CMU-ITC-90-104 4 January 1990
Contents
1. access the i th element of a marker value see Algorithm 1 In practice this algorithm is a primitive provided in the Ness system When it becomes common for users to store marker value objects within strings garbage collection will be necessary At the moment Storage for the underlying strings is released when no markers refer to them subscript m i Returns the i th element of m If the length of m is less than i the function returns an empty marker at the end of m If i is less than zero the function returns an empty marker at start m function subscript m i marker s if i lt 0 then return start m end if s next start m first element of m while extent s m do we are not at the end of m ifi 1then returns end if S next s next element of m 1 i 1 end while we are at the end of m return start next m end function Algorithm 1 The i th element of a marker value This example also illustrates typographical formatting of Ness code The desire for simplicity in Ness has led to a number of decisions to defer or not implement popular semantic tools The two major such decisions are that Ness is not object oriented and functions are not first class objects Ness does provide access to objects but these must be written in C as augmented with the class preprocessor 6 If functions were first class the extend functionality could be expressed as an assignment to an attribute of the o2. originally referred to will contain a copy of the value initially in r Table 2 The five primitive operations in the string algebra The string algebra solves other problems in addition to convenience in writing string processing as function calls The programmer need not be concerned with allocating Storage for strings because that is handled by the system Strings are not restricted to the array model found in some languages and programmers need not resort to integers pointers or some other non string data type in order to refer to substrings Strings also provide a data structuring form that may be more amenable to non programmer computation than traditional programming constructs which are designed more for the convenience of hte machine than the human For data structuring Ness markers provide not only string processing but also all the capabilities of structures and arrays A string is a structure when it has multiple objects embedded in it It is an array when the embedded objects are all the same type and integer subscription functions are used to access the object It should be noted that integer accessing does not reflect the majority of applications of arrays in many applications an array is accessed sequentially varying the subscript by one at each step This corresponds to sequencing through a marker value with the next function For non sequential access however it is trivial to write a function in the string algebra to 10
3. end function In function showcake the raster_xxx functions are proctable entires defined by the Taster inset The copy replace sequence copies the cake to the cut buffer and then replaces the visible cake with the new image The text inset named song text initially contains a blank and a newline The expression currentselection inset song text returns a marker value for the currently selected portion of the text in this inset this will usually be the empty string at the beginning It is immaterial which marker value is returned because the base function is applied to the value to get a marker for the entire contents of song text im_ForceUpdate causes the display to update to make visible the preceding change to the text The function play_notes is inserted in the proctable by a package which interprets notes strings and plays them on the keyboard speaker in the IBM RT PC 19 Appendix 2 The Warning to novices The warning text given below is wrapped around a Ness script when it appears in a document for user perusal The user may set a preference option to get a dialog box instead Such a user is presumed to know what he or she is doing NESS This inset is a Ness script If you choose the empower option at the end of this inset the script may alter the behavior of this window It may respond in new useful exciting or bizarre ways to your mouse clicks keystrokes and menu selections Warning Empowering a Nes
4. interloper perhaps a student about to take that examination Since a Ness script is a program and since it can do anything a user can its execution is a security loophole Hypercard offers a security level scheme of a sort users may choose to execute at one of five levels of privilege However these levels restrict the user from dangerous operations while not restricting scripts a script may even reset the level itself One reason this is not more of a problem in the Hypercard environment is that the equipment is less frequently connected to networks However stackware is shared and we can expect virus attacks via stackware in the future Some mainframe operating systems have implemented capabilities permissions that can be granted to limit the operations available to programs These would ease the security problems though they will still exist Consider for example the user who offers a brand new spiffy shell which gives graphical access to files This shell will have to be given enough capabilities that it could be dangerous Ultimately the best and only protection is Trust the reader must trust the person from whom he or she got a document In a small closed community such trust is an important factor in the free and open exchange of software Unfortunately the spread of networking is widening our communities and exacerbating the security problems The Ness implementation has features that make it more difficult though by no mean
