Scheme_9_Streams1

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1、StreamsinScheme(A&S3.5)•Consideraprocessthatconsumesitsinputsoneatatime–i.e.sequentially•onlypartofaninputsequenceneedstobeinexistenceforitsconsumingprocesstobeabletouseit.StateandStreams•structureswiththispropertyarecalledstreamsinScheme•Inabstractterms,astreamissomething

2、whichhasa–headwhichisavalueandaAbelson&Sussman&Sussmansections:–tailwhichisastream(3.4),3.5•becauseweaccessthestreamoneitematatime,thetailneednotbefullyevaluated•Thus,weexplorealternativemodesofevaluation:non-strictandlazyevaluation•Also,streamsprovideanimplicitnotionofsta

3、teStreams…Streams…©UniversityofAdelaide/1.0Scheme-9/Slide1©UniversityofAdelaide/1.0Scheme-9/Slide2ObjectsrepresentedasstreamsaGISprocessingchain•Variousobjectscanberepresentedasstreams.•Examples–Asequentialfile»Headisthefile“window”»TailistherestofthefileConversion–Thevalu

4、esproducedbyaniterationRectification»Headisthecurrentvalue.Conversion»Tailistheproceduralobjectcapableofproducingtherestofthestream–Datastreamsinsatellitedataoraudio/visualapplicationsStreams…Streams…©UniversityofAdelaide/1.0Scheme-9/Slide3©UniversityofAdelaide/1.0Scheme-9

5、/Slide4StreamsinSchemeImplementingstreamsinScheme(car(cdr(filterprime?(enumerate-interval100001000000))))•WewanttorepresentstreamevaluationinScheme..•Thestreamoperationsarecons-streamheadtail[10007,10009,…][10000…10^6]EnumFilter•Bookusestheequivalentterms:prime?10000cons-s

6、treamstream-carstream-cdr10^6•Wecouldtryrepresentingastreamasalistso:cons-streamiscons•Usingstandardlistbuilding/decompositionoperatorsisinadequate.headiscar–Theseoperatorsforcetheentireinputlisttobebuiltwheneverwepassastreaminasanargument.tailiscdr–i.e.thetailisevaluated.

7、empty-streamisnil–Wewouldpreferittobeleftun-evaluated.•Usingjustcons,car,andcdrprecludestheuseofinfinitestreams.•Thisisinadequate-forexample,considerastreamofprime•Usingonlyfinitestreamsisinefficient.numbers…•Sowelookforanalternative.Streams…Streams…©UniversityofAdelaide/1

8、.0Scheme-9/Slide5©UniversityofAdelaide/1.0Scheme-9/Slide6Page1ImplementingStreamsinScheme