计算机语言历史与评估课件

计算机语言历史与评估

OutlineDescribingLanguagesLearningNewLanguagesEvaluationRulesMyofficehoursarenowscheduled: Wednesdays,1-2pm(rightafterclass) Thursdays,3:30-4:30pmIfyoucan’tmakethem,sendemailtoarrangemeetingsatothertimes.ENIAC:ElectronicNumericalIntegratorandComputerEarlyWWIIcomputerButnottheworld’sfirst(PS4)BuilttocalculatebombingtablesMemorysize:twenty10decimaldigitaccumulators=664bitsApolloGuidanceComputer(1969):1inchYou:4.4milesENIAC(1946):½mmMathematicsPhDYale,1934EnteredNavy,1943FirsttoprogramMarkI(first“large”computer,51feetlong)Wrotefirstcompiler(1952)–programforprogrammingcomputersCo-designerofCOBOL(mostwidelyusedprogramminglanguageuntilafewyearsago)AdmiralGraceHopper

(1906-1992)“NobodybelievedthatIhadarunningcompilerandnobodywouldtouchit.Theytoldmecomputerscouldonlydoarithmetic.”NanostickHowfardoeslighttravelin1nanosecond?>(definenanosecond(/1(*10001000

1000)));;1billionthofas>(definelightspeed299792458);m/s>(*lightspeednanosecond)149896229/500000000>(exact->inexact(*lightspeednanosecond))0.299792458=justunder1footDellmachinesinSmallHallhave“1.8-GHzPentium4CPU”

GHz=GigaHertz=1BilliontimespersecondTheymustfinishastepbeforelighttravels6.6inches!CodewrittenbyhumansCompilerCodemachinecanrunCompilertranslatesfromcodeinahigh-levellanguagetomachinecodeDrSchemeusesaninterpreter.Aninterpreterislikeacompiler,exceptitrunsquicklyandquietlyonsmallbitsofcodeatatime.JohnBackusChemistrymajoratUVA(entered1943)FlunkedoutaftersecondsemesterJoinedIBMasprogrammerin1950DevelopedFortran,firstcommerciallysuccessfulprogramminglanguageandcompilerIBM704Fortranmanual,1956DescribingLanguagesFortranlanguagewasdescribedusingEnglishImpreciseVerbose,lotstoreadAdhocDO10I=1.10 Assigns1.10tothevariableDO10IDO10I=1,10 LoopsforI=1to10 (OftenincorrectlyblamedforlossofMariner-I)WantedamoreprecisewayofdescribingalanguageBackusNaurFormsymbol::=replacement Wecanreplacesymbolwithreplacement

nonterminal–symbolthatappearsonleftsideofrule terminals–symbolthatneverappearsontheleftsideofaruleA::=BmeansanywhereyouhaveanA,youcanreplaceitwithaB.LanguageElements

Whenlearningaforeignlanguage,whichelementsarehardesttolearn?Primitives:lotsofthem,andhardtolearnrealmeaningMeansofCombinationComplex,but,allnaturallanguageshavesimilarones[Chomsky]SOV(45%ofalllanguages) Sentence::=SubjectObjectVerb (Korean)SVO(42%) Sentence::=Subject

Verb

ObjectVSO(9%) Sentence::=VerbSubjectObject (Welsh) “Lladdoddyddraigydyn.”(Killedthedragontheman.)OSV(<1%): Tobati(NewGuinea)Schemish: Expression::=(Verb

Object)MeansofAbstraction:fewofthese,buttrickytolearndifferencesacrosslanguages English: I,we