5. the traffic A number of other lessons have been learned from this work o One can go quite far with static declaration of extends events and functions Simple scripts for enhancing documents do not seem to need a highly dynamic language o Ness shows how to do extensions in a more comprehensible manner than scattering scripts behind each individual object By giving names to objects they can be extended in the script and can serve as the targets of operations o The syntax of the extension language is far less a barrier to authors than is the size of the library of functions available o Strings can be dealt with functionally with the string algebra However the algebra is not as simple for non programmers as could be hoped The next step will be to define a pattern matching language to see if this can make clearer the description of string processing algorithms Ness is currently in daily use for maintenance of a data base of bugs and a bibliography Over time the number of applications will grow these will serve as the basis for a future report 14 References 1 Hypercard User s Manual Publication 030 3081 A Apple Computer Inc Cupertino Calif 1987 2 Wilfred J Hansen The Computational Power of an Algebra for Subsequences Information Technology Center Carnegie Mellon Univ 1989 3 Clayton Lewis and Gary M Olson Can Principles of Cognition Lower the Barriers to Programming Report on an informal w
6. CMU 1TC 90 104 4 January 1990 Enhancing documents with embedded programs How Ness extends insets in the Andrew ToolKit Wilfred J Hansen Information Technology Center Carnegie Mellon University Pittsburgh PA 15213 3890 Abstract An enhanced document responds to its reader in non traditional ways a button press may scroll the document play music Although such operations may be implemented as objects embedded in text full generality requires that a programming language be available to the author of a document This paper sketches the problems of embedding programs in documents and reviews the solutions adopted in the Ness component of the Andrew ToolKit A key question is the connection from user actions to program functions Other questions include the appropriate level of programming language its string processing capabilities and security Traditionally a computer document is an emulation of a paper one it sits there for the reader to explore at will This present paper is no different because even though I am writing it on a computer I expect most readers will view it on paper But suppose you were reading it at a computer how much more could it do Simulations calculations interactive examples waving flags music fireworks The report below describes a system in which it is possible to write documents that have most of these behaviors In order to provide the most general environment the system incorporates a program
7. as the syntax extend lt name gt lt attributes gt end extend 262 where the lt name gt must be a string constant giving the name of an object There are situations in which it might make sense that the lt name gt value be an expression rather than a constant and that the extend construction be executable rather than a declaration For instance an author may wish to have different extensions for an object at different times This design was not chosen for Ness because I felt that readers and authors would be best served by a language with relatively static semantics one where the reader could tell what the program refers to without a great deal of poking around the code There could be confusion for some authors if they were allowed an expression for lt name gt they might not know when the expression would be evaluated or whether a change in the value of the expression would cause the extend construct to refer to a different object Furthermore the static nature of the extend construct helps suggest to the author that the behavior of documents should be constant rather than dynamic once a reader has understood a given button it should not change its behavior without a clear cause Nonetheless a button can have differing behavior if an author really so desires the extension for the button need only test a global flag and behave differently for different values Suppose the author wants to dynamically create an object and insert i
8. bject extended During the design it was felt that this provision would make it possible to make more complex less intelligible programs For both functions and object orientedness only experience will suggest whether we need to augment the language Some observers have asked should Ness be Lisp or be more like Lisp One argument in favor of Lisp is that it has less syntax However this argument does not hold up when we notice that each special form in Lisp has its own unique syntax and that there are many more special forms in Lisp than language constructs in Ness Another argument for Lisp is that it is a well known language This is true but the domain for Ness is so different that a Lisp programmer must learn a new set of functions anyway this set of function names is a much higher hurdle to Ness than is the syntax of the language A sif final argument for Lisp is that lists are a convenient data structure which is simple to learn Ness counters this with the string algebra which is just as powerful and may be even more intuitive for non programmers 4 Security Embedding of scripts in documents does not introduce a new level of security problem but makes more obvious a common security problem The problem is that in small operating systems when I execute a program written by someone else it may do anything I myself may do in particular delete a file modify a file or send a copy of a file say a forth coming examination to an
