How is this context free grammar using difference lists in Prolog functioning?

Question

I\'m reading this tutorial on context free grammars in Prolog, and they mention at the bottom of the page implementing a context free grammar in Prolog using difference list

Answer 1

I know exactly what you mean. It is quite hard at first to think in terms of list differences. The good news is that you usually do not have to do this.

There is a built-in formalism called Definite Clause Grammars (DCGs) which make it completely unnecessary to manually encode list differences in cases like yours.

In your example, I recommend you simply write this as:

s --> np, vp.

np --> det, n.

vp --> v, np.
vp --> v.

det --> [the].
det --> [a].

n --> [woman].
n --> [man].

v --> [shoots].

and accept the fact that within DCGs, [T] represents the terminal T and, is read as "and then". That's how to describe lists with DCGs.

You can already use this to ask for all solutions, using the phrase/2 interface to DCGs:

?- phrase(s, Ls).
Ls = [the, woman, shoots, the, woman] ;
Ls = [the, woman, shoots, the, man] ;
Ls = [the, woman, shoots, a, woman] ;
etc.

It is hard to understand this in procedural terms at first, so I suggest that you do not try this. Instead, focus an a declarative reading of the grammar and see that it describes lists.

Later, you can go into more implementation details. Start with the way how simple terminals are translated, and then move on to more advanced grammar constructs: Rules containing a single terminal, then rules containing a terminal and a nonterminal etc.