脚本语言21世纪的高级编程语言.pdf

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1、Scripting:Higher LevelProgrammingfor the 21st CenturyJohn K.Ousterhouthttp:/ article appears in IEEE Computer magazine,March 1998)AbstractScripting languages such as Perl and Tcl represent a very differentstyle of programming than system programming languages such asC or JavaTM.Scripting languages a

2、re designed for gluingapplications;they use typeless approaches to achieve a higher levelof programming and more rapid application development thansystem programming languages.Increases in computer speed andchanges in the application mix are making scripting languages moreand more important for appl

3、ications of the future.Keywords:component frameworks,object-oriented programming,scripting,strong typing,system programming.1 IntroductionFor the last fifteen years a fundamental change has been occurring in theway people write computer programs.The change is a transition from systemprogramming lang

4、uages such as C or C+to scripting languages such asPerl or Tcl.Although many people are participating in the change,few peoplerealize that it is occurring and even fewer people know why it is happening.This article is an opinion piece that explains why scripting languages willhandle many of the prog

5、ramming tasks of the next century better thansystem programming languages.Scripting languages are designed for different tasks than systemprogramming languages,and this leads to fundamental differences in thelanguages.System programming languages were designed for building datastructures and algorit

6、hms from scratch,starting from the most primitivecomputer elements such as words of memory.In contrast,scriptinglanguages are designed for gluing:they assume the existence of a set ofpowerful components and are intended primarily for connecting componentsScriptinghttp:/ 共17页2009年05月05日 12:34together

7、.System programming languages are strongly typed to help managecomplexity,while scripting languages are typeless to simplify connectionsbetween components and provide rapid application development.Scripting languages and system programming languages are complementary,and most major computing platfor

8、ms since the 1960s have provided bothkinds of languages.The languages are typically used together in componentframeworks,where components are created with system programminglanguages and glued together with scripting languages.However,severalrecent trends,such as faster machines,better scripting lan

9、guages,theincreasing importance of graphical user interfaces and componentarchitectures,and the growth of the Internet,have greatly increased theapplicability of scripting languages.These trends will continue over the nextdecade,with more and more new applications written entirely in scriptinglangua

10、ges and system programming languages used primarily for creatingcomponents.2 System programming languagesIn order to understand the differences between scripting languages andsystem programming languages,it is important to understand how systemprogramming languages evolved.System programming languag

11、es wereintroduced as an alternative to assembly languages.In assembly languages,virtually every aspect of the machine is reflected in the program.Eachstatement represents a single machine instruction and programmers mustdeal with low-level details such as register allocation and procedure callingseq

12、uences.As a result,it is difficult to write and maintain large programs inassembly language.By the late 1950s higher level languages such as Lisp,Fortran,and Algolbegan to appear.In these languages statements no longer correspondexactly to machine instructions;a compiler translates each statement in

13、 thesource program into a sequence of binary instructions.Over time a series ofsystem programming languages evolved from Algol,including such languagesas PL/1,Pascal,C,C+,and Java.System programming languages are lessefficient then assembly languages but they allow applications to be developedmuch m

14、ore quickly.As a result,they have almost completely replacedassembly languages for the development of large applications.System programming languages differ from assembly languages in two ways:they are higher level and they are strongly typed.The term higher levelmeans that many details are handled

15、automatically so that programmers canwrite less code to get the same job done.For example:Scriptinghttp:/ 共17页2009年05月05日 12:34Register allocation is handled by the compiler so that programmers neednot write code to move information between registers and memory.Procedure calling sequences are genera

16、ted automatically:programmersneed not worry about moving arguments to and from the call stack.Programmers can use simple keywords such as while and if for controlstructures;the compiler generates all the detailed instructions toimplement the control structures.On average,each line of code in a syste

17、m programming language translatesto about five machine instructions,compared to one instruction per line inassembly language(in an informal analysis of eight C files written by fivedifferent people,I found that the ratio ranged from about 3 to 7 instructionsper line7;in a study of numerous languages

