27701472 introduction to flex

8/9/2019 27701472 Introduction to Flex

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Introduction to FlexIntroduction to Flex

From: Sec.1 , Flex Tutorial, by Clive Spenser,

LPA.


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Flex Tutorial 2

1.1 What Is Flex1.1 What Is Flex

Flex is a language specially designed for the

development and delivery of Expert Systems.

It is implemented in Prolog but looks much more like

standard English than a programming language.

This is a feature of its Knowledge Specification

Language (KSL).

Flex is very functional and can carry out most of theprocedures needed to build knowledge-based

systems.


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Flex Tutorial 3

1.1.1 What Are Flex Programs1.1.1 What Are Flex Programs

A Flex program is a collection of KSL statements.

Programs are stored in standard ASCII text files, and

can be edited using either the development

environments internal editor or any external text

editor or word-processor.


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Flex Tutorial 4

1.1.2 How Does Flex Relate to Prolog1.1.2 How Does Flex Relate to Prolog

Flex is implemented in Prolog; it extends the

traditional Prolog environment, and yet still gives

access to the underlying Prolog.

You do not need to know Prolog to use Flex, though

being familiar with Prolog syntax and basic Prolog

commands is useful.

For instance, by using the listing command, you can

actually see the internal (Prolog) representation of

your Flex (KSL) code.


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Flex Tutorial 5

1.1.3 Constructs in Flex1.1.3 Constructs in Flex

Flex contains many constructs ideal for building

knowledge-based systems (frames, instances, rules,

relations, groups, questions, answers, demons,

actions, functions).

You may wish to use some or all of these in building

your Knowledge-Based Systems (KBS).

You do not have to learn to use all of them at once!


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Flex Tutorial 6

1.1.4 How Extendable Is Flex?1.1.4 How Extendable Is Flex?

Flex can recognize and compile both Flex (.KSL files)

and Prolog (.PL files).

If you find you cannot do what you want in Flex, then

it can probably be accomplished in Prolog.

If it cannot be done in Prolog, then you can always

use the C interface and do it in C, or Java or some

other low-level language.


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Flex Tutorial 7

1.1.5 Naming Conventions in Flex1.1.5 Naming Conventions in Flex

Often Flex lets you re-use the same name to define

different constructs; e.g. you may have a question

named drink, and a group of the same name.

You do not have to make use of this facility and may

wish to employ a more explicit naming convention, for

instance, where the question is named

drink_question and the group named

drink_group.


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Flex Tutorial 8

1.2 Basic Flex Constructs1.2 Basic Flex Constructs

1.2.1 KSL Sentences1.2.1 KSL Sentences The basic unit of compilation in Flex is the sentence.

Sentences are made up of one or more KSL

statements containing conditions and/or directives,

and are terminated with a full stop.


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Flex Tutorial 9

1.2.2 Conditions and Directives1.2.2 Conditions and Directives

Conditionstest whether or not something iscurrently true.

Directiveschange the current state of an object tosome new state.

The context will determine which of these Flex will

expect.


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Flex Tutorial 10

1.3 Installing and Starting Flex1.3 Installing and Starting Flex

Load Flex with a double click on the Flex shortcut.

Flex loads in a few seconds.

If you have used Prolog before, you will recognize the

Console window but should notice an extra menu

named Flex.

The File and Edit menus are very similar to other

Windows applications. Open the menus to confirm

this.


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Flex Tutorial 11

1.4 Important Tips1.4 Important Tips

1.4.1 The Console window1.4.1 The Console window The first window you see is the Console window with

the standard Prolog ?- prompt.

This window is used for communication between you

and the system.


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Flex Tutorial 12

1.4.2 Create a New File1.4.2 Create a New File

File New"Untitled" Save As


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Flex Tutorial 13

1.4.3 The Run Menu1.4.3 The Run Menu

The Run menu is used to compile and run your

code.