9. context of the entire document See 8 for a good discussion of the advantages of hypertext and references to the growing literature on user disorientation Some discussion in the multi media mail community has focussed on using a language to describe mail documents With this facility new varieties of objects can be sent if the receiving system has no more capability than the language interpreter The work reported below is both more and less general It is not a language suitable for describing any object it assumes that a collection of objects will be available in the software of both sender and receiver It does however provide a language tailored for the author to ae describe a myriad of different forms of interconnection and behavior of objects It is important to distinguish the notion of enhancing a document with a script from the various authoring languages for educational tutorials good examples are cT and Best Course of Action see 7 With the latter the author constructs a program which generates a sequence of images this contrasts with the enhanced document approach of Ness where the program within the images The crucial difference is in user control with an enhanced document the reader is in control and can employ ordinary text operations to move through the text With program generated images control lies with the system the reader can move only to where the system allows Experience has shown that it is not easy for aut
10. elect the Add warning item on the Ness menu card to get 20 this help text back The final option is to decide to Empower this script To do so means that you trust the author of the script and the person who gave you this file it also means you are aware that the script may change how the system responds to your actions Scan for dangerous statements Author mode Let me edit the script Empower trust the source of this script 7
11. ents for an on event are executed whenever the object extended by the surrounding extend construct initates the event named by the lt specifier string gt In writing Ness scripts one of the early discoveries is that intercepting a mouse hit means that that hit will not go to the object In one example the object was a slider and the final value when the mouse is let up is to be transferred to a computation If the mouse is is ae intercepted the slider will not change its value in response to the mouse so the script must perform an additional operation to tell the extended inset of the mouse change A built in function in Ness provides for passing the event along to the inset Passing an intercepted event along to an inset may seem similar to the pass operation in Hypercard an operation that sends the current input message up the hierarchy to the next level button to card to stack to However in Ness and ATK events go down the hierarchy so the bottommost object will not see the event at all if the surrounding script so dictates In our estimation this gives the script and surroundings more control with less special purpose code within the objects Naming of insets and interception of mouse menu and keystroke events is handled by the cel and arbiter insets provided by the ADEW component of ATK 4 These mechanisms are outside the insets themselves so their facilities can be utilized without the collaboration of the subject ins
12. et Cels and arbiters are both examples of wrapper insets ones which have a single child to which they allocate their entire screen space Each cel wraps around a simple inset like a button so it can name the button and intercept events Each arbiter wraps around a substrate inset like text which can have multiple embedded objects from this vantage the arbiter collects the names from all enclosed cels and makes the names available to Ness Since an arbiter wraps a substrate and a substrate may contain arbitrary insets one subinset of an arbiter may be another arbiter together with its own enclosed substrate Ness script and collection of insets So how does a script refer to objects in an arbiter other than its own It doesn t At present a Ness script can only refer to objects that share the same surrounding arbiter as the script itself This restriction could be eliminated but not without some difficulty Consider the case where an author has defined a simulation with a collection of named insets and a Ness script that refers to them Suppose then that the author inserts two copies of this simulation in a document It is only because each is surrounded with its own arbiter that name conflicts are avoided the Ness script in each simulation still refers to the local insets As yet we have not enough experience with Ness to suggest that a capability of referring to objects within embedded arbiters is a necessary capability The statements with
13. have heard about Ness without seeing it have suggested that the script ought to include not only the extensions but also textual descriptions of the objects themselves Among the claimed advantages are that the script could be editted with a plain ASCII editor and that scripts could be generated by programs Ness has taken a very different approach Construction of objects and their assembly into a document or application are handled with the ordinary ATK object oriented editing facilities If a oe button is required in the upper left corner the author puts a button there and it is immediately visible just as the reader will eventually see it Ultimately ATK data streams are indeed encoded in ASCII so they can be editted with ASCII editors by incorrigible hackers It is also possible for a Ness script to insert an object in the document or application To select its size and place the script negotiates with the parent inset inset which the object is inserted In a fully developed system there is a point to having representations of the objects in the script so the author can click on one to request that it be highlighted in the document At present this is handled for Ness by an arbiter application part of the mechanism that handles naming of objects What can an extension do to its environment A guiding principal of Ness is that the instructions in an extension ought to have at least the same capabilities as the user sitting at the workstati
