This document introduces lazy lists, which are data structures that only calculate the needed parts of a list, unlike regular lists that calculate the entire list up front. It notes that Haskell supports lazy lists by default while Ruby can also support them. It describes different types of lazy lists, such as those with boundless, undefined, or defined lengths. It also discusses how to construct lazy lists and common operations like truncation, filtering, mapping, and folding.

1. Introduction to Lazy Lists
2008 UJIHISA, Tatsuhiro

2. UJIHISA Tatsuhiro
• Osaka University
• Ruby, Vim, Haskell and Math
• http://ujihisa.nowa.jp
• http://twitter.com/ujm
• To the US!

3. Targets
•
• Array, Enumerable, IO (File)
Haskell

4. Lazy lists

5. • Ruby Haskell
(Lazy list)
Live coding
Vim

6. 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.

7. 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]

8. List
• Data structure
• Insert, Delete, Access sequentially

9. c.f. Array
• Data structure
• Insert, Delete, Access randomly, Size

10. (List again)
• Data structure
• Insert, Delete, Access sequentially

11. Lazy List
• Data structure
• Insert, Delete, Access sequentially

12. Types of lazy lists
• Boundless length lists ( )
• Undefined length lists ( )
• Defined length lists ( )

13. How to construct
lazy lists?

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

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