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Flex Tutorial 14

1.4.4 Analysing Syntax Errors1.4.4 Analysing Syntax Errors

Pull down the Flex menu and make sure the Analyse

Syntax Errors item is enabled (ticked) before you

enter code.

If the system detects any mistakes during

compilation, it will make suggestions as to what is

wrong.

If Analyse Syntax Errors is not ticked, you will just

get a Cannot Parse Sentence... error without anyexplanations as to why.


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Flex Tutorial 15

1.4.5 How Are Flex Programs Compiled?1.4.5 How Are Flex Programs Compiled?

The KSL compiler translates Flex programs into an

internal Prolog-based representation which is

interpreted by the Flex run-time system.

In the case of a KSL syntax error, the parser attempts

to identify what caused the error and what alternative

words would have been acceptable.


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Flex Tutorial 16

1.4.6 How Are Flex Programs Executed?1.4.6 How Are Flex Programs Executed?

There is usually a top-level actionassociated with aFlex program to start it off.


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Flex Tutorial 17

1.4.7 Restarting Flex?1.4.7 Restarting Flex?

Closing a Flex file does not automatically remove

associated definitions.

You can clear memory by typing the initialise

command into the console window at the ?-prompt.

Alternatively, you can use the Initialise Workspace

item on the Flex menu.


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Flex Tutorial 18

1.4.8 How Do I Stop Flex (or Prolog) Looping1.4.8 How Do I Stop Flex (or Prolog) Looping

CONTROL + BREAK


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1.4.9 Use of Semicolons1.4.9 Use of Semicolons

Often, semicolons are used as delimiters, for

instance within actions, frames etc.


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1.4.11 Use of the $ Character1.4.11 Use of the $ Character

One important difference between the Flex andProlog execution models is that Flex will attempt toprove goals rather than directly execute them.

This involves a layer of interpretation such that Flexcan de-reference (evaluate) any arguments locatedwithin the scope of the goal statement, unless toldnot to by the $ symbol.

Flex de-references its arguments before a call ismade. The $ character can be used to inhibit this andforce the use of conventional Prolog pattern matching(called unification).


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Flex Tutorial 22

1.4.12 Debugging in Flex1.4.12 Debugging in Flex

Because the trace facility in Flex works at the Prolog

level, you may find it more helpful to insert write

statements within your flex code at the appropriate

points, followed by a new-line.


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Starting ProgrammingStarting Programming

From: Sec.2, Flex Tutorial, by Clive Spenser,

LPA.


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Flex Tutorial 24

2.1 First Program2.1 First Program

We will start by defining a question and anaction. The outline structure for these are:

question question_name

prompt text for question ;

input datatype [ ;

because explanation ] .

action action_name ;

do directive(s) .An example


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Flex Tutorial 25

question your_namePlease enter your name ;

input name .action hullo ;

do ask your_nameandwrite( 'hi there ' )

andwrite( your_name )andnl .

2.1 First Program2.1 First Program


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Flex Tutorial 26

2.2 Questions and Answers2.2 Questions and Answers

Flex has a built-in question and answer sub-system.

You can also generate basic questions using reservedwords such as choose from, input etc.

Alternatively you can build more sophisticated dialogsby supplying your own code to display them.

Asking a question generates a dialog box which, in theexample above, awaits keyboard input.

When you respond, the reply is stored in a variable

which has the same name as the question, in this caseyour_name.

You can then refer to your_name to access whateverwas entered.


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Flex Tutorial 27

2.3 Compiling and Running Queries2.3 Compiling and Running Queries

Made sure you do have a .KSL extension.

Use the Compile item on the Run menu to compileyour code.

In the Console window, type hullo. at the ?- promptand press .

You can try clicking on the Explain... button to seewhat happens.


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Frames and InstancesFrames and Instances


LPA.


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Flex Tutorial30

3.1 Frames and Instances3.1 Frames and Instances

In Flex, you store knowledge about objects using frame andinstance.

