If I use the predicates member or the nt1 in SWI-Prolog as follows:
?- member(X, [A]).
X = A.
or
nth1(N, [A], X).
N = 1,
A = X.
The interpreter unifies the variable A as X.
Is their some alternative version of this functions which does not use the unification. Means, if I call something like this:
?- _member(X, [A]).
it would give
false
as long as the call is not
?- member(X, [X]).
which would lead to
true
And in the same way
_nth1(N, [A], X).
would give false
but
_nth1(N, [X], X).
would give
N = 1
Seems like you just need to roll your own using ==/2 instead of =/2:
See https://swish.swi-prolog.org/p/DqhYGuEf.pl
is_member_of( X , [Y|_] ) :- X == Y .
is_member_of( X , [_|Ys] ) :- is_member_of(X,Ys) .
is_nth0( N , L , E ) :- is_nth(0,L,E,N) .
is_nth1( N , L , E ) :- is_nth(1,L,E,N) .
is_nth( N , [Y|_] , X , N ) :- X == Y .
is_nth( I , [_|Ys] , X , N ) :- J is I+1, is_nth(J,Ys,X,N) .