FUNCTIONAL PROGRAMMING TECHNIQUES

IN REGULAR JAVASCRIPT

ABOUT ME

Pavel KlimenkovSenior Software Developer @ Geotab Inc.Oakville, ON, Canada

WHAT THIS TALK IS ABOUT

• Pick few ideas from functional programming

• Simplify as much as possible

• Apply in JavaScript

• PROFIT!

WHAT THIS TALK IS ABOUT

• Pick few ideas from functional programming

• Simplify as much as possible

• Apply in JavaScript

• PROFIT!

WHAT THIS TALK IS ABOUT

• Pick few ideas from functional programming

• Simplify as much as possible

• Apply in JavaScript

• PROFIT!

WHAT THIS TALK IS ABOUT

• Pick few ideas from functional programming

• Simplify as much as possible

• Apply in JavaScript

• PROFIT!

WHAT THIS TALK ISN’T ABOUT

• OOP must die, FP will save the world

• Detailed FP manual

• “A monad is just a monoid in the category of endofunctors”

WHAT THIS TALK ISN’T ABOUT

• OOP must die, FP will save the world

• Detailed FP manual

• “A monad is just a monoid in the category of endofunctors”

WHAT THIS TALK ISN’T ABOUT

• OOP must die, FP will save the world

• Detailed FP manual

• “A monad is just a monoid in the category of endofunctors”

AGENDA•Immutability

•Simple functions

•Not that simple functions

•Combining OOP and FP

•Functors

•Monads

WHAT IS FP?

It’s a declarative programming style where all functions have no side effects and data is readonly.

BTW, SQL is also declarative

1. IMMUTABILITYObjects don’t change their state

Values are constants

IMMUTABILITY

• Use only read-only data

• If something needs to be changed - create new instance instead

EXAMPLE

Convert GPS coordinates to screen points

EXAMPLE: SOURCE DATA

EXAMPLE: CONVERSION

EXAMPLE: MUTABLE DATA

EXAMPLE: IMMUTABLE DATA

EXAMPLE

Edit Entity

EXAMPLE: ENTITY CONSTRUCTOR

EXAMPLE: MUTABLE DATA

EXAMPLE: IMMUTABLE DATA

HOW TO REDUCE TEMPTATION

• Use const or let instead of var

• filter/map/reduce instead of for

• Object.freeze - to prevent object mutation

IMMUTABILITY: PROS• Simpler code

• Less bugs

• Atomic object construction (object is either fully initialised, or doesn’t exist)

• Less temporal coupling (when order of initialisation matters)

• Simpler caching

• Thread safety*

IMMUTABILITY: CONS

• Higher CPU impact (more things to do)

• Higher memory impact (and more GC)

• Higher brain impact (OOP often comes with state)

2. SIMPLE FUNCTIONSFirst class functions

Pure functions

FIRST CLASS FUNCTIONS

Can be passed as arguments

Can be assigned as values

PURE FUNCTIONS• No side-effects

• Depend on arguments only (deterministic)

EXAMPLE: SIDE-EFFECTS

EXAMPLE: NONDETERMINISTIC FUNCTION

EXAMPLE: DETERMINISTIC FUNCTION

SIMPLE FUNCTIONS: PROS• No side-effects

• Less bugs

• Simpler testing

• Simpler code

• Absolute thread safety

SIMPLE FUNCTIONS: CONS

• No side-effects

• Our job is to create side-effects

• IO and Cache are side-effects

3. NOT THAT SIMPLE FUNCTIONSFunctions composition

Functions currying

FUNCTIONS COMPOSITION

Composition - gluing functions together

EXAMPLE: COMPOSITION

EXAMPLE: COMPOSITION

EXAMPLE: COMPOSITION

Two temporary arrays. Oh, the horror!

EXAMPLE: COMPOSITION

One temporary array. Harmony restored.

EXAMPLE: MORE COMPOSITION

EXAMPLE: MORE COMPOSITION

FUNCTIONS CURRYING

Currying - partial application of a function.