Frames define classes of objects and instances of framesdescribe individuals within those classes.

These are organized into a frame hierarchy with informationinherited by either explicit or implicit means.

The outline structure of a frame is:

frame frame_name ;

[default Attribute_Name is Value ]

[and default Attribute_Name1 is Value1 ] ;

[inherit Attribute_Name2 fromFrame ] .


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The following code defines a general class ofstudent in a frame.

frame student ;

default nationality is american and

default nature is studious and

default discipline is computing and

default residence is texas and

default major is undecided.

instance instance_name ;[ Attribute_Name is Value ][ andAttribute_Name1 is Value1 ] .


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instance maria is a studentnature is cheerful andnationality is spanish anddiscipline

isengineering

andstatus is sophomore.

instance anton is a student ;nature is frivolous andnationality is french and

discipline is mathematics andstatus is freshman andinterests are {tennis,computing,maria} andresidence is paris.



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The textual ordering of the attribute-value pairs isirrelevant.

The order in which frames and instances are written

affects the order in which they are accessed. Flex allows you to create instances with the same name

but belonging to different frames, however, the lastcreated instance will take preference.

The value of an attribute can be obtained by either ofthe following statements:

maria`s nationality

the nationality of maria


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3.2 Attribute Values3.2 Attribute Values

You can think of frames and instances as datastructures with a name, three columns and anunspecified number of rows.

Instances inherit their slot values from theirparent frames unless the values are over-ridden with local (current) definitions.

Frame/instance slot values, both default and

current, can be pre-defined in KSL files and/ordynamically created and updated at run-time.


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Flex Tutorial36


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More on ActionsMore on Actions


LPA.


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Flex Tutorial38

4.1 Using the Student Information4.1 Using the Student Information

BaseBase

Test a question by, say,?- ask( new_status ).

question the_student please choose a student ; choose fromanton, maria, john, mary, tom.

question new_status please select the new status ; choose fromfreshman, sophomore, junior, senior.

choose some of


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4.2 Scheme for changing a students status4.2 Scheme for changing a students status

and checking both the old and the new valueand checking both the old and the new value

1. Use the student frame and the five instancesyou have prepared

2. Choose a students name using the_student

question.3. Show the current status with echo or write

(just to check)

4. Query for the new status, using the questionnew_status

5. Use becomes to reset the_students statusattribute

6. echo or write the new status to screen.


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4.2 Scheme for changing a students status4.2 Scheme for changing a students status

and checking both the old and the new valueand checking both the old and the new value

action change_status ;do ask the_studentand echo( ' the status of', the_student,

' was ', the_student`s status ).

and ask new_statusandthe_student`sstatusbecomes new_statusand echo( ' the status of', the_student,

' is now', the_student`s status ).

action change_status ;

do ask the_student and check that S is the_student

and echo(' the status of', S, ' was', S`s status ) and ask new_status andS`s statusbecomes new_status

and echo( ' the status of', S, ' is now', S`s status ).


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4.3 A special frame called global4.3 A special frame called global

You sometimes need to store values to be usedanywhere in the program.

These can be put into a special frame called

global.

This is what the built-in question and answermechanism uses.

frame global;default current_year is 1996 anddefault college_name is 'Texas College'.


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4.4 Some aids to formatting4.4 Some aids to formatting

Use tab(N)to move text across the screen.

action action_name;

do write( 'something' )andtab( 10 ) % tab 10 spacesandwrite( 'Something else' ) .

The forloop is very much like that found inany other programming language.

action half_screen;do for N from0 to 10do nlend for .


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Flex Tutorial43


Modify the hullo program of sections 0 touse half_screen twice:

action hullo ;do half_screenandask your_nameand echo( 'Hi ', your_name )and ask your_age

andwrite( 'I think ' )andwrite( your_age )andwrite( 'is cool!' )andhalf_screen .


