Hello all,

I just published LazyList on squeaksource.  It is basically an ordinary 
collection, but with completely lazy semantics.  So you can create infinite 
lists like:

years := LazyElement enumerateFrom: 2006 with: [:year| year + interval ]

The list has many of the regular collection operations (well at least the 
ones I needed. :), as well as some for dealing with the lazy nature (drop: 
aNumber, dropWhile:, take: aNumber, takeWhile:).

All operations are completely lazy, so doing "aLazyList take: 500" just 
returns something that knows how to take 500 from aLazyList.  To get the 
list to actually evaluate use "asOrderedCollection" (it's easy to add more 
"as" methods if needed).

The list implementation has basically two elements: an actual value (the 
first value of the computation) and a function that knows how to generate 
the next value, and the next generator function.  So this makes messages 
like "allButFirst" fast since it is only required to compute one value.

So to give a quick usage example, imagine I had a list of dates I want to 
show in a web page, but the dates are generated based on some computation 
(recurrence rules for example) and are infinite in nature.  One might 
implement the list display component like this:

PRBoxWidget subclass: #OccurrencesSelector
	instanceVariableNames: 'dateSource dateList'
	classVariableNames: ''
	poolDictionaries: ''
	category: 'web site'

OccurrencesSelector>>initialize

  dateSource := self datesStartingAt: DateAndTime now.    "The lazy list"
  dateList := OrderedCollection new.
  1 to: 10 do: [:n| self appendNextDate ]                          "Generate 
initial list to display"

OccurrencesSelector>>appendNextDate
  dateList add: dateSource first.
  dateSource := dateSource allButFirst.     "This forces the next date to 
evaluate"

OccurrencesSelector>>renderOn: html

  html someListDrawingMethodWith: dateList.
  html downArrowButton onClickDo: [ self appendNextDate ]


The caveat for this implementation is, of course, debugging.  The first 
value is always calculated, but everything after that is buried in delayed 
calculations and wont be run unless "asOrderedCollection" gets called, or a 
select: gets ran that doesn't find anything.


I hope someone finds it useful.  Thanks,
Jason

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