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@jleo3
The Functional Rubyist
A primer

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@jleo3
joe leo (lucy’s daddy)
twitter/github/linkedin: @jleo3

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@jleo3
def
method()
TOGETHER WE WILL BUILD SOMETHING GREAT

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@jleo3

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@jleo3
DAVID A BLACK

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@jleo3

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@jleo3
(RUBYISTS / FRIENDS)

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@jleo3

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@jleo3
a note on purity
Ruby
Java 7
PHP
Lisp

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@jleo3
a purely functional language...
...guarantees referential transparency
RUBY DOES NOT

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@jleo3
A purely functional
language...
...guarantees immutability
RUBY DOES NOT

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@jleo3
{ haskell bullies }

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@jleo3
side effects are necessary to do interesting things
● I/O
● raising exceptions
● outputting data to the terminal
● updating a record in a database
● anything else that changes state

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@jleo3
what do most people mean when they fp?
Remove side effects!
Curry! Generics! Lazy Evaluation!
Recurse! Tail-call optimize!

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@jleo3
where to begin?

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@jleo3
let ruby be your guide!

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@jleo3
side-effect-free ruby
● String.upcase
● Enumerable.map
● Array.filter

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@jleo3
bang! a side effect
● “my string”.upcase!
● [1, 2, 3, 4] << 5
● { x: “ruby” }.fetch(:y)

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@jleo3
an example
Student#calculate_grade
def calculate_grade(scores, student)
case scores.sum / scores.size
when 90..100
student.update_grade("A")
when 80...90
student.update_grade("B")
when 70...80
student.update_grade("C")
when 60...70
student.update_grade("D")
else
student.update_grade("F")
end
end
Student#calculate_grade
def calculate_grade(scores)
case scores.sum / scores.size
when 90..100
"A"
when 80...90
"B"
when 70...80
"C"
when 60...70
"D"
else
"F"
end
end

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@jleo3
what is currying?
breaking down…
● 1 function with many arguments…
● ...into many functions, each with 1
argument

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@jleo3
what is currying?
simple example: add
add = -> (a, b) { a + b } #(1 function, 2 arguments)
curried_add = -> (a) { -> (b){ a + b } } #(2 functions, each with 1 argument)

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@jleo3
what is partial function application?
● Pass in a number of arguments less than the function’s arity
● Result: a new function with all of the passed-in arguments
applied
add = -> (a, b) { a + b }
add = -> (5, b) { 5 + b } # partially applied; NOT VALID SYNTAX

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@jleo3
but I’m lazy! i don’t want to have to think about
all of that!
● You don’t have to!
● curry handles both currying and
partial function application.
● add.curry

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@jleo3
into action with curry and partial function
application
find_multiples = -> (x, arr) {
arr.select { |el| el % x == 0 }
}

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@jleo3
Unique IRL; Generic in code
● Functions that return functions
● Building blocks for more
specific functions
● Generics are to FP what
Objects are to OOP

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@jleo3
into action with curry and partial function
application
Generic:
find_multiples_of = find_multiples.curry
Individuated:
find_multiples_of_3 = find_multiples_of.call(3)
find_multiples_of_5 = find_multiples_of.call(5)

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@jleo3
thinking in streams
Find infinite multiples!
find_first_multiples = -> (num, mult)
{
(1..).lazy.select {
|x| x % mult == 0
}.first(num)
}

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@jleo3
recurse!
Writing recursive functions is all about determining the “terminal
clause.”
Popular recursive functions:
● Factorial (terminal clause: x <= 1)
● Fibonacci (terminal clause: x <= 1)
● Sum Squares (terminal clause: x == 0)

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@jleo3
VM, optimize thyself!
RubyVM::InstructionSequence.compile_option =
{
tailcall_optimization: true,
trace_instruction: false
}
demystifying tail-call optimization

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@jleo3
demystifying tail-call optimization
def factorial(x)
return x if x == 2
x * factorial(x - 1)
end
def factorial(x, acc=1)
return acc if x <= 1
factorial(x - 1, x * acc)
end
(RECURSIVE) (TAIL RECURSIVE)

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@jleo3
Where can I learn more?
You guessed it!

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@jleo3
Where can I learn more?
But also...

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@jleo3
Thank you! Also, get @ me!
- joe leo
- twitter/github/linkedin: @jleo3
- defmethod.com
Thank you to…
- everyone at Rubyconf
- Matz
- Abby and the Program
Committee
- David A. Black