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action student_status ;do for every Student is some instance of student% Student is a local variable

% it starts with uppercase.do write( Student )andwrite( ' record is ' )andnlandtab( 5 )andwrite( 'Nationality is ' )andwrite( Student`s nationality )and% etc.end for


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Flex Tutorial45


action do_something ;do check that X is a studentand check that the nationality of X is Yandwrite( X-Y ) .

action do_something_else ;do check that [ X is a student

andnationality

ofXis

Y ]

andwrite( X-Y ) .

OR


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Some Technical NotesSome Technical Notes


LPA.


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Flex Tutorial 47

5 Some technical notes5 Some technical notes

In order to be able to talk intelligently about aspects ofprogramming in a language (whether in Flex or in anyother language) we all need to understand and be ableto name the basic units of the language.

The basic unit in Flex is a token.

Tokens are treated as single items by the compiler.

Flex uses various types of Prolog token: punctuation,number, atom, string, byte list and variable, and built

on top of these Prolog tokens are the concepts of a KSLname and a KSL value.


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5.1 Comments and punctuation5.1 Comments and punctuation

/* */a block comment.

%a single line comment

( ) [ ] { } | ! ; ,

are all treated asseparate items and each one is a token.


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5.2 Numbers5.2 Numbers

Numbers are either integers or floating-pointnumbers.

211327 -32768 0 2.34 10.3e99 -0.81


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5.3 Atoms5.3 Atoms

Atoms are of three types:alphanumeric, symbolicand quoted. alphanumeric

apple aPPLE h45j apple_cartorangesAndApples

symbolic

a contiguous sequence of symbols such as

*, >, or #. quoted

'Apple' '123' 'The green man' '^h''ht'


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Flex Tutorial 51

5.4 Byte Lists5.4 Byte Lists

Characters enclosed in double quotes aretreated as lists of ASCII characters.

A boy is shorthand for [65,32,98,111,121].

For the technically minded: LPA is a 32-bitsystem. Each character occupies 4 bytes butonly one of them is used - this is wasteful. Inaddition, there is a 4 byte pointer to eachcharacter - add the type tags and it turns outthat each character uses 10 bytes ofmemory!!


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5.5 Strings5.5 Strings

LPA offers a special dedicated string data typefor text items denoted by the use ofbackwards quotes.

The maximum length of these `strings` is 64K(much bigger than regular atoms).

For the technically minded: Thismethod of string storage uses

approximately 1.3 bytes per character.This is quite efficient. Maximum length65535 bytes - compare atoms.


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5.6 Variables5.6 Variables

A (logical) variable is an uppercase letter (A-Z) or an underscore (_), followed by asequence of zero or more alphabetic

characters (a-z, A-Z or _) or digits (0-9).MrSpock Apple APPLE _23 X.

The underscore on its own _ is the don`tcare variable. Its contents are not stored.


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5.7 Names5.7 Names

A name is any quotedatom, or any atomwhich is not a reserved word (i.e. an atomwhich does not appear in the glossary of the

KSL).brick brick32 'The' 'the brick'


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5.8 Values5.8 Values

A value is any number, any string or anyname.


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Forward-chaining Production RulesForward-chaining Production Rules


LPA.


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6.1 Rules6.1 Rules

Forward-chaining (production) rules are usedby the (meta-level) forward-chaining engine inFlex.

rule rule_name

if condition(s)

then directive(s) ;

[because explanation ] ;[ score score_expression ] .


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rule check_residence1if the_student`s residence is not included in

{london,texas,kensington}

andthe_student`s status is included in{freshman,sophomore}then echo( 'it will be necessary to book accommodation

at texas for ', the_student ).

rule check_residence2if the_student`s residence is included in{london,texas,kensington}then echo( 'it will not be necessary to book

accommodation at texas for ', the_student ).

