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Functional Programming in Dart A practical approach

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Course of talk • Functions & Variables • First class & Higher order functions • Lambdas • Code quality and quantity • List APIs

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Functions & Variables

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Properties of functions • Purity • Only depends on data provided to it • add(a, b) => a + b; //Pure • int a;
 add(b) => a + b; //Impure

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Properties of functions • Referential transparency • A function always returns the same value on same input, no matter when you call it • add(a,b) => a + b; //transparent • int a;
 add(b) => a + b; //not transparent • a = 5, add(5) => 10; a = 10, add(5) => 15 • Facilitated by purity of functions

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Properties of variables • Immutable, final, const. • Functional Programming principles are language independent • All math functions are referentially transparent, i.e. Math.sin(s), Math.sqrt(x), Math.max(a,b).

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Why, bruh? • Why should variables be immutable? • No race conditions. • Less cognitive load. • Why should functions be pure? • Concurrency and parallel evaluation. • Why should functions be referentially transparent? • Lazy evaluation.

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First class & Higher order functions

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First class functions • What does it mean to be first class? • First class = type system can make sense of it. • Imperative: a = 5; a = "string"; a = new Object(); • Functional: var f = (a) => (a + 1) • Type: (int) -> int • In general: (parameter_types) -> return_type

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Higher order functions • Simply put, it is a function which takes function as argument and/or returns a function. • apply(Function f, int a) { return f(a); } • Function createAdder(int a) { return (b) => a + b; } • Function chaining: createAdder(10)(20)

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Lambdas

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Lambdas • OOP => Inheritance, top-down approach • Functional => composition, bottom-up approach • Naming is one of the biggest problems in programming • Lambda = anonymous function • (param1, param2, ...) => single_statement • (param1, param2, ...) { //multiple statements } • var adder = (a, b) => a + b;

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Code quality and quantity

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Code quality and quantity • Functional APIs help you write less code • Not laziness, but conciseness • More code = More chances of error • APIs = tried, tested and stable code • "Best code is no code at all" - Unknown • "We use code like violence. if it doesn't work, we use more of it" - Venkat Subramaniam

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–Albert Einstein “e = (mc) ” 2

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–Albert Einstein “errors = (more code) ” 2

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FP APIs for Iterable

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removeWhere List list = List.generate(10, (i) => i); for (int i in list) { if (i % 2 == 0) list.remove(i); }

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removeWhere List list = List.generate(10, (i) => i); for (int i in list) { if (i % 2 == 0) list.remove(i); }

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removeWhere List list = List.generate(10, (i) => i); for (int i in list) { if (i % 2 == 0) list.remove(i); }

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removeWhere List list = List.generate(10, (i) => i); for (int i in list) { if (i % 2 == 0) list.remove(i); }

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removeWhere List list = List.generate(10, (i) => i); for (int i in list) { if (i % 2 == 0) list.remove(i); }

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removeWhere List list = List.generate(10, (i) => i);
 
 list.removeWhere((f) => f % 2 == 0);

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); bool isFound = false; for (int i in list) { if (i % 2 == 0) { isFound = true; break; } } if (isFound) print("Found");

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any List list = List.generate(10, (i) => i); list.any((i) => i % 2 == 0) ? print("Found") : print(“Not Found”);

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int index = -1; for (int i in list) { if (i % 2 == 0) { index = i; break; } } if (index != -1) print("Found ${list[index].toString()}");

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firstWhere List list = List.generate(10, (i) => i); int element = list.firstWhere((i) => i % 2 == 0, orElse: () => -1); if(element != -1) { print("Found $element"); }

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); bool isAllA = true; for (String follower in followers) { if (!(follower.startsWith("a"))) isAllA = false; } if (isAllA) print("Good to go"); else print("Uh oh");

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every List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); if(followers.every((s) => s.startsWith("a"))) print("Good to go"); else print("Uh oh");

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z")); StringBuffer buffer = StringBuffer(); for (int i = 0; i < followers.length; i++) { if (i == followers.length - 1) buffer.write(followers[i]); else buffer.write(followers[i] + ", "); } print(buffer.toString());