18、 Capers Jones found that for agiven task,assembly languages require about 3-6 times as many lines of codeas system programming languages3).Programmers can write roughly thesame number of lines of code per year regardless of language1,so systemprogramming languages allow applications to be written mu

19、ch more quicklythan assembly language.The second difference between assembly language and system programminglanguages is typing.I use the term typing to refer to the degree to whichthe meaning of information is specified in advance of its use.In a stronglytyped language the programmer declares how e

20、ach piece of information willbe used and the language prevents the information from being used in anyother way.In a weakly typed language there are no a priori restrictions on howinformation can be used:the meaning of information is determined solely bythe way it is used,not by any initial promises.

21、1Modern computers are fundamentally typeless:any word in memory can holdany kind of value,such as an integer,a floating-point number,a pointer,or aninstruction.The meaning of a value is determined by how it is used:if theprogram counter points at a word of memory then it is treated as aninstruction;

22、if a word is referenced by an integer add instruction then it istreated as an integer;and so on.The same word can be used in differentways at different times.In contrast,todays system programming languages are strongly typed.Forexample:Each variable in a system programming language must be declared

23、witha particular type such as integer or pointer to string,and it must be usedin ways that are appropriate for the type.Data and code are totally segregated:it is difficult or impossible tocreate new code on the fly.Variables can be collected into structures or objects with well-definedScriptinghttp

24、:/ 共17页2009年05月05日 12:34substructure and procedures or methods to manipulate them;an objectof one type cannot be used where an object of a different type isexpected.Typing has several advantages.First,it makes large programs moremanageable by clarifying how things are used and differentiating betwee

25、nthings that must be treated differently.Second,compilers can use typeinformation to detect certain kinds of errors,such as an attempt to use afloating-point value as a pointer.Third,typing improves performance byallowing compilers to generate specialized code.For example,if a compilerknows that a v

26、ariable always holds an integer value then it can generateinteger instructions to manipulate the variable;if the compiler doesnt knowthe type of a variable then it must generate additional instructions to checkthe variables type at runtime.To summarize,system programming languages are designed to ha

27、ndle thesame tasks as assembly languages,namely creating applications fromscratch.System programming languages are higher level and much morestrongly typed than assembly languages.This allows applications to becreated more rapidly and managed more easily with only a slight loss inperformance.See Fig

28、ure 1 for a graphical comparison of assembly languageand several system programming languages.Scriptinghttp:/ 共17页2009年05月05日 12:343 Scripting languagesScripting languages such as Perl9,Python4,Rexx6,Tcl8,Visual Basic,and the Unix shells represent a very different style of programming thansystem pro

29、gramming languages.Scripting languages assume that therealready exists a collection of useful components written in other languages.Scripting languages arent intended for writing applications from scratch;theyare intended primarily for plugging together components.For example,Tcland Visual Basic can

30、 be used to arrange collections of user interface controlson the screen,and Unix shell scripts are used to assemble filter programsinto pipelines.Scripting languages are often used to extend the features ofcomponents but they are rarely used for complex algorithms and datastructures;features like th

31、ese are usually provided by the components.Scripting languages are sometimes referred to as glue languages or systemintegration languages.In order to simplify the task of connecting components,scripting languagestend to be typeless:all things look and behave the same so that they areinterchangeable.

32、For example,in Tcl or Visual Basic a variable can hold astring one moment and an integer the next.Code and data are ofteninterchangeable,so that a program can write another program and thenexecute it on the fly.Scripting languages are often string-oriented,since thisprovides a uniform representation

33、 for many different things.A typeless language makes it much easier to hook together components.There are no a priori restrictions on how things can be used,and allcomponents and values are represented in a uniform fashion.Thus anycomponent or value can be used in any situation;components designed f

34、orone purpose can be used for totally different purposes never foreseen by thedesigner.For example,in the Unix shells,all filter programs read a stream ofbytes from an input and write a string of bytes to an output;any twoprograms can be connected together by attaching the output of one programto th

