Non-context-free Grammars
In pure Prolog, normal DCG rules with no extra arguments on the functors, such as the previous example, can only express context-free grammars; there is only one argument on the left side of the production. However, context-sensitive grammars can also be expressed with DCGs, by providing extra arguments, such as in the following example:
s --> a(N), b(N), c(N). a(0) --> . a(M) -->, a(N), {M is N + 1}. b(0) --> . b(M) -->, b(N), {M is N + 1}. c(0) --> . c(M) -->, c(N), {M is N + 1}.This set of DCG rules describes the grammar which generates the language that consists of strings of the form .
s --> symbols(Sem,a), symbols(Sem,b), symbols(Sem,c). symbols(end,_) --> . symbols(s(Sem),S) -->, symbols(Sem,S).This set of DCG rules describes the grammar which generates the language that consists of strings of the form, by structurally representing
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