Adding Arbitrary Prolog Goals

Grammar rules are simply expanded to Prolog goals. We can also insert arbitrary Prolog subgoals on the right hand side of a grammar rule but we must tell Prolog that we do not want them expanded. This is done with the help of braces --- i.e. { }. For example, here is a grammar rule which parses a single character input as an ASCII code and succeeds if the character represents a digit. It also returns the digit found.

 
digit(D) -->

		      [X],

			 SPM_quot" 		 X >= 48,

					 X =< 57,

					D is X-48 SPM_quot".

The grammar rule looks for a character at the head of a list of input characters and succeeds if the Prolog subgoals

 
		       SPM_quot" 		 X >= 48,

		         		 X =< 57,

		         		 D is X-48 SPM_quot".