35、e input of the other.The following shell command stacks three filterstogether to count the number of lines in the selection that contain the wordscripting:select|grep scripting|wcThe select program reads the text that is currently selected on the displayand prints it on its output;the grep program r

36、eads its input and prints on itsoutput the lines containing scripting;the wc program counts the number oflines on its input.Each of these programs can be used in numerous othersituations to perform different tasks.Scriptinghttp:/ 共17页2009年05月05日 12:34The strongly typed nature of system programming l

37、anguages discouragesreuse.Typing encourages programmers to create a variety of incompatibleinterfaces(interfaces are good;more interfaces are better).Each interfacerequires objects of specific types and the compiler prevents any other typesof objects from being used with the interface,even if that w

38、ould be useful.Inorder to use a new object with an existing interface,conversion code must bewritten to translate between the type of the object and the type expected bythe interface.This in turn requires recompiling part or all of the application,which isnt possible in the common case where the app

39、lication is distributedin binary form.To see the advantages of a typeless language,consider the following Tclcommand:button.b-text Hello!-font Times 16-command puts helloThis command creates a new button control that displays a text string in a16-point Times font and prints a short message when the

40、user clicks on thecontrol.It mixes six different types of things in a single statement:a commandname(button),a button control(.b),property names(-text,-font,and-command),simple strings(Hello!and hello),a font name(Times 16)thatincludes a typeface name(Times)and a size in points(16),and a Tcl script(

41、puts hello).Tcl represents all of these things uniformly with strings.In thisexample the properties may be specified in any order and unspecifiedproperties are given default values;more than 20 properties were leftunspecified in the example.The same example requires 7 lines of code in two methods wh

42、en implementedin Java.With C+and Microsoft Foundation Classes,it requires about 25 linesof code in three procedures(see 7 for the code for these examples).Justsetting the font requires several lines of code in Microsoft FoundationClasses:CFont*fontPtr=new CFont();fontPtr-CreateFont(16,0,0,0,700,0,0,

43、0,ANSI_CHARSET,OUT_DEFAULT_PRECIS,CLIP_DEFAULT_PRECIS,DEFAULT_QUALITY,DEFAULT_PITCH|FF_DONTCARE,Times New Roman);buttonPtr-SetFont(fontPtr);Much of this code is a consequence of the strong typing.In order to set thefont of a button,its SetFont method must be invoked,but this method mustScriptinghttp

44、:/ 共17页2009年05月05日 12:34be passed a pointer to a CFont object.This in turn requires a new object tobe declared and initialized.In order to initialize the CFont object its CreateFontmethod must be invoked,but CreateFont has a rigid interface that requires14 different arguments to be specified.In Tcl,

45、the essential characteristics ofthe font(typeface Times,size 16 points)can be used immediately with nodeclarations or conversions.Furthermore,Tcl allows the behavior for thebutton to be included directly in the command that creates the button,whileC+and Java require it to be placed in a separately d

46、eclared method.(In practice,a trivial example like this would probably be handled with agraphical development environment that hides the complexity of theunderlying language:the user enters property values in a form and thedevelopment environment outputs the code.However,in more complexsituations su

47、ch as conditional assignment of property values or interfacesgenerated programmatically,the developer must write code in the underlyinglanguage.)It might seem that the typeless nature of scripting languages could allowerrors to go undetected,but in practice scripting languages are just as safeas sys

48、tem programming languages.For example,an error will occur if the fontsize specified for the button example above is a non-integer string such asxyz.The difference is that scripting languages do their error checking at thelast possible moment,when a value is used.Strong typing allows errors to bedete

49、cted at compile-time,so the cost of run-time checks is avoided.However,the price to be paid for this efficiency is restrictions on how information canbe used:this results in more code and less flexible programs.Another key difference between scripting languages and system programminglanguages is tha

50、t scripting languages are usually interpreted whereas systemprogramming languages are usually compiled.Interpreted languages providerapid turnaround during development by eliminating compile times.Interpreters also make applications more flexible by allowing users to programthe applications at run-t