Egor Mihnevich elfenlaid

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

New Build System Tricks

Command Line

alias xcbuild=$(xcode-select -p)/../SharedFrameworks/XCBuild.framework/Versions/A/Support/xcbuild
# THIS DOESNT WORK YET: xcbuild openIDEConsole  # … then switch to Xcode ➡️
xcbuild showSpecs
xcbuild build <foo.pif> [—target <target>]

A composable pattern for pure state machines with effects

State machines are everywhere in interactive systems, but they're rarely defined clearly and explicitly. Given some big blob of code including implicit state machines, which transitions are possible and under what conditions? What effects take place on what transitions?

There are existing design patterns for state machines, but all the patterns I've seen complect side effects with the structure of the state machine itself. Instances of these patterns are difficult to test without mocking, and they end up with more dependencies. Worse, the classic patterns compose poorly: hierarchical state machines are typically not straightforward extensions. The functional programming world has solutions, but they don't transpose neatly enough to be broadly usable in mainstream languages.

Here I present a composable pattern for pure state machiness with effects,

Put this in your config.fish

run

code path/to/project

	defmodule MyApp.AccountLookup.Cache do
	@moduledoc """
	Provides a cache that can be used for account lookups. This cache is backed by
	Cachex for local storage and pg2 for remote distribution. Keys are set to expire
	after 7-10 days (randomly distributed) in order to prevent stale data in our cache
	over a long time period.
	"""

	use Cachex.DistributedCache

	/// Withable is a simple protocol to make constructing
	/// and modifying objects with multiple properties
	/// more pleasant (functional, chainable, point-free)
	public protocol Withable {
	init()
	}

	public extension Withable {
	/// Construct a new instance, setting an arbitrary subset of properties
	init(with config: (inout Self) -> Void) {

	FROM elixir:1.5.3-alpine

	WORKDIR /app
	ADD . /app

	# Install hex & rebar
	RUN mix local.hex --force && \
	mix local.rebar --force && \
	mix hex.info

	// Taken from the commercial iOS PDF framework http://pspdfkit.com.
	// Copyright (c) 2014 Peter Steinberger, PSPDFKit GmbH. All rights reserved.
	// Licensed under MIT (http://opensource.org/licenses/MIT)
	//
	// You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.

	#import <UIKit/UIKit.h>
	#import <objc/runtime.h>
	#import <objc/message.h>

	/// Observes a run loop to detect any stalling or blocking that occurs.
	///
	/// This class is thread-safe.
	@interface GHRunLoopWatchdog : NSObject

	/// Initializes the receiver to watch the specified run loop, using a default
	/// stalling threshold.
	- (id)initWithRunLoop:(CFRunLoopRef)runLoop;

	/// Initializes the receiver to detect when the specified run loop blocks for