flex Expert System Toolkit - LPIS

7. Knowledge Specification Language 69
The precedence of operators can be overridden by enclosing the expression
within brackets.
Examples
( 1 - 2 ) * ( 3 + 4 )
( its temp times its volume ) to the power of 2
Dereferencing
Unlike Prolog, expressions in flex are automatically dereferenced and
evaluated when required. For example, any reference to the name of a slot
or a global variable will automatically cause that slot or variable's current
value to be accessed and used.
This dereferencing can be suppressed by prefixing the term with the $
symbol. Any term prefixed with $ will be taken literally and no attempt will
be made to evaluate it as a flex entity. This means that the name of a frame
or attribute when prefixed by $ will be treated simply as text with no
reference to its flex status.
For instance, consider the following code.
action writer ;
do ask toys
and write( $toys )
and nl
and write( toys ) .
question toys
Which toys would you like? ;
choose some of doll, teddy, train, jigsaw, paints .
If we now execute the writer action on the Prolog command line, we will
see that we get back two written values. The first is the absolute value of
toys, namely the atom itself (because of the use of the $ symbol), and the
second is the current value that toys evaluates to, which in this case, is the
list of items selected from the toys question.
?- writer.
toys
[teddy, train, paints]
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