14. hors to always imagine where readers will want to go in reviewing a tutorial so the reader many be sometimes stymied in trying to get to a desired place The best known system with a programming language for enhancing what the reader sees is Hypercard 1 However Hypercard operates in a far more limited environment its images are constrained to a certain small size and only two forms of object may be embedded buttons and fields In contrast the system described below is implemented within the far richer environment of the Andrew ToolKit ATK 5 Underlying ATK is an object architecture complete with inheritance of methods Two principle forms of object are data object this provides for storage and manipulation of information every data object has at least the methods for writing the object to a data stream and reading it back view this provides user access to the contents of a data object by displaying it in a rectangle on the screen it also provides user interface operations for scrolling or modifying the information The ATK architecture is specifically designed so that a view cannot know whether it occupies an entire window or whether it is a subrectangle in another view If a view has embedded child views the architecture provides for the parent view to completely control the events seen by the child thus views can be nested arbitrarily To describe what the user sees of the combination of view and data object this combinat
15. in an on construct may refer to currentinset to refer to the inset whose event has triggered the current execution they may refer to inset lt string expression gt to refer to the inset whose name is given by the lt string expression gt Both constructions yield objects as their values objects on which it is possible to invoke two classes of function that are defined in C code First the script may refer to methods and instance variables of the object Since this is not well protected and can lead to incorrect behavior it is discouraged but there are situations in which it is crucial More safely the script can call functions in what is called the proctable Each inset defines a set of procedures in this table from which they are available to be called from Ness functions or to be bound to keystrokes or menu options as a user customizes his environment 3 The String Algebra Other than for string values the Ness language is as simple and traditional as possible The seven statement forms currently implemented provide for variables functions and flow of control as sketched in Table 1 and illustrated in Appendix 1 Semicolons are optional between statements Parentheses are used only for function call and expression nesting Since it is implemented with ATK programs may be typographically formatted for clarity There are five types of data in Ness integer real boolean object and marker The first three of these are as in other common la
16. ion is called an inset It is worth noting that the architecture proved sufficiently general that little change to the underlying system was required to add Ness In starting Ness it was not clear how script pieces should interface with insets and the user The many design alternatives are outlined in the next section and the following section describes the choices made for Ness A sketch of other facets of Ness especially the string algebra is in the third section followed by a discussion of security issues in the last section 1 How can documents be enhanced Define an extension as a sequence of code to be executed at some particular time during a user s perusal of a document The principal semantic questions are 1 What sorts of events can trigger the extension 2 What effect can the extension have on the containing text and surrounding objects It is apparent that in the usual workstation environment the set of events that can trigger an extension must include user actions with the keyboard and the mouse In ATK an important subset of mouse actions are selection from the popup menu Another set of trigger events are those defined by each inset class For instance text objects may initiate an event when some portion is selected or when the text scrolls both of which may be caused by any of several user actions In principle inset defined events would be a sufficient set of triggers However user inputs are an important add
17. itional form of trigger because they enable the extension language to deal with any object whether or not it has been constructed with the language in mind How is each object to be connected to the portion of the script which specifies the response to events on that object One answer to this is given by Hypercard which associates a script with each object The difficulty with this approach is that scripts and their functionality get scattered all over the place In trying to understand what happens in response to a mouse click on a button it may be necessary to look at as many as five different scripts button card background stack and home stack and trying to understand the interaction of several buttons requires looking at each in turn The situation is similar to the difficulties of trying to understand a spreadsheet of even modest complexity 3 In Ness all the scripts for a collection of objects are gathered together Each object is named and the extensions for an object appear together in a group identified with the name of the object Review of the script is thus reduced to looking in a single place though it is now not as easy to see which object each name refers to especially if many objects are extended Ness authors can avoid this by using multiple scripts in the document one for each section It is the author s responsibility to clarify as far as possible the relation between script portions and objects A few people who