EXAMPLE: CURRYING

EXAMPLE: CURRYING

EXAMPLE: CURRYING

EXAMPLE: CURRYING

EXAMPLE: CURRYING

EXAMPLE: CURRYING

NOT THAT SIMPLE FUNCTIONS: PROS• Flexibility in using and reusing functions

• Functions tend to be smaller

• Memoization (“Simple functions” PROS list was already too long)

NOT THAT SIMPLE FUNCTIONS: CONS• Sometimes code looks cryptic

• Might introduce negative performance impact

COMBINING OOP AND FPShell - OOP, IO, side-effects

Core - FP, logic, pure functions

Core

Shell

MAIN PRINCIPLES• Imperative shell

• Deals with IO

• Thin layer

MAIN PRINCIPLES• Functional core

• deals with immutable data and pure functions

• contains application logic

EXAMPLE: MOVING VEHICLES

EXAMPLE: MOVING VEHICLES

IO, shell

EXAMPLE: MOVING VEHICLES

Logic, pure functions,

core

SHELL/CORE: PROS

• Main bugs will come from the shell, which is small

• Core is easy to test. Unit tests should be enough

• Shell is the one that needs integration tests (shell is small!)

• Command query responsibility segregation out of the box

SHELL/CORE: CONS• Anything?

FUNCTORSSmart containers for values

EXAMPLE: INCREMENTING A PICTURE

EXAMPLE: INCREMENTING A PICTURE

EXAMPLE: INCREMENTING A PICTURE

EXAMPLE: INCREMENTING A PICTURE

EXAMPLE: INCREMENTING A PICTURE

??? 3.png ???

FUNCTOR• Is a container around some data,

• that knows how to apply functions to that data,

• keeping result in the container

FUNCTOR• Container - array, async, nullable - anything

• Some data - numbers, objects, other functors - anything

• Functions are applied through map()

ARRAY IS A FUNCTOR!

PROMISE IS ALMOST A FUNCTOR!

BACK TO EXAMPLE

.png

USEFUL FUNCTORS• Maybe

• Either

• IO

• Writer

• State

MAYBE - ALMOST LIKE NULLABLE

MAYBE - ALMOST LIKE NULLABLE*

MAYBE - ALMOST LIKE NULLABLE

MAYBE EXAMPLE

MAYBE EXAMPLE

EITHER = LEFT ♥ RIGHT

EITHER = LEFT ♥ RIGHT

EITHER = LEFT ♥ RIGHT

EITHER = LEFT ♥ RIGHT

EITHER = LEFT ♥ RIGHT

EITHER = LEFT ♥ RIGHT

EITHER ♥ RAILWAY ORIENTED

PROGRAMMING

Right

Left

f(x) g(x) h(x)

Picture and concept are shamelessly borrowed from here

TO BE HONEST…

Traditional Maybe = Just ♥ Nothing

OTHER FUNCTORS

• IO - for… IO

• Writer - for logs

• State - for… state

FUNCTORS: PROS• Convenient control over computation

• Logic and context separation

FUNCTORS: CONS• Steep learning curve

• Temptation to use functors for everything

SKIPPED CHAPTERS• Applicatives

• Monoids

MONADS

MONADS ARE CURSED: EITHER YOU DON’T GET THEM, OR YOU CANT EXPLAIN THEM

EXAMPLE: NESTED ARRAYS

EXAMPLE: NESTED MAYBE

EXAMPLE: NESTED MAYBE

MAYBE WILL BECOME A MONAD IF

MAYBE WILL BECOME A MONAD IF

MAYBE WILL BECOME A MONAD IF

NOW YOU KNOW KUNG FU MONADS

Whoa!

ARRAY WILL BECOME A MONAD IF

ECMAScript gets flatten or flatMap for arrays (soon)

BTW, PROMISE IS ALMOST A MONAD

SUMMARY• Immutability - everything is read-only, less bugs

• Pure functions - no side-effects, less bugs

• Composition, currying - reusing functions

• Functors - separating context and logic

• Monads - smart functors

THANK YOU!

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/pasha.klimenkov

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Questions?