Object Oriented Functional Programming — Saul Mora

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Object Oriented Functional Programming Saul Mora [email protected] @casademora

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Functional Object Oriented Programming Saul Mora [email protected] @casademora

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Programming Oriented Functional Object Saul Mora [email protected] @casademora

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Objects

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C++ Objective C C# Java Ruby Python Javascript PHP

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Scala Clojure Swift

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Will it blend

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What a Functional Language is NOT

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Statically Typed

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Generics

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Overloaded Operators

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Optionals

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A language where functions are value types

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Objective C is a Functional Language

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F(x) = y

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f(x: Int) -> Int

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map

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let numbers = [1, 2, 3, 4, 5] var calculatedNumbers: [Int] = [] for value in numbers { calculatedNumbers.append(value * 2) }

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let numbers = [1, 2, 3, 4, 5] let calculatedNumbers = map(numbers) { $0 * 2 }

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Declarative vs. Imperative

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What vs. How

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Deep Dive

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func old_and_busted_expired(fileURL: NSURL) -> Bool

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func old_and_busted_expired(fileURL: NSURL) -> Bool { let fileManager = NSFileManager() if let filePath = fileURL.path { if fileManager.fileExistsAtPath(filePath) { var error: NSError? let fileAttributes = fileManager.attributesOfItemAtPath(filePath, error: &error) if let fileAttributes = fileAttributes { if let creationDate = fileAttributes[NSFileModificationDate] as? NSDate { return creationDate.isBefore(NSDate.oneDayAgo()) } } else { NSLog("No file attributes \(filePath)") } } } return false }

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let fileManager = NSFileManager() if let filePath = fileURL.path { if fileManager.fileExistsAtPath(filePath) { var error: NSError? let fileAttributes = fileManager.attributesOfItemAtPath(filePath, error: &error) if let fileAttributes = fileAttributes { if let creationDate = fileAttributes[NSFileModificationDate] as? NSDate { return creationDate .isBefore(NSDate.oneDayAgo()) } } else { NSLog("No file attributes \(filePath)") } }

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unwrapping the file path check if the file exists extract and unwrap the file attributes extract, unwrap and cast the NSModifiedDateAttribute compute one day ago return comparison to file last modified date

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let filePath = fileURL.path

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let fileExists : (_) -> (_)

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let fileExists : (String) -> (String?)

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let fileExists : (String) -> (String?) = { path in fileManager.fileExistsAtPath(path) ? path : nil }

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let retrieveFileAttributes : (_) -> (_)

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let retrieveFileAttributes : (String) -> ([NSObject: AnyObject]?)

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let retrieveFileAttributes : (String) -> ([NSObject: AnyObject]?) = { path in var error : NSError? let attributes = fileManager.attributesOfItemAtPath(path, error: &error) if attributes == nil { NSLog("Unable to check \(filePath): \(error)") } return attributes }

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let extractCreationDate : (_) -> (_)

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let extractCreationDate : ([NSObject:AnyObject]) -> (NSDate?)

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let extractCreationDate : [NSObject:AnyObject] -> NSDate? = { $0[NSFileModificationDate] as? NSDate }

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let filePath: String? let fileExists: String -> String? let extractFileAttributes: String -> [NSObject:AnyObject]? let extractCreationDate: [NSObject:AnyObject] -> NSDate?

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Composition

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bind

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bind(A?, f(A) -> B?) -> B?

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func bind(a: A?, f: A -> B?) -> B? { if let x = a { return f(x) } else { return .None } }

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bind(filePath, fileExists)

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bind(bind(filePath, fileExists), retrieveFileAttributes)

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bind(bind(bind(filePath, fileExists), retrieveFileAttributes), extractCreationDate)

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let creationDate: NSDate? = bind(bind(bind(filePath, fileExists), retrieveFileAttributes), extractCreationDate)

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>>= operator

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func >>=(a: A?, f: A -> B?) -> B? { return bind(a, f) }

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bind(filePath, fileExists)

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filePath >>= fileExists

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if let creationDate = filePath >>= fileExists >>= retrieveFileAttributes >>= extractCreationDate { return creationDate.isBefore(NSDate.oneDayAgo()) }

