Suppose I call an "a-list" a list of zero or more "a"s:
% as = [a,a,a]
as --> [].
as --> [a], as.
Suppose I want to represent a "b-list”, a list of zero or more a-lists:
% bs := [ as, as ]
% bs = [ [a,a,a], [a] ]
bs --> [].
bs --> [A], { phrase(as,A) }, bs.
Is there some more idiomatic way of saying this which doesn't require use of the curly braces to drop out of DCG into "normal Prolog", only to call phrase() again?
An alternative, that I don't think is better as it's arguably less readable, is to use a lambda expression for the definition of the bs//0 non-terminal. Using Logtalk (which you can run with most Prolog systems) or SWI-Prolog (via library(yall), which implements Logtalk lambda expressions):
bs --> [].
bs --> [[A|As],As]>>phrase(as,A), bs.
The lambda expression provides access to the grammar rule implicit arguments.
Sample calls:
?- phrase(bs, [[a],[a,a,a],[a,a]]).
true.
?- phrase(bs, [[a],[a,b,a],[a,a]]).
false.