18. ming language Ness the design of which has been kept simple so as to reach a broad range of authors The underlying system is the Andrew ToolKit ATK Typical applications of such a system include C 1990 IEEE Reprinted with permission from Proceedings of IEEE Computer Society 1990 Interna tional Conference on Computer Languages March 12 15 1990 New Orleans to appear Permission to copy without fce all or part of this material is granted provided that the copies are not made or distributed for direct commercial advantage the IEEE copyright notice and the ttle of the publication and its date appear and notice is given that copying is by permission of the Institute of Electrical and Elec tronics Engincers To copy otherwise or to republish requires a fee and specific permission uO enhanced documents with various animations and simulations to illustrate the points of the document parameterized letter after the user fills in a few fields in a form letter other fields are calculated and the full letter is constructed personal data base addresses appointments course records bibliography directory editor click on a file to see its attributes or select it for moving or deleting system status monitor a user builds a personal monitor for work station Status by selecting from a library of system measurement tools and a library of ways to view dynamically changing values dials strip charts and so on dunge
19. ng the interconnection of a language embedded in a document is to identify those user events which initiate the operations described by the language With the extend construct Ness associates event handlers with named insets If the inset signals appropriate events they may be handled via the on event construct otherwise the script can intercept user events destined for the inset with the on menu on mouse and on keys constructs Next the design must specify how the language can affect the document One general tool is to allow the script to perform all possible user operations In addition Ness provides a full set of functions for manipulating insets especially the text inset for which Ness provides a string algebra Finally the design must provide some control so nefarious authors are not as free to produce programs which can damage readers files With Ness the reader has the option to empower a script or not and also the Scan mode which aids in reviewing the script for potentially dangerous statements sis Although apriori it may seem that enhanced documents would be excellent for mail they turn out not to be used in mail very much The world of electronic mail is much more a world of short immediate messages than it is a world of carefully crafted communication Plans for multi media mail must satisfy the requirement for transmission of a variety of kinds of bulk information including scripts but this will not be the majority of
20. nguages An object value is a pointer to an object it is principally of use for values that are to be passed to methods and proctable functions written in C variables declaration booleanp q_ realx y z integer i j assignment x y z p notqor True function call f x p i sin z return returnx y_ exit function flow of control while do exit while if then elif else while i lt j do x 25 if x gt y then exit while end if i i l1 end while Table 1 The seven statement forms in Ness Other flow of control statements are planned for the future String values in Ness had to differ from those in other languages for several reasons Most importantly strings in any user level language must be able to include full typography fonts indentation italic non ASCII characters and so on as well as embedded objects In a functional language it is also important that substrings be first class objects which can be passed as arguments and returned as values In other programming languages it is difficult to write parsing and other string processing functions because substring references are clumsy If substrings are not an integral part of the language each substring reference needs three components a reference to the underlying string an indication of the start of the substring and an indication of the length of the substring It is possible to retain such values by keeping three separate variables it is even reasonable to pass them a
21. on Consequently primitives are provided for simulating the user actions of mouse hits menu selections and key strokes In addition each object makes available a set of operations that Ness extensions may invoke One of the serious problems with allowing scripts to pretend to be the user is that ATK permits users to alter the binding from keystrokes and menu options to function calls If the author writes a script which invoke the sequence ESC N it may work for the author but fail for others who have ESC N bound to a different operation The correct approach is for the author to instead call the function which he or she has bound to the ESC N key Functions behave the same for all users How does the extension script refer to objects in the environment For each execution of an extension there is one unique object namely the one which triggered the extension this object can be referred to by a special name There is also a special name for the text in which the script is embedded via this name the program can access any portion of the document and any of its other objects Most commonly however objects are referenced by the same name which is used to associate extensions with objects 2 The Ness Extend construct A Ness script is a sequence of attribute specifications declarations of global variables global functions and extend blocks An extend block associates a set of contained attributes with some named object An extend block h