Tok

Pisin(PapuaNewGuinea):mi(I),mitupela(he/sheandI),mitripela(bothofthemandI),mipela(allofthemandI),yumitupela(youandI),yumitripela(bothofyouandI),yumipela(allofyouandI)PagesinRevised5ReportontheAlgorithmicLanguageSchemePrimitivesMeansofCombinationMeansofAbstraction48pagestotal(includesformalspecificationandexamples)PagesinRevised5ReportontheAlgorithmicLanguageSchemePrimitivesStandardProceduresPrimitiveexpressionsIdentifiers,numerals1821MeansofCombinationExpressionsProgramstructure22MeansofAbstractionDefinitions½48pagestotal(includesformalspecificationandexamples)PagesinRevised5ReportontheAlgorithmicLanguageSchemePagesinC++LanguageSpecification(1998)PrimitivesStandardProceduresPrimitiveexpressionsIdentifiers,numerals1821MeansofCombinationExpressionsProgramstructure22MeansofAbstractionDefinitions½48pagestotal(includesformalspecificationandexamples)PagesinRevised5ReportontheAlgorithmicLanguageSchemePagesinC++LanguageSpecification(1998)PrimitivesStandardProceduresPrimitiveexpressionsIdentifiers,numerals1821StandardProceduresPrimitiveexpressionsIdentifiers,numerals3563010MeansofCombinationExpressionsProgramstructure22Expressions,StatementsProgramStructure 19735MeansofAbstractionDefinitions½Declarations,Classes17348pagestotal(includesformalspecificationandexamples)776pagestotal(includesnoformalspecificationorexamples)C++Corelanguageissueslisthas469items!PagesinRevised5ReportontheAlgorithmicLanguageSchemeEnglishPrimitivesStandardProceduresPrimitiveexpressionsIdentifiers,numerals1821MorphemesWordsinOxfordEnglishDictionary?500,000MeansofCombinationExpressionsProgramstructure22GrammarRulesEnglishGrammarforDummiesBook100s(?)384pagesMeansofAbstractionDefinitions½Pronouns~2048pagestotal(includesformalspecificationandexamples)EvaluationExpressionsandValues(Almost)everyexpressionhasavalueHaveyouseenanyexpressionsthatdon’thavevalues?Whenanexpressionwithavalueisevaluated,itsvalueisproducedPrimitiveExpressionsExpression::=

PrimitiveExpressionPrimitiveExpression

::=NumberPrimitiveExpression

::=#t

|

#fPrimitiveExpression

::=PrimitiveProcedureEvaluationRule1:Primitives

Iftheexpressionisaprimitive,itevaluatestoitspre-definedvalue.>22>#t#t>+#<primitive:+>NameExpressionsExpression::=NameExpressionNameExpression

::=NameEvaluationRule2:Names

Iftheexpressionisaname,itevaluatestothevalueassociatedwiththatname.>(definetwo2)>two2ApplicationExpressionsExpression::=ApplicationExpressionApplicationExpression

::=(ExpressionMoreExpressions)MoreExpressions

::=εMoreExpressions

::=ExpressionMoreExpressionsEvaluationRule3:ApplicationIftheexpressionisanapplication:Evaluateallthesubexpressions(inanyorder)Applythevalueofthefirstsubexpressiontothevaluesofalltheothersubexpressions.(Expression0Expression1Expression2…)RulesforApplicationPrimitives.Iftheproceduretoapplyisaprimitive,justdoit.ConstructedProcedures.Iftheprocedureisaconstructedprocedure,evaluatethebodyoftheprocedurewitheachformalparameterreplacedbythecorrespondingactualargumentexpressionvalue.EvalApplyEvalandApplyaredefinedintermsofeachother.WithoutEval,therewouldbenoApply,WithoutApplytherewouldbenoEval!MakingProcedures

lambdameans“makeaprocedure”Expression::=ProcedureExpressionProcedureExpression::=

(lambda

(Parameters)

Expression)

Parameters::=εParameters::=NameParametersEvaluationRule4:Lambda4.Lambdaexpressionsevaluatetoa procedurethattakesthegiven parametersandhastheexpressionas itsbody.LambdaExample:TautologyFunction(lambda()#t)>((lambda()#t)150)#<procedure>:expectsnoarguments,given1:150>((lambda()#t))#t>((l