Functional Programing Android architecture approaches using Arrow
The library being used here to fetch super heroes is the MarvelApiClientAndroid
from Karumi. Since it's targeting the real Marvel API,
you will need to add marvelPublicKey=your_public_key and marvelPrivateKey=your_private_key to
your home gradle.properties to be able to compile or run it. You can also add them by command line:
./gradlew detektCheck build -PmarvelPublicKey="\"whatever\"" -PmarvelPrivateKey="\"whatever\""
To achieve functional programing over Kotlin I am using a library that we have been working on in the spanish dev community. It's called Arrow and its first official release is around the corner!
Big thanks to all the lib contributors which I am part of. Here they are.
This project showcases a different functional architecture approach per module. These are the strategies / gradle modules available.
This module showcases an approach based on a stack of monads resolving all the concerns that an app could have, including Dependency Injection, error handling, asynchrony and threading, IO computations, and so on.
- Read this detailed post to use it as the best documentation out there for this approach.
Tagless-Final style is focused on never depending on concrete types like Option or Try, but use Higher Kinds to make our code depend on typeclass constrained behaviors, leaving the decision about which concrete types to use to the moment when we want to run the code. You will really want to look at this PR to have a very good and detailed description of what tagless-final is.
This approach is equivalent to the Tagless-Final one but fixing the Typeclasses parametrized type F to IO. It's a simplification
about how to solve the app concerns without the need for depending on abstractions and therefore for the typeclass inherent ad-hoc polymorphism.
This FP style is very trendy. We are applying it over Android thanks to Arrow here, on the free-monads project module. It's highly recommended to take a look at this PR in order to understand the approach.
Free Monads is based on the idea of composing an AST (abstract syntax tree) of computations with type Free<S, A>, where S is your algebra, which will never depend on implementation details but on abstractions defined by an algebra, which is an algebraic data type (ADT). We are defining it through a sealed class on this sample.
Those ops can be combined as blocks to create more complex ones. Then, we need an interpreter which will be in charge to provide implementation details for the moment when the user decides to run the whole AST providing semantics to it and a Monad instance to resolve all effects / perform execution of effects in a controlled context. The user has the power of chosing which interpreter to use and which monad instance he wants to solve the problem. That enables testing, since we can easily remove our real side effects in the app at our testing environment by switching the interpreter by a fake one.
Referential transparency from a function perspective means that it should be clearly defining the work it's going to do based on its name, input parameter types and return type. It's a quite important concept on functional programing. We are quite used to model our error cases based on exceptions and callbacks in many cases, but exceptions do not surpass thread limits, and callbacks completely destroy referential transparency.
To sum up, callbacks also break referential transparency, since a void return type on a method means that it could be applying side effects and we wouldn't really know. We don't have any clue about how the result is going to look like by looking at the function declaration.
The main goal I have for error handling here is to be able to integrate errors with successful results using Monads. The purpose is to benefit from functional structures to manage use case results in a very readable way, so all the concerns about what the function is going to return and how it's going to work for it.
This approach helps me to program in a simple imperative style about how to process asynchronous results that could be produced by an API query.
A result of type Reader<Future<Either<Error, Success>> is clearly defining a deferred computation
(Reader) that will require some context to work. When we decide to run this block, we will run the
reader passing a context to it to provide all it's required dependencies, and it will run an
async task using a Future that will produce a result of either Error type or Success type on
completion. And that can be the return type for an use case, for example.
If we define the different types of expected Errors with a sealed class we should be able to
close the hierarchy to limit the amount of expected errors inside our domain and do pattern matching
to it (when statement) to achieve different behaviors depending on that.
From centuries ago, Android devs have been using complex frameworks like Dagger to achieve dependency injection. But DI is just a concept not bound to any library. It's is all about passing collaborators to your classes from the outside world. That means DI would also be to just add some setters or a constructor with some collaborator arguments to your class.
The Reader or the Kleisli used on this project are just an alternative to achieve DI that
is gonna play a good role in terms of psynergy along with the resting Monads being used.
Trying to achieve purity on this repo, as much as I can. Purity means determinism. Functional behaviors are usually abstracted to functions, not to classes. I don't need to play with instances most of the time since all the operations and transformations over the data can always be pure and just wrapped in functions as first class citizens.
The library being used here to fetch super heroes is the MarvelApiClientAndroid from Karumi.
- Jorge Castillo Pérez - jorge.castillo.prz@gmail.com
Copyright 2018 Jorge Castillo Pérez
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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