6.1 Rules6.1 Rules


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Flex Tutorial 59

6.2 Rulesets6.2 Rulesets

You will appreciate that in a large application there maybe many hundreds of rules.

It is convenient to group sets of rules which belongtogether and may be relevant at one particular stage ofprocessing.

Such a group is called a ruleset.

However, if you dont want to have discrete groups, youcan inform Flex of this by using the general definition:

ruleset set1

contains all rules.

frame student; % the name of the frame


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default nationality is american and % frame attributes always Odefault nature is studious and % O have default keyword.default discipline is computing anddefault residence is texas anddefault major is undecided.

instance maria is a student; % parent frame is studentnature is cheerful andnationality is spanish anddiscipline is engineering andstatus is sophomore andresidence is madrid.

instance anton is a student;nature is frivolous andnationality is french anddiscipline is mathematics andstatus is freshman andinterests are {tennis,computing,maria} andresidence is paris.

instance margaret is a student;nature is sporty andnationality is finnish anddiscipline is art andstatus is freshman andinterests are {tennis,computing,anton} andresidence is texas.

Part 1

rule check_residence1if S is an instance of student and not checked( S )


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if S is an instance of student and not checked( S )and S`s residence is not included in {london,texas,kensington}and S`s status is included in {freshman,sophomore}then echo( 'it will be necessary to book accommodation at texas for', S )and S`s residence becomes texasand echo( 'The residence for ', S, ' is now', S`s residence, 'by rule1' )

and remember that checked( S ).rule check_residence2

if S is an instance of student and not checked( S )and S`s residence is not included in {london,texas,kensington}and S`s status is not included in {freshman,sophomore}then echo( 'it will be necessary to book accommodation at kensington for ', S )and S`s residence becomes kensington

and echo('The residence for ', S, ' is now ', S`s residence, 'by rule2' )and remember that checked( S ).rule check_residence3

if S is an instance of student and not checked( S )and S`s residence is included in {london,texas,kensington}and S`s status is included in {freshman,sophomore,junior,senior}then echo( 'it will not be necessary to book accommodation attexas for ', S, 'by rule3' )

and remember that checked( S ).ruleset set1

contains all rules;select rule using first come first served.

action residence;do restart % this clears any remembered factsand echo( starting, check )and invoke ruleset set1and echo( finished, check ).

Part 2


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6.2 Rulesets6.2 Rulesets

Flex provides three methods of determiningthe order in which to select rules: first come first served, in which it picks the

rules in the order in which they appear in the file; conflict resolution, where the best rule is picked

according to a score; and

conflict resolution with thresholdwhich picksthe first rule whose score is above a user supplied

threshold. This score is defined using the scoreclause in rules.


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6.3 Facts and exceptions6.3 Facts and exceptions

Everything the expert system knows about the currentstate of its world is contained in frames and instancesand in facts and exceptions.

Flex maintains two databases: one of facts, which are

true, and one of exceptions, which are not true. These are entered during run-time and exist only for

the duration of the session.

They are entered with the keywords remember thatand removed with the keywords forget that.

The facts and exceptions will either be obtained asanswers to questions or as consequences of applyingproduction rules - where the consequences will containa line such as remember that and/or forget that...


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6.4 Groups6.4 Groups

Groups are name collections like sets.

Think of them as type declarations but with abuilt in ordering.

Groups are often used to collect items to feedinto a question.

group wall_colours

magnolia, coffee, apple_white, barley, buttermilk .

question wall_colour

Please choose a colour for your room ;

choose fromwall_colours .


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6.5 Relations6.5 Relations

In Flex, relations are used to representbackward-chaining rules and have the form:

relation relation_name(arg1,arg2,..., argN)

if condition(s) .


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6.5 Relations6.5 Relations

Relations are like actions. However, whereasactions can only have one definition, relationscan have multiple (alternative) definitions.

Relations can be called directly from within anaction (like the system predicates echo andwrite).

