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)