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prologdcg

Prolog predicate calling

In the following tutorial: http://www.csupomona.edu/~jrfisher/www/prolog_tutorial/7_3.html

There is the part:

test_parser :- repeat,
               write('?? '), 
               read_line(X),
               ( c(F,X,[])   | q(F,X,[])  ),
               nl, write(X), nl, write(F), nl, fail.

Now I'm extremely confused about the c(F,X,[]) and q(F,X,[]) part because it doesn't seem to match any thing that I have seen, c only takes one parameter from what I can tell and these parameters don't seem to make sense for q. Please help me understand what is going on here.

Solution

  • c is defined with -->, which actually adds two hidden arguments to it. The first of these is a list to be parsed by the grammar rule; the second is "what's left" after the parse. c(F,X,[]) calls c on the list X to obtain a result F, expecting [] to be left, i.e. the parser should consume the entire list X.