bitwalker

The Indexed State Monad in Haskell, Scala, and C#

Have you ever had to write code that made a complex series of succesive modifications to a single piece of mutable state? (Almost certainly yes.)

Did you ever wish you could make the compiler tell you if a particular operation on the state was illegal at a given point in the modifications? (If you're a fan of static typing, probably yes.)

If that's the case, the indexed state monad can help!

Motivation

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Mensch font, Webkit, Chrome, Firefox, Kaleidoscope, iTerm, Transmit, Coda, Sublime Text, Sequel Pro,

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/** 
 * JavaScript Module Pattern Example with "Passive Attachment"
 */
(function (root, builder, undefined) {
    if (typeof define === 'function') {
        // CommonJS AMD
        define('MyModule', ['jQuery', 'DependencyA', 'DependencyB'], function($, a, b) {
            return builder(root, $, a, b, null);
        });
    }

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import scala.util.parsing.combinator._

/**
 * This trait provides the mathematical operations which the calculator can perform.
 */
trait Maths {
  def add(x: Float, y: Float) = x + y
  def sub(x: Float, y: Float) = x - y
  def mul(x: Float, y: Float) = x * y
  def div(x: Float, y: Float) = if (y > 0) (x / y) else 0.0f

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#!/bin/sh
# AUTHOR: Paul Schoenfelder
#
# This post-receive hook validates that committed files use proper (LF) line endings.

verify_lf()
{
    echo "Validating line endings..."
    oldrev=$(git rev-parse $1)
    newrev=$(git rev-parse $2)

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/**
 * Dead Center an element with display: inline-block
 */

html,
body { margin: 0; height: 100%; }

.container {
	letter-spacing: -0.31em;
	text-rendering: optimizespeed;

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/**
 * Extracts values from a string using a pattern mask.
 *
 * Example:
 *  var greetFormat = 'Hello {{name}}! My name is {{greeter}}!'
 *  extract(greetFormat, 'Hello Paul! My name is Robotron!') => { name: 'Paul', greeter: 'Robotron'}
 */
function extract(format, s) {
    var parser = getParser(format);
    var parsed = parser.mask.exec(s);

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function withinRange(sourceLong, sourceLat, targetLong, targetLat, range) {
    var nauticalMilesPerStatuteMile = 0.868976;
    // Latitude is always the same distance. Divide degrees by 60 to get arcminutes
    var milesPerMinLat = 69.047 / 60;
    // Distance in miles per arcminute longitude depends on the cosine of the arcminutes of latitude
    var milesPerMinLong = (1 * Math.cos(sourceLat)) / nauticalMilesPerStatuteMile;
    // Get delta in minutes between source and target longitude, convert to statute miles
    var deltaLong = Math.abs((sourceLong * 60) - (targetLong * 60));
    // Convert `deltaLong` to statute miles, rounding to 2 decimal places
    var deltaLong = Math.round((milesPerMinLong * deltaLong) * 100) / 100;

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declare @hexstring varchar(max)
declare @bin       varbinary(max)

set @hexstring = '0x10.....'
set @bin = (
  select cast('' as xml).value('xs:hexBinary(substring(sql:variable("@hexstring"), sql:column("t.pos")))', 'varbinary(max)')
  from (select case substring(@hexstring, 1, 2) when '0x' then 3 else 0 end) as t(pos)
)
select @bin

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using System;

namespace Types
{
    public static class Option
    {
        public static Option<T> ToOption<T>(this T @this)
        {
            if (@this == null)
                return Option<T>.None;