The "undo" Stack

4.1i Standard Text Editor Compatibility 4.6 Language Translator Possibilities

APPENDIX As Program Description Language (PDL)

APPENDIX B: Summary o£ System Levels ...

APPENDIX C: Glossary of terms used ...

LIST OF FIGURES

1.1 An example of low and high-level POL descriptions 2

2.1 The Logical Screen Partitions 6

2.2 A Sample Editing Situation 7

2.3 An example of a predefined block construct 8

2.4 System Level Hierarchy 10

3.1 Using the Line Editor's character delete functions 13 3.2 The Line Editor's Insert mode operation 13

3.3 The Front-end tree structure 15

3.4 The Base Level tree structure 16

3.5 The scrolling functions1 tree structure 17

3.6 Using the EditLn function 18

3.7 The Insert mode tree structure 19

3.8 Ihe Delete mode tree structure 20

3.9 The Copy mode tree structure 20

3.10 The Move mode tree structure 21

3.11 The tree structure for Line Insert mode 21 3.12 An example using the Insert Line facility 22 3.13 The tree structure of the Data Description Insert mode 24 3.14 The Window Screen in Data Description Insert mode 25

3.15 Inserting a block construct 26

3.16 The tree structure for Construct Insert mode 26 3.17 The tree structure for Line Delete mode 27

3.18 Using the Line Delete function 28

3.19 The tree structure for Construct Delete mode 29 3.20 The tree structure for Block Delete mode 30

3.21 Using the Block Delete function 31

3.22 The tree structure for Line Copy mode 33 3.23 The tree structure for Block Copy mode 34 3.24 The tree structure for Line Move mode 35

3.25 Using the Line Move function 36

3.26 The tree structure for Block [love mode 37 4.1 Using the "undo" stack

1 INTRODUCTION

In this chapter some of the important terms and concepts involved in the package are introduced. The first section is introduced mainly to give the user a feel for the structure of the Program Description Language (PDL) for which the package was designed. Secondly, a broad descriptive account is given of the system, paying particular attention to the definition of new terms. In this second section, the advantages of using the package are also emphasized.

1.1 A Brief Summary of the Features of PDL

PDL (Program Description Language) is a machine-independent language written in a structured english format which is used to express a design in a logical, high-level notation. Due to its descriptive nature, a well-defined model of the design of a project can be obtained using conventional programming concepts. The use of comments in PDL is encouraged so that a design can be developed in program form via a series of stepwise refinement iterations.

This top-down design approach aids the designer to view the system as a whole initially, and to slowly expand the view to include more detail until the required implementation level is reached. A design that is fully analyzed using a PDL approach will result in a structured Pascal-like high-level program. This can then be implemented using any suitable technology. Although such an approach is useful in both hardware and software designs, the full power of PDL can only be appreciated in the latter. A very high-level description of a project can he written in a few lines of PDL using a couple of descriptive comments. This is known as high-level PDL (See Fig.1.1 (a)). PDL in its lowest level form is similar to any modern programming language in structural strength and integrity. As can be seen from Fig.1.1 (b), a low-level PDL description is in many ways similar to a computer program.

A PDL program is usually divided into a single program module and an associated set of procedures or subroutines. The procedures will list all their relevant input and output variables as well as any external procedures accessed by them. Extensive use of procedures can be made for the design of large systems, so that program complexity can be controlled. This type of fragmentation also aids in error detection and assists in system building by providing reusable modules.

Strong typing of the constants and variables is enforced in the Data Description segment before the start of the program or procedure body. The five characteristics used for this purpose are known as: Function, Type, Structure, Scope and Name. A detailed description of these terms can be found in Appendix A

which gives the formal specification for PDL. Rigid indentation disciplines in the area of data description ensure ease of readability as is evident from Pig.1.1. -User-defined types may also be specified; the familiar and useful "record" concept in Pascal is also available in PDL.

(a) Program ROOTS

*Get the variable co-efficient B* If (Determinant is positive)

♦Determine the two roots* *Output the two roots* ‘Output an error message*

End Program: >) Program ROOTS Constants; Real: Single: Local: A = 5 Variables: Single: B XI X2 Keyboard: B End Get: Determinant :=> B**2 - 4*A*C If (Determinant >= 0) then: XI := (-B + SQR (Determinant)) / (2*A) X2 t- (-B - SQR (Determinant)) / (2*A)

Console: 'The two roots are*, XI, 'and1, X2 End Put:

Console: ’The roots are imaginary.1 End Put:

End Program:

Pig.1.1: An example of low and high-level PDL descriptions A high-level PDL description of a program for determining the roots of a quadratic equation is shown in figure (a). The corresponding low-level or detailed PDL for this same program can be seen in figure (b).

The Algorithm segment which comprises the program body makes use of assignment, selection, and iteration constructs. The control constructs which will be familiar to the programmer include the following:

If - then - else

Repeat - until — - While - do

Get and Put routines are used for input and output purposes respectively. Sections of code which run concurrently (or "in parallel"}, as opposed to the conventional sequential order, can be distinguished via the Co-begin and Co-end keywords. Comments, enclosed by asterisks, are freely allowed at any point in the program.

trtientation is vital 'co the understanding of an*' program, thus >0L should always be appropriately indented Pig.1.1 (b)). Entering such a program manually thus involves *ge amount of effort. This is especially true in the Data -option segment or in the ease of inserting or deleting an imbedded construct. The clerical effort involved in producing a readable PDL program may overshadow the use of PDL as an effective design aid. It is mainly for this reason that this PDL generator package has been produced.

1.2 A General Description of the Package

The package presented here is aimed at producing a tool for simplifying the input of a program written in Program Description Language (PEL)-. A syntax-directed editor package such as this, knows the syntax rules for the language for which it will be used. Such a system combines the text manipulation facilities of a general-purpose editor with the syntax or error-checking functions of a compiler. A user can thus write a program, being interactively warned of any language structure errors that would be made.

The aim of this editor package is to enable the user to put together and edit a PDL design with the minimum of effort. The user should not be constantly preoccupied with pettv issues such as indentation correction and program structure, uut rather be left with the actual task of designing the required system. This package will thus ensure that programs written using this system will emerge well formatted, readable, and syntactically correct. The syntax-directed editor was built with many modern editor improvement concepts in mind, so that the resulting system should prove easy to learn and operate.

The entire system is function-key driven, saving the user unnecessarily wasted time for typing (eg. The insertion of a construct with associated indentation can be achieved by the depression of a single function key.) The keyboard is used only

when necessary for entering text lines or for specifying filenames or other required responses. Otherwise, the entire system is driven with the use of a dynamic set of ten function keys.

Indentation (or prettyprinting) is done automatically by the editor, leaving the user more time to concentrate on the true design rather than the program format. Also, a PDL program produced using this editor will have a consistent format. In this way, the documentation layout can be standardized.

The editor can also perform syntax and limited semantic error detection. This means that compilation errors such as "missing end of construct" or "undefined variable name" would be flagged as errors at edit time. Thus, a completed program, if it could be run, would be free of the most common compilation errors. Such an editor will prove to be useful as a teaching aid as it emphasizes good programming practices and allows the user to learn the rules of PDL quickly. Because of its time-saving nature, such an editor is of great use as a design tool. Programs produced by this system will not only be correct syntactically and semantically, but will also be in a standard prettyprinted format and thus readable form.