22. ons and dragons the description of the world is a large text through which the reader can scroll as the reader solves puzzles descriptions of new rooms and objects are added to the text The remainder of this paper discusses the system as though it were intended for extending document objects are inserted in the document at various places and their behavior is controlled by a script written in the programming language In all cases the discussion applies equally well to programming an application such a situation is just an image that cannot be scrolled off the screen as parts of a document can For documents the substrate is a text for applications the substrate is a drawing editor or some other tool for laying out the contents of a window Hypertext documents can be created easily within a system that supports authorship of enhanced documents The author need only insert in the text a button extended to respond to a button press by scrolling elsewhere in the same document or another Nor is the link constrained to always branch to the same place the author can arrange the script so the destination is chosen among several depending on what the reader has seen so far Because lengthy linear documents are possible the Ness ATK combination may reduce the disorientation readers sometimes encounter in hypertext systems with small nodes The scroll bar in the view of the document serves as a visual indication of where the current image is within the
23. orkshop University of Colorado July 1986 4 Thomas P Neuendorffer ADEW The Andrew Development Environment Workbench An Overview presented at the X Conference Boston MA 1989 5 Andrew J Palay Wilfred J Hansen et al The Andrew Toolkit An Overview presented at the Usenix Conference Dallas TX January 1988 6 Paul G Crumley The Andrew Class System Information Technology Center Carnegie Mellon University file andrew doc Class doc 1989 7 Bruce A Sherwood and Jill H Larkin New tools for courseware production Journal of Computing in Higher Education vol 1 no 1 pp 3 20 1989 8 Ben Shneiderman and Greg Kearsley Hypertext Hands On Addison Wesley 1989 15 Appendix 1 A Ness extended birthday card After empowering the Ness in the birthday card below the reader can click the mouse on the cake the card plays Happy Birthday shows the words and lights the candle on the cake Before wthlatkinessidemosihappybday d Happy Birthday Empower the Ness below and click on the cake for Birthday greetings For dest results use an IBM RT PC workstation Happy Birthday to Ness 16 After clicking on the cake thiatkiness demosihappybday d Happy Birthday Happy birthday to you Happy birthday to you Happy birthday Dear Nessie Happy birthday to you Empower the Ness below and click on the cake for Birthday greetings For best results u
24. s impossible for a villain to damage an unwary user In particular no script is ever executed or even compiled without permission from the reader Users may choose among two options for this protection The default option automatically invoked for any user who has not chosen otherwise is that the Ness script is surrounded with a text that describes the dangers of executing a script see Appendix 2 The tail end of this text has buttons which allow the reader several options including that of Empowering the script which compiles the script and activates any extensions it specifies To be absolutely sure the user wants to empower the script a click on the Empower button pops up a dialog box asking whether the user really intends to empower the script It may be no surprise that despite considerable early design work actual implementation of Ness security began November 4 1988 two days after the infamous Morris internet virus s12 A villain should also be intimidated by the fact that Ness scripts are stored only in source form The villain cannot know which readers will take the time to examine the script before empowering it an examination which might ferret out any suspicious code Few readers will read scripts in their entirety but enough will to provide a deterent Such examination of the code is aided by Scan another option among the buttons at the end of the warning text This option compiles the script but generates an error mes
25. s arguments to functions Returning such a triple of values from a function however is awkward at best The situation is so bad that it is difficult to see how to write a satisfactory string package as either a set of functions 9 or a preprocessor for C the algebra needs to be incorporated as a fundamental part of a language In Ness marker values serve as string values Each such value refers to a substring of some underlying base string In particular for documents one base string will be the document itself and a Ness script can refer to and modify an associated text via marker values A formal algebra underlies marker values as detailed in 2 where it is proven that the algebra is Turing equivalent Within this algebra constants and concatenation serve traditional roles each returns a marker value for an entire newly created string Five functions provide for all possible manipulations on strings base start next extent and replace See table 2 base s returns a marker for the entire base string underlying s start s returns a marker for the empty string which starts where s does next s returns a marker for the single character which starts where s ends extext S r returns a marker for the portion of the base between start s and start next r if the latter precedes the former the value is an empty marker at start next r replace s r modifies the base string underlying s so the portion that s
