Functional Programming in Dart - A practical approach

Transcript

Functional Programming
in Dart
A practical approach

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
if (i % 2 == 0)
list.remove(i);
}

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removeWhere
if (i % 2 == 0)
list.remove(i);
}

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removeWhere
if (i % 2 == 0)
list.remove(i);
}

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removeWhere
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
bool isFound = false;
if (i % 2 == 0) {
isFound = true;
break;
}
}
if (isFound) print("Found");

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any
if (i % 2 == 0) {
isFound = true;
break;
}
}

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any
if (i % 2 == 0) {
isFound = true;
break;
}
}

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any
if (i % 2 == 0) {
isFound = true;
break;
}
}

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any
if (i % 2 == 0) {
isFound = true;
break;
}
}

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any
if (i % 2 == 0) {
isFound = true;
break;
}
}

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

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}
if (index != -1) print("Found ${list[index].toString()}");

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}

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firstWhere
int index = -1;
if (i % 2 == 0) {
index = i;
break;
}
}

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firstWhere
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 =
bool isAllA = true;
}

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every
List followers =
bool isAllA = true;
}

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every
List followers =
bool isAllA = true;
}

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every
List followers =
bool isAllA = true;
}

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every
List followers =
bool isAllA = true;
}

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every
List followers =
if(followers.every((s) => s.startsWith("a")))
print("Good to go");
else
print("Uh oh");

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join
List followers =
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 =
}

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join
List followers =
}

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join
List followers =
}

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join
List followers =
}

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join
List followers =
}

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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
double sum = 0;
sum += e.salary;
}
}

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fold
double sum = 0;
sum += e.salary;
}
}

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fold
double sum = 0;
sum += e.salary;
}
}

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fold
double sum = 0;
sum += e.salary;
}
}

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fold
double sum = 0;
sum += e.salary;
}
}

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fold
double sum = 0;
sum += e.salary;
}
}

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fold
return employees.fold(0, (s, e) => s + e.salary) / employees.length;
}

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fold
}

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fold
}

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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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Employee {
name: a
salary: 0
}
Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
0

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Employee {
name: a
salary: 0
}
Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
0

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Employee {
name: a
salary: 0
}
Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
0

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Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 30
}
0

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Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 30
}
0

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1000
Employee {
name: az
salary: 1000
}
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 30
}

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1000
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 3000
}
Employee {
name: azzz
salary: 40
}

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1000
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 3000
}
Employee {
name: azzz
salary: 40
}

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3000
Employee {
name: azz
salary: 2000
}
Employee {
name: azzz
salary: 3000
}
Employee {
name: azzz
salary: 40
}

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3000
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));
print(capitalize(employees));
List capitalize(List employees) {
List names = List();
names.add(e.name.replaceRange(0, 1, e.name[0].toUpperCase()));
}
return names;
}

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