4.14.3 An Example

As an example, here is a simple grammar that parses an arithmetic expression (made up of digits and operators) and computes its value. Create a file containing the following rules:

                                 
grammar.pl
     
     
     expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.
     expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.
     expr(X) --> term(X).
     
     term(Z) --> number(X), "*", term(Y), {Z is X * Y}.
     term(Z) --> number(X), "/", term(Y), {Z is X / Y}.
     term(Z) --> number(Z).
     
     number(C) --> "+", number(C).
     number(C) --> "-", number(X), {C is -X}.
     number(X) --> [C], {"0"=<C, C=<"9", X is C - "0"}.

In the last rule, C is the character code of a decimal digit.

This grammar can now be used to parse and evaluate an expression by means of the built-in predicates phrase/[2,3]. See mpg-ref-phrase. For example,

     | ?- [grammar].
     | ?- phrase(expr(Z), "-2+3*5+1").
     
     Z = 14
     
     | ?- phrase(expr(Z), "-2+3*5", Rest).
     
     Z = 13,
     Rest = [] ;
     
     Z = 1,
     Rest = "*5" ;
     
     Z = -2,
     Rest = "+3*5" ;
     
     no