26. s script is just like running a program The author of the script or program if malicious can write it in such a way that it can destroy your files If you do not trust the place or person from which you got this script DO NOT EMPOWER IT To learn what this script is supposed to do you should read the surrounding document for a description Or you can read the script itself if you are familiar with Ness After reading you have four choices do nothing empower the script scan it for potentially dangerous statements or change it The last three options appear after the script To learn about Ness give the command help ness or see the files in usr andrew doc ness The Ness Script lt lt the script is nested here gt gt End of the Ness Script Your Options If you are uncertain whether to empower this script the safest choice is to select NONE of the options below Also safe is the Scan option which you can choose if you are familiar with the Ness language The scan highlights each statement in the script which might conceivably change this file or other files After choosing the Scan option select the Next danger item on the Ness menu card to cycle through all the potentially dangerous statements If you know Ness and wish to modify the script you can choose the Author mode option If you do so this help text surrounding the script will vanish and you will be able to edit the script You can s
27. sage for each operation which might conceivably modify any of the reader s files whether in memory or on the disk Without artificial intelligence this scan is forced to be quite paranoiac it flags many statements which are completely harmless Nonetheless it typically selects less than a fifth of all statements More experienced readers may wish a direct approach to empowering scripts They may specify in their personal preferences that they wish to see a dialog box instead of the warning text Then whenever a Ness appears for the first time the reader is presented a dialog box which offers the same options as the buttons at the end of the warning text The necessity for security adds an unfortunate complexity It would be preferable if users did not have to know about the script and the notion of empowering it Worse the requirement means that an author must position the script in such a way that it will be visible on the screen because otherwise the reader will never see it to Empower it This can Clutter the design of applications with an unwanted element In the future Ness and ATK will have mechanisms to reveal the script at the outset and later hide it Evaluation This paper has shown that document extension has considerable potential for bringing the computer revolution to information delivery It has described the Ness language which permits an author to construct a document with a variety of behaviors The first problem in defini
28. se an IBM RT PC workstation p Happy Birthday to Ness One the next page is the Ness script for the birthday card The image area at the top of the card has two named insets visible cake is the raster on the left and to its right is a text inset called song text The visible cake inset is extended so mouse clicks on it can be intercepted siTe Happy Birthday to Ness boolean lit False visible cake is initially unlit marker Cakes n extend visible cake on mouse any if mouseaction mouseleftup then lit not lit if lit then showcake FirstObject second Cakes sing else showcake FirstObject Cakes replace base currentselection inset song text n end if end if end mouse end extend function showcake object cake raster_copy_subraster cake raster_select_entire inset visible cake raster_replace_subraster inset visible cake raster_center_image inset visible cake end function function sing marker m m last base currentselection inset song text m last replace m nHappy birthday to you n im_ForceUpdate play_notes L7 CC L4 DCF E P4 m last replace m nHappy birthday to you n play_notes L7 CC L4 DCG F P4 im_ForceUpdate m last replace m nHappy birthday n tDear Nessie n 18 play_notes L7 CC L4 gt C lt A FED P4 im_ForceUpdate m last replace m nHappy birthday to you n play_notes L7 A A L4 AFG F
29. t in the document How can extensions be specified for it With Ness this can be accomplished if the name of the future object is pre known The lt name gt in the extend construct can refer to an object which does not yet exist When it is created the extensions will be automatically applied to it For complete generality however it will probably be necessary to add to Ness some form of executable extend operation In additional to declarations and function definitions the attributes in a extend block may include event specifications Each such specifies a trigger event and a list of statements to be executed when the event occurs In a manner similar to the extend construct an event specification is a static declaration with the form on lt event type gt lt specifier string gt lt statements gt end lt event type gt The lt event type gt s are mouse menu keys and event where the first three intercept the various user events and the fourth reacts to named events initiated by objects The interpretation of the lt specifier string gt which must be a constant depends on the lt event type gt For mouse it says which button is to be intercepted and whether the interception is for the down stroke movement while down upstroke or all of the above For menu and keys it specifies which operations are to intercepted or provided for the user In particular the lt specifier string gt for menu can add new options to the menu The statem
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