From OOP to FP

Object-oriented programming makes code understandable by encapsulating moving parts. Functional programming makes code understandable by minimizing moving parts.

-- Michael Feathers

Functions

• Math: mapping inputs to outputs
• Programming: decompose and reuse

Functional programming

• Referential transparency
• Functions as first-class

Referential transparency

val a = <expr>
List(a, a)

List(<expr>, <expr>)

Referential transparency

val a = 1 + 2
List(a, a)

List(1 + 2, 1 + 2)

Referential transparency

val a = println("TMTOWTDI")
List(a, a)

List(println("TMTOWTDI"), println("TMTOWTDI"))

Referential transparency

val a = i += 1
List(a, a)

List(i += 1, i += 1)

Referential transparency

val a = ???
List(a, a)

List(???, ???)

Referential transparency

val a = new MyObject
List(a, a)

List(new MyObject, new MyObject)

Pure functional programming

• Total: an output for every input.
• Deterministic: Same output for the same input.
• Pure: only effect is computing the output.

Life without mutation

• Recursion
• Persistent data structures
• Local mutation

Recursion

Tail recursion

• @tailrec
• accumulators

Immutability

Classes should be immutable unless there's a very good reason to make them mutable

-- Joshua Bloch, Effective Java

Immutable objects are simple. Immutable objects are also safer. Immutable objects are always threads-safe.

-- Brian Goetz, Java Concurrency in Practice

Persistent data structures

Implement a tail-recursive function

• size: takes a list, returns size
• concat: takes 2 lists, returns concatenated list
• reverse: takes list, returns reversed list

Functions

• Local functions
• Function literals

Higher-order functions

Combinators are arguably the most reusable constructs we have in programming

-- Dean Wampler

• filter
• map
• foldLeft

filter

map

Syntax sugar

Additional resources

• Scastie
• Scala Fiddle
• API Docs
• Cube composer