Arrow_FP_Kotlin.pdf - Speaker Deck

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Leveraging the power of Arrow to build Android(Kotlin) apps using Functional Programming MONIKA KUMAR JETHANI

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About Me

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Agenda 1- Overview of Functional Programming 2- Functional Constructs of Kotlin 3- Introduction to Arrow 4- Deep dive into Arrow Data Types: Option, Either, Try 5- Deep dive into Arrow Type Classes: Functor, Monad, Applicative 6- More FP Concepts 7- Sample Apps

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What is Functional Programming??? Functional Programming is a programming paradigm—a style of building the structure and elements of computer programs—that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data. Its main focus is on “what to solve” in contrast to an imperative style where the main focus is “how to solve”. Source: Wikipedia

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Defining a Pure Function Input Output F Characteristics of a Pure function: 1- Provides the same output for the same input at all times. 2- Works only on the parameters supplied to it, doesn’t modify anything outside function.

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Advantages of Functional Programming over OOP  Brevity  Modularization of code  Easier to test  Code becomes predictable  Encourages safe programming by providing constructs such as immutability and pure functions.

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Functional Programming Languages  Any Language that treats functions as first class citizens.  Functions can be passed to other functions, returned from other functions and functions can be assigned to a variable.  Examples: Scala, Haskell, Kotlin.

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Functional Constructs of Kotlin  Immutability  Higher-Order Functions  Lambdas  Lazy Evaluation  Tail Call Recursion  Extension Functions  Null Safety  Pattern Matching

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Limitations of Kotlin w.r.t. FP  Doesn’t support typed functional programming.  Doesn’t enforce FP.

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Arrow  Functional Companion to Kotlin’s standard library.  Library that provides Typed Functional Programming in Kotlin.  Introduced in 2017.  Fusion of 2 most popular Kotlin libraries: Kategory and funcKTionale.  Documentation available at : https://arrow-kt.io/  Latest release: 0.7.3

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Data Types  Option  Either  Try

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Option Option is a container for an optional value of type A. If the value of type A is present, the Option is an instance of Some , containing the present value of type A. If the value is absent, the Option is the object None.

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More to Option

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Option with getOrElse getOrElse() allow us to provide default value if None.

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Use case for Option  A Registration form filled by users where entities like Name, Date-of-Birth are optional. Searching the user by name, would results in a type Optional.  Can also be used in when clause of Kotlin.

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Either  As its name implies, Either uses either of the two values.  By convention, right hand side of Either holds successful values and the left side holds the exceptional case.

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Extraction of values of Either

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Using getOrElse with Either getOrElse allows us to provide a default value for the left-side.

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Use case for Either Displaying Validation Messages: A form contains valid patterns for email, phone, name. The input entered by the user can be either valid or invalid.

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Try  Try checks if a particular value exists and then routes towards success or failure.  A Try instance where the operation has been successful, which is represented as Success  a Try instance where the computation has failed is represented with a Throwable, which represents Failure.

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Extracting value of Try We can extract Try inner values using ‘when’ clause.

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Using recover Recover allows us to handle the exception if it’s a Failure.

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Use case for Try Catching exceptions- User pass two values to the function for divide. Either the function would return the result or throw an exception.

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Type Classes  Functor  Monad  Applicative

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Functor Allow us to apply a function to a wrapped value using map. Here, ‘f’ is a function transforming the wrapped value. And map() returns the transformed value wrapped in same context. Present at: Arrow.typeclasses.Functor

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How Functor works? fmap :: (a -> b) -> f(a) -> f(b) fmap takes a Function And a Functor And returns a new Functor

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How Functor works? Continued… 2 (+3) 2 5 5 Unwrap value from Context. Apply Function. Rewrap value in Context. No value. (+3) Don’t apply function. End up with Nothing.

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Functor applied to Array (+3) (+3) (+3) 2 4 6 5 7 9 Apply the function to each value.

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Functor applied to a function Source: https://hackernoon.com/kotlin-functors- applicatives-and-monads-in-pictures-part-1-3- c47a1b1ce251

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Polymorphism with Functor

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Applicative Allow you to apply a wrapped function to a wrapped value using apply.

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How Applicative works? (+3) <*> 2 5 Function wrapped in a Context Value in a Context (+3) 2 Unwrap both and apply the function to the value New value in a context

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Applying Applicative to an array of values Source: https://hackernoon.com/kotlin-functors-applicatives-and- monads-in-pictures-part-2-3-f99a09efd1ec

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Applicative Builder Preserves Type Information when computing over different Option types. Callback is invoked when all values are completed, if all values are Some.

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Monads Monads apply a wrapped value to a function that returns a wrapped value using flatMap.

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How does Monad work? (>>=) :: ma -> (a -> mb) -> mb >>= takes a Monad And a function that returns a Monad And it returns a Monad

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Monad Chaining

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Monad Binding

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An interesting relation Functor Applicative Monad

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More Functional Programming Concepts  Function composition  Memoization  Pipes  Logical Complement

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Function Composition  Process of building a function using existing functions.  Compose is an infix function.  Compose takes the value from the function on the right, applies it to the function on the left and continues to pass it to the left.

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Piping  Reverse of ‘compose’.  Takes a value from the function on the left and sequentially applies it to the function on the right.

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Logical Component  Takes any function that returns a Boolean type and negates it.

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Memoization  A technique to cache results of pure functions.  A function that is memorized behaves as a normal function, but stores the result of previous computations associated with the parameters supplied to produce that result.

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Integration of Arrow with Android Studio project Add dependencies

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Calculator app using Arrows FP constructs

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Calculator app using Arrows FP constructs

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Modelling division as Try When Divisor is zero When Divisor is non-zero

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Form validation using Arrow

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Modelling data with Either When no input is provided When input is provided

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Best practices when using Arrow for Android  Push side effects to a single point on your system to keep the rest of the architecture completely pure.  Be clear on the concepts of different data types and their differences.  Be clear on the concepts of different type classes and incorporate them in your computations accordingly.

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Projects using Arrow  raulh82vlc/FunctionalKotlin  dcampogiani/AndroidFunctionalValidation  47deg/helios You can check the projects that are being done/have being done using Arrow here: https://arrow-kt.io/docs/quickstart/projects/

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Are you READY to try out Arrow? THANK YOU For queries, please reach out to me at [email protected]