Generate lists with independent variables in Prolog - prolog

Question

I am having a problem while creating a list which I will the use to fill. I am trying to create it with independent variables, so that I can then unify with a maplist with some other lists.
For this, I did:
length(Subs, Len),
maplist(=([_, _, _]), Subs),
however, this results in this:
Subs = [[_22998, _23020, _23042], [_22998, _23020, _23042], [_22998, _23020, _23042]].
And so, when I try to unify the next way:
maplist(nth1(1), Subs, Com1),
maplist(nth1(2), Subs, Perm_sub),
maplist(nth1(3), Subs, Com3).
with:
Perm_sub = [5, 7, 6],
Com1 = [P23, P34, P36],
Com3 = [1, 3, 2].
It fails, because 5 unifies with _22998, and thus collides with 7. Is there a simple, elegant way of doing what I intend, or do I have to create a new function to handle this, and if so, how do I do the function?

Answer 1

You can create an auxiliary predicate as following:
triple([_, _, _]).
And ask:
?- length(L, 3), maplist(triple, L).
L = [[_2404, _2410, _2416], [_2422, _2428, _2434], [_2440, _2446, _2452]].
Alternatively, you can use a lambda expression as following:
?- length(L, 3), maplist([[_,_,_]]>>true, L).
L = [[_7198, _7204, _7210], [_7276, _7282, _7288], [_7354, _7360, _7366]].
EDIT 10/05/2021
Notice that variables used in a term are shared with their environment (they are "global", not lambda bound) and, consequently, they persist through all calls of maplist. For example, in the following query, variable X is instantiated with 1 and, after that, its value will persist through the next calls.
?- trace, maplist(=(X), [1,Y,1]).
^ Call: (11) apply:maplist(=(_1882), [1, _1892, 1]) ? creep
Call: (12) apply:maplist_([1, _1892, 1], user: =(_1882)) ? creep
Call: (13) _1882=1 ? creep
Exit: (13) 1=1 ? creep
Call: (13) apply:maplist_([_1892, 1], user: =(1)) ? creep
Call: (14) 1=_1892 ? creep
Exit: (14) 1=1 ? creep
Call: (14) apply:maplist_([1], user: =(1)) ? creep
Call: (15) 1=1 ? creep
Exit: (15) 1=1 ? creep
Call: (15) apply:maplist_([], user: =(1)) ? creep
Exit: (15) apply:maplist_([], user: =(1)) ? creep
Exit: (14) apply:maplist_([1], user: =(1)) ? creep
Exit: (13) apply:maplist_([1, 1], user: =(1)) ? creep
Exit: (12) apply:maplist_([1, 1, 1], user: =(1)) ? creep
^ Exit: (11) apply:maplist(user: =(1), [1, 1, 1]) ? creep
X = Y, Y = 1.
On the other hand, variables used as predicate arguments are "local" and their instantiations do not persist from one call to another. For example:
?- [user].
| equal(_X).
| ^Z
% user://1 compiled 0.02 sec, 1 clauses
true.
?- trace, maplist(equal, [1,Y,1]).
^ Call: (11) apply:maplist(equal, [1, _1752, 1]) ? creep
Call: (12) apply:maplist_([1, _1752, 1], user:equal) ? creep
Call: (13) equal(1) ? creep
Exit: (13) equal(1) ? creep
Call: (13) apply:maplist_([_1752, 1], user:equal) ? creep
Call: (14) equal(_1752) ? creep
Exit: (14) equal(_1752) ? creep
Call: (14) apply:maplist_([1], user:equal) ? creep
Call: (15) equal(1) ? creep
Exit: (15) equal(1) ? creep
Call: (15) apply:maplist_([], user:equal) ? creep
Exit: (15) apply:maplist_([], user:equal) ? creep
Exit: (14) apply:maplist_([1], user:equal) ? creep
Exit: (13) apply:maplist_([_1752, 1], user:equal) ? creep
Exit: (12) apply:maplist_([1, _1752, 1], user:equal) ? creep
^ Exit: (11) apply:maplist(user:equal, [1, _1752, 1]) ? creep
true.