FUNCTIONAL RUBYNOT THE RUBY YOU'RE USED TO... OR IS IT?

Dave Hackett / @tallguy_hackett

WHAT IS FUNCTIONAL PROGRAMMING?Funtional Programming (FP) is a style of building the structure

and elements of computer programs, that treats computation asthe evaluation of mathematical functions and avoids state andmutable data. It is a declarative programming paradigm, which

means programming is done with expressions. In functional code,the output value of a function depends only on the arguments

that are input to the function, so calling a function f twice with thesame value for an argument x will produce the same result f(x)

both times. —Wikipedia

TL;DRFuntional Programming is like math equations.

pinterest

IT'S NOT THAT BADThere's just a few things to remember.

IMMUTABILITYValues are values are values. They don't change.

PERSISTENCEValues aren't forgotten, overwritten, or removed over time.

EXPRESSIONSFP is declarative not imperative.

EXAMPLES IN CLOJURE(doseq [e [1 2 3]] (println e))

; => 1; => 2; => 3

(def orig [1 2 3])

(def added (map inc orig))

; => orig == [1 2 3]; => added == [2 3 4]

(reduce + [1 2 3])

; => 6

DOES RUBY HAVE SOME OF THIS ALREADY?Yes... kind of... sometimes.

CHANCES ARE YOU'RE ALREADY APPLYINGSOME FP-ISH TECHNIQUES

ENUMERABLE MODULE

FP IN RUBY![1, 2, 3].each do |e| puts eend

# => 1# => 2# => 3

orig = [1, 2, 3]added = orig.map do |e| e + 1end

# => orig == [1, 2, 3]# => added == [2, 3, 4]

[1, 2, 3].reduce(:+)

# => 6

OOPS!orig = [1, 2, 3]added = orig.map! do |e| e + 1end

# => orig == [2, 3, 4]# => added == [2, 3, 4]

greeting = "hello world"

def end_with_a_bang(phrase) phrase << "!"end

puts end_with_a_bang(greeting)# => hello world!puts greeting# => hello world!puts end_with_a_bang(greeting)# => hello world!!puts greeting# => hello world!!

DOES RUBY HAVE IMMUTABLE VALUES?

WHY DOESN'T RUBY SUPPORT ++ OR --?

NUMBERS ARE IMMUTABLEEVERYTHING IS MESSAGE PASSING IN RUBY

explanation

SYMBOLS ARE IMMUTABLE

RANGES ARE IMMUTABLE

DATE AND TIME ARE IMMUTABLE

VALUES GEMrequire 'values'

Point = Values.new(:x, :y)p = Point.new(1, 2)p.x = 2# => NoMethodError

Values

WHAT ABOUT DATA STRUCTURES?

BUILT IN... EPIC FAIL!

HAMSTER GEMrequire 'hamster'

v = Hamster.vector(1, 2, 3)v1 = v.add(4)# => v == [1, 2, 3]# => v1 == [1, 2, 3, 4]

v2 = v << 5# => v == [1, 2, 3]# => v2 == [1, 2, 3, 5]

v3 = v.set 0, 5# => v == [1, 2, 3]# => v3 == [5, 2, 3]

hamster

EXPRESSION COMPOSITION

LIKE IN MATH...FP is composed of a bunch of functions. Complex functions are

functions composed of other functions.

MATHSo for instance is an example of function

composition. Another way to write the same thing is

f(g(h(x)))

(f ∘ g ∘ h)(x)The story is that the result of is fed as input to whose

result is then fed into for the final result

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

CODESo in code this [[...],[...],...][3].drop(2).first

There are 3 method calls (functions)

[3] occurs first

.drop(2) occurs next

.first occurs last

It makes sense to us that results are passed down the messagechain

EXERCISES4clojure

THE ENDBY DAVE HACKETT