Parallel Universes Reader & Coreader for Dependency Injection Ì INCL. COMONADS, MONADS & THE DISTANT PAST Elmar Kretzer @elmkretzer 

- talk to network - use Date() - persist data Does your app even: - you name it Why care? 

HOW TO? ... wire it together? ... test it peacefully? ... set it up? 

highly flexible duplication of code mental overhead error prone Keep It Pass On 1 2 

3 OPTIONS: „DO IT MANUALLY“ „USE A FRAMEWORK“ „DO SOMETHING ELSE“ 

Let’s talk about „something else“ Functional Programming Weirder names for weird concepts. Some/None/Few/All people like it. 

But Swift does use/allow/force functional patterns. Sometimes you don’t even notice. Swift Guess who’s not a Functional PL: 

empowered by fp 

FP & DI – Cool & How? 

http://hackage.haskell.org/package/mtl-1.1.0.2/docs/Control-Monad-Reader.html „Represents a computation, which can read values from a shared environment, pass values from function to function, and execute sub-computations in a modified environment.“ 1/2 READER MONAD 

very abstract return sth. based on sth. else afterwards pass it on to get sth. that itself returns sth. based on sth. else 

Describe dependent operations. Provide How at the end. 100 + 10 + 5 + 2 100 * 10 * 5 * 2 100 - 10 - 5 - 2 100 / 10 / 5 / 2 

„In more technical language the Reader comonad is left adjoint to the Reader monad.“ 2/2 READER COMONAD https://hackage.haskell.org/package/category-extras-0.53.0/docs/Control-Comonad-Reader.html 

very co-abstract hold sth. and sth. else afterwards get sth. new from sth. and sth. else and return the new with sth. else 

Describe dependent access. Provide from WHAT at the end. RealWorld -> Api -> Server -> Time MockWorld - > Api -> Server -> Time 

Reader vs Coreader Comonad extend M a -> M a -> b -> M b Produce -> Consume Cokleisli Monad flatMap M a -> a -> M b -> M b Consume -> Produce Kleisli 

Coreader No. B/C Controller, Storyboards and so forth. Reader Yes. No one is holding you back. Worth it & simple to use in iOS? 

What do you do? Corridor
 µFramework for a Coreader-like 
 access to dependencies. Combined with Reader: full-fledged control. github.com/symentis/Corridor 

Pro? Unified Handling of DI. No business code for passing on Dependencies. Dependencies are transparent. Testing simply takes different Environments. Less code. More safety. 

Follow-Up article including gists on: http://elm4ward.github.io/ Corridor on GitHub: https://github.com/symentis/Corridor Me on Twitter: @elmkretzer thanks for your time!