Thinking Functionally in Ruby

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THINKING FUNCTIONALLY IN RUBY. @tomstuart

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Functional programming is a pretty neat idea. 1:

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Enumerable contains some useful methods. 2:

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I hope these things are connected.

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(Functional programming is a pretty neat idea) 1.

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What is functional programming?

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A programming style.

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Two broad families of languages:

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1. Lisp-like •Scheme •Common Lisp •Dylan •Clojure •Dynamically typed •Homoiconic (code is data) •Lists are fundamental

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2. ML-like •Standard ML •Haskell •OCaml •Scala (sort of) •Statically typed (+ reconstruction) •Code is not data •ADTs and pattern matching

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Functions as values

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What is a function?

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1 7 2 3 6 4 5 4 2 6 8 3 7 1

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Functions as values

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“First-class functions” Implies: higher-order functions “closures”, “lambdas”, “anonymous functions”

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No side effects

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Values are immutable All functions do is return their result No state No I/O

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(In reality, different languages accomplish this to varying degrees.)

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Implies: recursion Implies: persistent data structures

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So what?

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Unfortunately...

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Declarative programming is counterintuitive when youʼre accustomed to imperative programming WHAT! HOW!

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Copying is expensive

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But!

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Referential transparency “an expression can be replaced with its value” • Good for efﬁciency • caching/memoisation/inlining • ultimately more efﬁcient • Good for programmer understanding • state is incredibly hard to deal with • ultimately more intuitive • Good for code reuse and testing

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Highly expressive Deeply satisfying Strongly compositional

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Future-proof •Mooreʼs Law is running out of steam •similar transistor density, more cores •The futureʼs concurrent •Concurrent access to mutable state is hard... •but not if you donʼt have mutable state! •Parallelising an algorithm is hard... •but itʼs easier if your code isnʼt overspeciﬁed

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OK seriously: so what?

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Ruby can do some of this. Not really a functional language, but we can pretend and get some of the beneﬁts.

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Use function values lambda { |x| x + 1 } blocks, procs

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Consider treating your values as immutable State is rarely worth it. It will kill you in the end.

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http://clojure.org/state

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Use the functional-ﬂavoured parts of the standard library

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Be declarative. Think functionally. What, not how.

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2. (Enumerable contains some useful methods)

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Enumerable#zip

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[1, 2, 3, 4]. zip([5, 6, 7, 8]) [1, 2, 3, 4]. zip([5, 6, 7, 8])

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2 1 3 4 6 5 7 8

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2 1 3 4 6 5 7 8

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[ ] 2 1 3 4 6 5 7 8 [ ] , , , , [ ] [ ] [ ] , , ,

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[1, 2, 3, 4]. zip([5, 6, 7, 8], [9, 10, 11, 12])

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Enumerable#select (a.k.a. #find_all)

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{ |x| x.odd? }

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1 1 ?

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1 1 ? !

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1 1 ? ! 2 2 ?

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1 1 ? ! 2 2 ? "

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[1, 2, 3, 4]. select { |x| x.odd? }

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1 2 3 4 2 1 3 4 ? ? ? ?

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1 2 3 4 2 1 3 4 ? ? ? ? ! " " !

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1 3 1 3 ? ? ! ! 1 3

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1 3 [ ] ,

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Enumerable#partition

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[1, 2, 3, 4]. partition { |x| x.odd? }

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1 2 3 4 2 1 3 4 ? ? ? ?

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1 2 3 4 2 1 3 4 ? ? ? ? ! " " !

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1 ? ! 4 ? " 2 ? " 3 ? !

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1 ? ! 4 ? " 2 ? " 3 ? ! 1 3 2 4

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1 3 [ ] , 2 4 [ ] , , [ ]

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Enumerable#map (a.k.a. #collect)

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{ |x| x * 3 }

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2 2 6 ×3

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2 2 6 ×3

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[1, 2, 3, 4]. map { |x| x * 3 }

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1 2 3 4 2 1 3 4 6 3 9 12 ×3 ×3 ×3 ×3

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1 2 3 4 2 1 3 4 6 3 9 12 ×3 ×3 ×3 ×3

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6 3 9 12 [ ] , , ,

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Enumerable#inject (a.k.a. #reduce)

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{ |x, y| x + y }

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3 3 5 5 8 +

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3 3 5 5 8 +

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[1, 2, 3, 4]. inject(0) { |x,y| x+y } [1, 2, 3, 4]. inject(0) { |x,y| x+y }

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1 2 3 4 1 2 3 4 0 1 3 6 10 + + + +

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1 1 2 3 3 6 4 0 1 2 3 4 0 1 3 6 10 + + + +

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module Enumerable def inject(initial) result = initial for element in self result = yield(result, element) end result end end

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a.k.a. “left fold” (foldl, fold_left)

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1 2 3 4 0

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10 (((0 + 1) + 2) + 3) + 4

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...versus “right fold” (foldr, fold_right)

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1 2 3 4 0

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10 1 + (2 + (3 + (4 + 0)))

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The initial argument is optional...