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join List followers = List.generate(10, (i) => "a".padLeft((i % 2) * 5, "z") print(followers.join(", "));

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { double sum = 0; for (Employee e in employees) { sum += e.salary; } return sum / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { return employees.fold(0, (s, e) => s + e.salary) / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { return employees.fold(0, (s, e) => s + e.salary) / employees.length; }

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fold List employees = List.generate( 10, (i) => new Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(averageSalary(employees)); double averageSalary(List employees) { return employees.fold(0, (s, e) => s + e.salary) / employees.length; }

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employees.fold(0, (s, e) => s + e.salary) Employee { name: a salary: 0 } Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } 0

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employees.fold(0, (s, e) => s + e.salary) Employee { name: a salary: 0 } Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } 0

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employees.fold(0, (s, e) => s + e.salary) Employee { name: a salary: 0 } Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } 0

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employees.fold(0, (s, e) => s + e.salary) Employee { name: a salary: 0 } Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } 0

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employees.fold(0, (s, e) => s + e.salary) Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } Employee { name: azzz salary: 30 } 0

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employees.fold(0, (s, e) => s + e.salary) Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } Employee { name: azzz salary: 30 } 0

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1000 employees.fold(0, (s, e) => s + e.salary) Employee { name: az salary: 1000 } Employee { name: azz salary: 2000 } Employee { name: azzz salary: 30 }

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1000 employees.fold(0, (s, e) => s + e.salary) Employee { name: azz salary: 2000 } Employee { name: azzz salary: 3000 } Employee { name: azzz salary: 40 }

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1000 employees.fold(0, (s, e) => s + e.salary) Employee { name: azz salary: 2000 } Employee { name: azzz salary: 3000 } Employee { name: azzz salary: 40 }

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3000 employees.fold(0, (s, e) => s + e.salary) Employee { name: azz salary: 2000 } Employee { name: azzz salary: 3000 } Employee { name: azzz salary: 40 }

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3000 employees.fold(0, (s, e) => s + e.salary) Employee { name: azzz salary: 3000 } Employee { name: azzzz salary: 4000 } Employee { name: azzz salary: 50 }

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Fold as (almost) universal function • employees.max()
 employees.fold(0, (s, e) => (e.salary > s) ? e.salary : s) • employees.any(predicate)
 employees.fold(false, (s, e) => 
 (s == false) ? predicate(e) : true) • employees.every(predicate)
 employees.fold(true, (s, e) =>
 (s == true) ? predicate(e) : false)

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { List names = List(); for (Employee e in employees) { names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase())); } return names; }

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map List employees = List.generate(10, (i) => Employee("a".padRight(i + 1, "z"), i * 1000)); employees.shuffle(); print(capitalize(employees)); List capitalize(List employees) { return employees .map((e) => e.name.replaceRange(0, 1, e.name[0].toUpperCase())).toList(); }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); for (int number in numbers) { if (number % 2 == 1) { print("$number is odd"); } } for (int number in numbers) { if (number % 2 == 0) { print("$number is even"); } }

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where List numbers = List.generate(10, (i) => i); numbers .where((number) => number % 2 == 1) .forEach((number) => print("$number is odd")); numbers .where((number) => number % 2 == 0) .forEach((number) => print("$number is even"));

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More Functional APIs • https://api.dartlang.org/stable/2.2.0/dart- core/Iterable-class.html#instance-methods • https://api.dartlang.org/stable/2.2.0/dart- async/Stream-class.html#instance-methods

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A Combined Example

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words.txt art bread dartisan mart data dartium earth datamart eggs state of the art dart spinach dart rider part

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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main.dart File('words.txt') .readAsStringSync() .split('\n') .where((word) => word.contains('dart')) .map((word) => '${String.fromCharCodes(Runes('\u2764'))} -> $word') .forEach((word) => print(word));

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Output ❤ -> dartisan ❤ -> dartium ❤ -> state of the art dart ❤ -> dart

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Thank You! Questions?

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• github.com/samvidmistry • github.com/piedcipher • twitter.com/@mistrysamvid • twitter.com/@piedcipher • medium.com/@samvidmistry • medium.com/@piedcipher