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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