Term Parser Example

This demonstrates the capabilities of dynamic operators.

The term grammar specifies as a simple interactive system for reading and printing algebraic terms. The parser repeatedly reads a term folowed by a dot ('.') from the standard input, and prints its canonical form - i.e., f(t₁, ... ,t_n). One types the command exit. to exit the system. Prolog's dynamic operators are predefined, but - as in Prolog - may be redefined at parse time. Still as in Prolog, new dynamic operators may also be defined and redefined at parse time as well.

We define a simple tokenizer and a driver for our needs, and run Jacc on the term grammar:

% jacc Term
*** Reading grammar in file Term.grm ... 
*** Starting grammar analysis ... 
*** Grammar analysis completed in 70 ms.
*** Building parsing tables ... 
*** Writing parser file TermParser.java
*** Parser generation completed in 100 ms.
*** Total processing time: 271 ms.

This generates the term parser. We then compile all the relevant java files:

% javac Term*.java

and run the Term reader:

% java TermMain
*** Welcome to the dynamic operator Term Parser!
*** Version of Mon Dec 20 18:49:39 PST 2004
> a+b*c-d.
-(+(a,*(b,c)),d)

The system read in the algebraic term "a+b*c-d" and printed it in its canonical form. With the canonical form, we can verify the effect of operator precedence and associativity. For example, the operator + was declared in the grammar to be left associative:

> a+b+c.
+(+(a,b),c)

We can change an operator's specification dynamically. Indeed, the term grammar defines a semantic action so that the parser interprets a term of the form "op(P,F,O)" to (re-)define the operator O to have precedence level P, and fixity F. We can use this to make the operator + become right-associative. To wit:

> op(500,xfy,+).
op(500,xfy,+)
> a+b+c.
+(a,+(b,c))

We can also define fresh operators. For example, $ is not known as an operator. Indeed:

> a $ b.
*** Syntax Error: unexpected 'FUNCTOR'($) - see stdin (line 5, column 3)

But now, let us define it on the fly as a binary left associative operator:

> op(400,yfx,$).
op(400,yfx,$)

and verify that it is now indeed understood to be so:

> a $ b $ c.
$($(a,b),c)

Finally, we can also verify the behavior of the if...then...else construct as an algebraic term:

> if a then b else c.
then(if(a),else(b,c))
> if a<b then x+y else (if a = b then 1 else 0).
then(if(<(a,b)),else(+(x,y),then(if(=(a,b)),else(1,0))))
> exit.
*** Bye bye...
%

Author:: Hassan Aït-Kaci
Version:: Last modified on Wed Dec 29 19:21:38 2004 by hak