Introduction to Lazy Lists2008 UJIHISA, Tatsuhiro

UJIHISA Tatsuhiro• Osaka University

• Ruby, Vim, Haskell and Math

• http://ujihisa.nowa.jp

• http://twitter.com/ujm

• To the US!

Targets

• ★★☆☆☆• Array, Enumerable, IO (File)

Haskell

Lazy lists

• RubyやHaskellを用いて遅延リスト (Lazy list) の概念とその有用性の議論を展開する。説明にはLive codingが含まれる。もしも時間が余れば、全てのプログラマにとって有用である伝統的ツールVimについて簡単な紹介を行う。

Lazy Lists• Lazy lists (or streams)

• List (= Array in Ruby)

• Only needed parts are calculated.

• Haskell supports lazy lists by default.

• Ruby can support it.

List (=Array) in Ruby• Array class

• [0, 1, 2, 3], Array.new(4) {|i| i }

• a #=> [0, 1, 2, 3]a.map {|i| i * 2 } #=> [0, 2, 4, 6]

List• Data structure

• Insert, Delete, Access sequentially

c.f. Array• Data structure

• Insert, Delete, Access randomly, Size

(List again)• Data structure

• Insert, Delete, Access sequentially

Lazy List• Data structure

• Insert, Delete, Access sequentially

Types of lazy lists

• Boundless length lists (長さが無限)

• Undefined length lists (長さが未定義)

• Defined length lists (長さが定義済み)

How to constructlazy lists?

1 class LazyList 2 def initialize(&b) 3 @b = b 4 @i = -1 5 end 6 7 def succ 8 @b[@i+=1] 9 end 10 end

l = LazyList.new {¦i¦ i * 2 } 10.times do p l.succend

def each loop { yield succ }end

l = LazyList.new {¦i¦ i * 2 } l.each do ¦i¦ p i break if i > 10end

Notations• array = 1000+ length array

heavy = lambda { heavy procedure }

• array.take(2).map(&heavy)

• array.map(&heavy).take(2)

File.open(__FILE__) do ¦io¦ a = nil while a = io.gets do p a endend

File.open(__FILE__) do ¦io¦ io.each do ¦a¦ p a endend

File.open(__FILE__) do ¦io¦ io.each do ¦a¦ if a != "\n" p a end endend

File.open(__FILE__) do ¦io¦ io.select {¦a¦ a != "\n" }.each {¦a¦ p a }end

File.open(__FILE__) do ¦io¦ io.each do ¦a¦ if a != "\n" p a end endend

File.open(__FILE__) do ¦io¦ io.select {¦a¦ a != "\n" }.each {¦a¦ p a }end

File.open(__FILE__) do ¦io¦ io. select {¦a¦ a != "\n" }. each {¦a¦ p a }end

Common case• truncation (break)

• filtering (if, unless; select, reject)

• mapping (=, map)

• processing (???, each)folding (<<, inject)

list.each {¦a¦ next if ... b = ... ...}

list. select { ... }. map { ... }. each { ... }

result = 0list.each {¦a¦ next if ... b = ... result += b}

result = list. select { ... }. map { ... }. inject(0) { ... }

$ sudo gem install lazylist

sq = list { x * x }.where :x => 1..Infinity

# Haskell!# sq = [ x * x ¦ x <- [1..] ]