Kof2008 Itll

Introduction to Lazy Lists 2008 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 Haskell (Lazy list) Live coding Vim

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 ( ) • Undeﬁned length lists ( ) • Deﬁned length lists ( )

How to construct lazy lists?

Notations • array = 1000+ length array heavy = lambda { heavy procedure } • array.take(2).map(&heavy) • array.map(&heavy).take(2)

Common case • truncation (break) • ﬁltering (if, unless; select, reject) • mapping (=, map) • processing (???, each) folding (<<, inject)

Kof2008 Itll

by ujihisa on Nov 08, 2008

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