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let checkExpired: NSDate -> Bool? = { $0.isBefore(NSDate.oneDayAgo()) }

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return filePath >>= fileExists >>= retrieveFileAttributes >>= extractCreationDate >>= checkExpired ?? false

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func expired(fileURL: NSURL) -> Bool { let fileManager = NSFileManager() var error : NSError? let filePath = fileURL.path let fileExists : (String) -> (String?) = { path in fileManager.fileExistsAtPath(path) ? path : nil } let retrieveFileAttributes : (String) -> ([NSObject: AnyObject]?) = { path in var error : NSError? return fileManager.attributesOfItemAtPath(path, error: &error) } let extractCreationDate : ([NSObject:AnyObject]) -> NSDate? = { $0[NSFileModificationDate] as? NSDate } let checkExpired: NSDate -> Bool? = { $0.isBefore(NSDate.oneDayAgo()) } return filePath >>= fileExists >>= retrieveFileAttributes >>= extractCreationDate >>= checkExpired ?? false }

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Railway Oriented Programming fsharpforfunandproﬁt.com/posts/recipe-part2/

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Readability

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bind flatMap (fmap) apply >>= >>- <^> <$> <*>

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func <^>(f: A -> B, a: A?) -> B? { return fmap(f, a) } func fmap(f: A -> B, a: A?) -> B? { if let x = a { return f(x) } else { return .None } }

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func <*>(f: (A -> B)?, a: A?) -> B? { return apply(f, a) } func apply(f: (A -> B)?, a: A?) -> B? { if let x = a, let fx = f { return fx(x) } return .None }

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Argo http://github.com/thoughtbot/Argo

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What about objects?

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Objects > Functions

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Objects ≈ Functions

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Caching

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retrieve

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Data Client RemoteDataSource LocalDataSource

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Data Client RemoteDataSource LocalDataSource

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Data Client RemoteDataSource LocalDataSource DataSourceCache

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Data Client RemoteDataSource LocalDataSource DataSourceCache Cache Strategy

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Strategy

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protocol DataSource { func fetchItem() -> Int? } func retrieve(from: S, or: S) -> T? { return from.fetchItem() ?? or.fetchItem() }

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protocol DataSource { func fetchItem() -> Int? } func retrieveItem(from: S, or: S) -> T? { return from.fetchItem() ?? or.fetchItem() }

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from.fetchItem()

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from .fetchItem() Object Instance Selector

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func retrieve (from: S, or: S, using: S -> T?) -> T? { return using(from) ?? using(or) }

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let localDataSource: DataSource let remoteDataSource: DataSource return retrieve(from: localDataSource, or: remoteDataSource) { $0.fetchItem() }

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protocol DataSourceCacheStrategy { func retrieve(from: [DataSource], using: DataSource -> T?) -> T? } struct DataSourceCache { let strategy: DataSourceCacheStrategy let localDataSource: DataSource let remoteDataSource: DataSource func fetch(applyFunction: DataSource -> T?) -> T? { let sources = [localDataSource, remoteDataSource] return strategy.retrieve(sources, using: applyFunction) } }

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struct BasicDataSourceCacheStrategy: DataSourceCacheStrategy { func retrieve(from: [DataSource], using: DataSource -> T?) -> T? { for dataSource in from { if let result = using(dataSource) { return result } } return nil } }

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struct MoreFancyDataSourceCacheStrategy: DataSourceCacheStrategy { func retrieve(from: [DataSource], using: DataSource -> T?) -> T?{ let cacheValidation: T? -> Bool = { $0 != nil } for dataSource in from { let result = using(dataSource) if cacheValidation(result) { return result } } return nil } }

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Encapsulation

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Abstraction

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SOLID

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Object Oriented in the Large, Functional in the Small

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Stuff that changes vs. Stuff that doesn’t

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Functional Friendly Problem Domains

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- Alan Kay, The Early History of Smalltalk I was less interested in programs as algebraic patterns than I was in a clear scheme that could handle a variety of styles of programming.

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Object Oriented Programming Functional Programming

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Programming Object Oriented Programming Functional Programming

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–Alan Kay Programming is at heart a practical art in which real things are built, and a real implementation thus has to exist.

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Object Oriented Functional Programming Saul Mora [email protected] @casademora