches the second pair's first element. I was thinking about a more complex problem I could solve with this toolkit, one

FP Training 4: Recursion

Agenda

Example 1: factorial
Example 2: map
Example 3: join

1. We implemented the factorial example in JavaScript imperatively:

Endless streams

Disclaimer: This is a satire, and not meant to be taken any more seriously than Java itself. It is not about Java 8 streams. This satire just shows how easy is to define a very complex endless stream functionally in Java using lambdas.

In Scala:

def naturals: Stream[Int] = 0 #:: naturals.map(_ + 1)

	{-# LANGUAGE NoImplicitPrelude #-}
	module Rng where

	import GHC.Show (Show)
	import GHC.Base (Eq, ($))
	import GHC.Num ((*), (+), Integer)
	import GHC.Float (Float, Double)
	import GHC.Real (fromIntegral, (/))
	import GHC.Enum (maxBound)
	import Data.Int (Int64, Int32, Int16, Int8)

	data Day = Monday \| Tuesday \| Wednesday \| Thursday \| Friday \| Saturday \| Sunday
	deriving (Eq, Ord, Show, Read, Bounded, Enum)

	-- and that's it. we have everything we need
	-- david@mac:~% stack ghci
	-- λ> Monday == Tuesday
	-- False
	-- λ> Monday == Monday
	-- True
	-- λ> Tuesday > Monday

	#!/usr/bin/env node

	'use strict'

	var mysql = require('mysql')
	var fs = require('fs')
	var path = require('path')

	function exitOnError (f) {
	return function (err) {

	'use strict'

	var crypto = require('crypto')
	var promise = Promise.resolve()

	var data = []

	for (var i = 0; i < 1e5; ++i) {
	promise = promise.then(function () {
	return new Promise(function (resolve, reject) {

	function dfs (vertex, visited) {
	if (visited.indexOf(vertex.id) !== -1) { // cut visited nodes
	return null
	}
	if (!vertex.children) { // leaf, return its value
	return { id: vertex.id, sum: vertex.value }
	} else { // inner node, traverse leaves and aggregate their values
	var sum = 0
	var childSums = []
	for (var child in vertex.children) {

	#/usr/bin/env sh
	function killport() {
	lsof -i :$1 \| tail -n 1 \| awk '{ printf $2 }' \| xargs kill -9
	}