Functional Programming Concepts and Ruby

Adrian Francke (@adrian2112)

Why I started to learn functional programming1. Interesting and useful concepts for general software

development 2. For fun

Structure and Interpretation of Computer Programs (SICP) http://en.wikipedia.org/wiki/Structure_and_Interpretation_of_Computer_Programs

1. Interesting and useful concepts for general software development

2. For fun Structure and Interpretation of Computer Programs (SICP) http://en.wikipedia.org/wiki/Structure_and_Interpretation_of_Computer_Programs

(3. Hipster)

Why I started to learn functional programming

Goal of this talk1. Show some functional programming concepts which

can be used in your day to day programming in a non-functional programming language

2. Get you interested in functional programming

What is functional programming? Declarative vs Imperative

Functional Programming is a kind of declarative programming

Imperative “Describes computation in terms of statements that change a program state (...) describing how a program operates.”

http://en.wikipedia.org/wiki/Imperative_programming

Declarative “A style of building the structure and elements of computer programs, that expresses the logic of a computation without describing its control flow. (...) describing what the program should accomplish (...), rather than describing how”

http://en.wikipedia.org/wiki/Declarative_programming

ExamplesImperative Declarative

From: http://latentflip.com/imperative-vs-declarative/

ExamplesImperative Functional

Imperative: http://www.siafoo.net/snippet/306 Functional: http://en.literateprograms.org/Quicksort_%28Haskell%29

Functional Programming concepts: Recursion● Is a method where the solution to a

problem depends on solutions to smaller instances of the same problem

● Recursion, see Recursion

● Recursion is heavily used in functional programming as it is the canonical and often the only way to iterate

Image from: http://learnyouahaskell.com/

Functional Programming concepts: Recursion

Functional Programming concepts: Higher order functions

● Functions that take other functions as their arguments

● Functions that returns other functions

Functional Programming concepts: Higher order functions● Higher order function example

● accumulate from http://mitpress.mit.edu/sicp/full-text/book/

book-Z-H-15.html#%_sec_2.2.3

Functional Programming concepts: Purity

● Referential transparency ● Lazy evaluation

Functional Programming concepts: Referential transparency

● A program (or function) that has the same effects and output on the same input

● Allows the programmer and the compiler to reason about program behavior

● Try to make all functions referential transparent and keep the side effects in one place

Functional Programming concepts: Referential transparency

Useful

Useless

Unsafe (side effects everywhere)

Safe (Limited side effects)

• C • Java • … “normal” languages

• Haskell • (probably other pure funcional laguages)

https://www.youtube.com/watch?v=iSmkqocn0oQ

Future

Functional Programming concepts: Referential transparency

● A pure function can always be replaced by its result. ● There’s no difference between Math.cos(Math::PI)

and -1.0 ● A function which calls an impure function is impure as

well. Impurity is contagious

http://www.sitepoint.com/functional-programming-pure-functions/

Functional Programming concepts: Referential transparency

Example from: http://valve.github.io/blog/2014/07/04/from-object-to-functional-immutability/

Referential opaque Referential transparent

Functional Programming concepts: Referential transparency

● Easy to test (it does not depend on anything but its input)

● Easy maintenance ● Concurrency/parallelization

Functional Programming concepts: Referential transparency

Example from: http://www.sitepoint.com/functional-programming-pure-functions/

Same function. Only if it is a pure function

Functional Programming concepts: Lazy evaluation

● Pure functions can be performed at any time and still yield the same result.

● Defer the computation of values until they are needed

● Example

Which of this concepts I use in ruby and how● Higher order functions

o map o reduce o select o find

● Pure functions (referential transparency)

Use attr_reader instead of attr_accessor so once defined it cannot be changed

Keep side effects in one place

Use of higher order functions

This functions depend on self.date. Should I send the date as parameter?

Current stand = Lets take this functions as “curried” or partially applied with the objects date. self.date not modifiable

Conclusions● Learn new approaches to solve problems ● Try functional programing!! ● It will help you to create better code.

A language that doesn’t affect the way you think about programming, is not worth knowing. — Alan Perlis

Resources● Structure and Interpretation of Computer Programs (SICP) http://

mitpress.mit.edu/sicp/full-text/book/book-Z-H-38.html#%_index_start

● Programming with Nothing http://codon.com/programming-with-nothing

References● Haskell wiki https://wiki.haskell.org/Functional_programming

● Composing programs (Higher order functions) http://www.composingprograms.com/pages/16-higher-order-functions.html

● Learn You a Haskell for Great Good! http://learnyouahaskell.com

● http://www.sitepoint.com/functional-programming-pure-functions/