Relations can be used in the if or then part of

(forward-chaining) rules. Relations can call other relations.

frame student


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frame student.

instance maria is a student ;nature is cheerful and

nationality is spanish andstatus is freshman andresidence is madrid.

relation check_residence( S )if S`s status is included in {freshman,junior}

and S`s residence is not included in {texas,london}and echo( 'we need to book accommodation for', S, 'at texas' ).

/* this definition is used when the first one fails */relation check_residence( S )if echo( S, ' does not need to have accommodation booked' ).

action test;do for every S is an instance of studentdo check_residence( S ) and write( S ) and nlend for.


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relation check_residence1( S, Residence )if S`s status is included in {freshman,junior}

and S`s residence is not includedin {texas,london}and check that Residence is S`s residenceandecho( 'we need to book accommodation for', S, 'at texas' ).

/* this definition is used when the first one fails */.

relation check_residence1( S, Residence )if echo( S, ' does not need to have accommodation booked' )and check that Residence is S`s residence.

action test1 ;do for every S is an instance of student

docheck_residence1( S, Residence )

andwrite( S - Residence ) andnlend for .


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6.6 Templates6.6 Templates

Flex provides a template facility to make KSL morereadable.

Templates are replaced at compile time by textsubstitution. Carets (^) are used to indicate where anyvariables may be.

The outline structure for a template is:

template label_to_replace

positive_template ;

negative_template .


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ProjectsProjects


LPA.


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Data Driven ProgrammingData Driven Programming


LPA.


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8. Data Driven Programming8. Data Driven Programming

There are four types of procedure which takeplace automatically when data in frames orinstances is added or changed. These are:

launches, constraints,

demons and

watchdogs.

Sometimes this technique is called proceduralattachment and is often found in objectoriented systems.


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8.1 Launches8.1 Launches

A launch is a procedure which can be attached to aframe and is automatically invoked whenever a newinstance of that frame is created.

Its primary use is in setting up the initial characteristicsof frame instances.

The directives associated with a launch are executedimmediately afterthe instance is created.

A launch can be tailored such that it fires only undercertain circumstances.

The outline structure for a launch is:launch launch_name

when Instance is a new [ instance of ] Frame

andcondition(s)

then directive(s) .


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8.1 Launches8.1 Launches

launch new_studentwhen S is a new studentandS is not exit

then ask s_nationality.


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8.2 Constraints8.2 Constraints

A constraint is a validity check which can be attached toan attribute of a frame.

It is automatically invoked whenever the value for thatslot changes.

The checks associated with a constraint are executedimmediately before the value of the slot is to bechanged, and the value only changes if the checkssucceed.

If any check fails then the slot is notupdated, and theupdate itself will fail.

Note that a demon may be used to perform checks aftera slot value has changed.


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8.2 Constraints8.2 Constraints

The outline structure of a constraint

constraint constraint_namewhen Attribute changes[ fromExpression1 to Expression2 ][ andcondition1(s) ]then check that condition2(s)[ otherwise directive(s) ] .


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8.3 Demons8.3 Demons

A demon is a procedure which can be attached toan attribute of a frame.

It is automatically invoked whenever the valuefor that slot changes.

The directives associated with a demon areexecuted immediately afterthe slot valuechanges.

A demon can be tailored such that it fires only for

given values and/or only under certaincircumstances.

Note that a constraint may be used to performchecks before a slot value is changed.


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8.3 Demons8.3 Demons

The outline structure

demon demon_namewhen Attribute changes[ fromExpression1 to Expression2 ][ andcondition1(s) ]

then directive(s) .


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8.4 Watchdogs8.4 Watchdogs

A watchdog checks the access rights to anattribute of a frame.

It is automatically invoked whenever there is a

request for the currentvalue (not the defaultvalue) of that slot.

The checks associated with a watchdog areexecuted immediately before the value is

accessed. If the check fails then the access call also fails.


