Prolog - generate alternating characters on the way back: [a]; [A, b]; [A, b, a]; [A, b, a, b]

Question

I thought a lot and could not understand. Is it possible to make a script that with backtrack generates lists in this format:

[a]
[a,b]
[a,b,a]
[a,b,a,b]
...

I created one that generates two elements at a time, but my head started to hurt, trying to create one that generates "a", and next time "b", then "a", etc.

Here's the script for two elements at a time:

ab([a]).
ab([b,a|T]):-ab([a|T]).
ab([a,b|T]):-ab([b|T]).

Answer 1

When describing lists, always use the DCG entry .

, , .

, :

abs --> [a], abs_rest.

abs_rest --> [].
abs_rest --> [b], ( [] | abs ).

:

?- phrase(abs, ABs).
ABs = [a] ;
ABs = [a, b] ;
ABs = [a, b, a] ;
ABs = [a, b, a, b] ;
ABs = [a, b, a, b, a] ;
ABs = [a, b, a, b, a, b] .

. dcg !

Answer 2

@mat, dcg, .

.

abs --> [a].
abs --> [a,b].
abs --> [a,b], abs.

?- phrase(abs, Ls).
Ls = [a] ;
Ls = [a, b] ;
Ls = [a, b, a] ;
Ls = [a, b, a, b] ;
Ls = [a, b, a, b, a] ;
Ls = [a, b, a, b, a, b] ;
Ls = [a, b, a, b, a, b, a] ;
Ls = [a, b, a, b, a, b, a, b] ;
Ls = [a, b, a, b, a, b, a, b, a] 

,

abs2 --> [].
abs2 --> [a].
abs2 --> [a,b], abs2.

?- phrase(abs2, Ls).
Ls = [] ;
Ls = [a] ;
Ls = [a, b] ;
Ls = [a, b, a] ;
Ls = [a, b, a, b] ;
Ls = [a, b, a, b, a] ;
Ls = [a, b, a, b, a, b] ;
Ls = [a, b, a, b, a, b, a] ;
Ls = [a, b, a, b, a, b, a, b] 

SWI-Prolog

DCG, @mat listing/1, DCG Prolog.

listing(abs).

abs([a|A], A).
abs([a, b|A], A).
abs([a, b|A], B) :-
        abs(A, B).

listing(abs2).  

abs2(A, A).
abs2([a|A], A).
abs2([a, b|A], B) :-
        abs2(A, B).

Prolog, :

abs(X,[]).
X = [a] ;
X = [a, b] ;
X = [a, b, a] ;
X = [a, b, a, b] ;
X = [a, b, a, b, a] ;
X = [a, b, a, b, a, b] ;
X = [a, b, a, b, a, b, a]

abs2(X,[]).
X = [] ;
X = [a] ;
X = [a, b] ;
X = [a, b, a] ;
X = [a, b, a, b] ;
X = [a, b, a, b, a] ;
X = [a, b, a, b, a, b] 

Answer 3

, , :

?- ab([b,a]).
true

ab([b,a|T]) :-
  ab([a|T]).

, , . , , ab a, - ba, ba ab :

ab([a]).
ab([a|Xs]) :-
    ba(Xs).

ba([b]).
ba([b|Xs]) :-
    ab(Xs).

abs ,   a , b .   , :

abs(a,[a]).
abs(b,[b]).
abs(a,[a|Xs]) :-
    abs(b,Xs).
abs(b,[b|Xs]) :-
    abs(a,Xs).

Now we define the sequence abas the last one added a:

ab(Xs) :-
    abs(a,Xs).

Good luck :)