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[1, 2, 3, 4]. inject { |x,y| x+y }

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[1, 2, 3, 4]. inject { |x,y| x+y } [2, 3, 4]. inject(1) { |x,y| x+y }

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...but only if the output is the same type as the input...

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>> ['El', 'rug']. inject(0) { |l,s| l + s.length } => 5 >> ['El', 'rug']. inject { |l,s| l + s.length } TypeError: can't convert Fixnum into String from (irb):1:in `+' from (irb):1 from (irb):1:in ìnject' from (irb):1:in èach' from (irb):1:in ìnject' from (irb):1

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...and itʼs meaningful to get nil when the collection is empty

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>> [].inject { |x,y| x+y } => nil >> [].inject(0) { |x,y| x+y } => 0 >> [].inject(1) { |x,y| x*y } => 1

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COMPOSE!

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[1, 2, 3, 4]. map { |x| x * 3 }. inject(0) { |x| x+y }

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1 2 3 4 2 1 3 4 6 3 9 12 ×3 ×3 ×3 ×3 0 0 3 9 18 30 + + + +

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3 6 9 12 1 2 3 4 2 1 3 4 6 3 9 12 ×3 ×3 ×3 ×3 3 9 18 0 0 3 9 18 30 + + + +

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I am so excited. What now?

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Review your Ruby code. Func it up.

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result = '' for name in names unless result.empty? result << ', ' end result << name end result

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result = '' for name in names unless result.empty? result << ', ' end result << name end result names.join(', ') !

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def count_mines_near(x, y) count = 0 for i in x-1..x+1 for j in y-1..y+1 count += 1 if mine_at?(i, j) end end count end

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def count_mines_near(x, y) count = 0 for i in x-1..x+1 for j in y-1..y+1 count += 1 if mine_at?(i, j) end end count end def count_mines_near(x, y) ((x-1..x+1).entries * 3).sort. zip((y-1..y+1).entries * 3). select { |x, y| mine_at?(x, y) }. length end !

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def count_mines_near(x, y) ((x-1..x+1).entries * 3).sort. zip((y-1..y+1).entries * 3). select { |x, y| mine_at?(x, y) }. length end [1, 2, 3, 1, 2, 3, 1, 2, 3] [1, 1, 1, 2, 2, 2, 3, 3, 3] [7, 8, 9, 7, 8, 9, 7, 8, 9] [[1, 7], [1, 8], [1, 9], [2, 7], [2, 8], [2, 9], [3, 7], [3, 8], [3, 9]] .sort = .zip( ) = # = (2, 8)

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def count_mines_near(x, y) ((x-500..x+500).entries * 1001).sort. zip((y-500..y+500).entries * 1001). select { |x, y| mine_at?(x, y) }. length end [ [1, 7], [1, 8], …, [1, 1007], [2, 7], [2, 8], …, [2, 1007], …, …, …, [1000, 7], [1000, 8], …, [1000, 1007], [1001, 7], [1000, 8], …, [1001, 1007]]

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[ [1, 7], [1, 8], …, [1, 1007], [2, 7], [2, 8], …, [2, 1007], …, …, …, [1000, 7], [1000, 8], …, [1000, 1007], [1001, 7], [1000, 8], …, [1001, 1007]] 173 96 121 78 237 705

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def numbers_near(n, radius) ((n - radius)..(n + radius)).entries end def squares_near(x, y, radius) diameter = (radius * 2) + 1 (numbers_near(x, radius) * diameter). sort. zip(numbers_near(y, radius) * diameter) end def count_mines_near(x, y, radius = 1) squares_near(x, y, radius). select { |x, y| mine_at?(x, y) }. length end

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Learn a functional language •OCaml •Scheme (Google “SICP”) •Clojure •Scala •Haskell? Erlang?

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Thanks! @tomstuart / tom@experthuman.com

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http://www.ﬂickr.com/photos/somemixedstuff/2403249501/ http://en.wikipedia.org/wiki/File:Modiﬁed-pc-case.png