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8.4 Watchdogs8.4 Watchdogs

The outline structure of a watchdog :

watchdog watchdog_name

when Attribute is requestedandcondition1(s)then check that condition2(s)otherwise directive(s) .

This is a natural for password protection.

when the status of student is requestedthen do password....


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Directives and ConditionsDirectives and Conditions


LPA.


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9.1 Directives9.1 Directives

Directives are used to change the current stateto some new state, where a state consists ofthe global variables, frames, instances, facts

and exceptions.


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9.1.1 Assignments9.1.1 Assignments

The changing of global variables, frames andinstances is known as assignment.

There are two kinds of assignment, direct

assignments and assignments which happenas the result of creation of a new frame orinstance.


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9.1.1.1 Direct Assignments9.1.1.1 Direct Assignments

A direct assignmentconsists of a variant onthe left hand side and an expression on theright hand side of an assignment operator.

Variant := Expression

Variantbecome ExpressionVariantbecomes Expression

Examples

methane_level := highthe kettle`s temperaturebecomes 45the cinema`s filmsbecome

{ 'Gone With the Wind', 'Star Wars' }


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9.1.1.2 New instances9.1.1.2 New instances

Directives can dynamically create newinstances of frames with local attributes.

All other attributes of the parent frame will

automatically be inherited by the instance.

Instance is a new FrameInstance is another Frame

'Tiddles' is another cat whose owner is alexanderplant33 is a new plant whose size is medium


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9.1.2 Database Maintenance9.1.2 Database Maintenance

The addition and removal of facts and exceptions isclassed as database maintenance.

Database maintenance is accomplished by directiveswhich add assertions to, or delete assertions from boththe positive (facts) and the negative (exceptions)

databases. Facts may be added and removed using the following

KSL keywords:

remember remember that forget forget that

Examples

remember that pregnant( P )

remember likes( alexander, harvey )

remember not raining

forget danger_level( red )

forget that not boiling


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Flex Tutorial 91

9.2 Conditions9.2 Conditions

A condition is used to test the current state(for example of global variables, frames orfacts).

Conditions either test for the existence of avariant or compare the value of twoexpressions; a condition may also be aprocedure call.


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Flex Tutorial 92

9.2.1 Equality Comparison9.2.1 Equality Comparison

isareis equal toExamples

alpha = beta / 2jugA`s contents are jugA`s capacitythe size of some brick is equal to 4the employee`s name is phil

not alpha = betanot the pupil`s mark is 70X is an elephantX is a kind of animal whose ears are small


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Flex Tutorial 93

9.2.2 Direct Comparison9.2.2 Direct Comparison

greater than [or equal to][at or] aboveless than [or equal to][at or] below

alpha > beta / 2the temperature =< the 'freezing point' of waterthe pupil`s mark is not below 50

the temperature is at or above boiling_pointthe likelihood of frost is less than probablethe food`s calories is less than or equal to 400


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Flex Tutorial 94

9.2.3 Relative Comparison9.2.3 Relative Comparison

its colour is at or above the colour of moneyaccording to { red , blue , white , green }

group fuzzy_orderingcertain, probable, possible, unlikely, impossible .

the likelihood of frost is less than probableaccording to fuzzy_ordering


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Flex Tutorial 95

9.2.4 Set Membership9.2.4 Set Membership

include(s)included indo(es) not include.

Examplesthe staff include { john and mary }a surprise is included in the contents of the box

the Rodent`s tail does not include bushy


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9.4 Context Switching9.4 Context Switching

If you wish to use a condition where a directiveis expected, then the context can be switchedby inserting the word(s) check[ that ].

For example, an action requires directives buta relation requires conditions.

Examples

relation emp_name( Emp, Name )

if Name is Emp`s name .action emp_name( Emp, Name ) ;

do check that Name is Emp`s name .

27701472 introduction to flex

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