Prolog - All different list not counting "_" variable

Question

I need to do a function in Prolog that, given a list, check if every member is different. The only particularity is that it must not count "_" variables. Here is an example of what I want :

unique([3,1,2]). 
> true

unique([3,1,2,_]).
> true

unique([3,1,2,_,_,_]).
> true

unique([3,1,2,1]).
> false

unique([3,1,2,1,_]).
> false

I already tried with the following function, using the predicate "var" to check if the variable is free or not, but it doesn't work :

element(X,[X|_]) :- !.
element(X,[_|Q]) :- element(X,Q).

unique([]).
unique([_,[]]).
unique([T,Q]) :- var(T), unique(Q), !.
unique([T|Q]) :- \+element(T,Q),unique(Q).

Thank you for your help

PS: I'm using GProlog

PS2: I know that the function fd_all_different could work, but I'd rather implement it myself

Basically, I do not have this case in my program, so it doesn't matter — bastien-r, May 30 '18 at 12:43
+1 for using this nice name in place of `all_different/1`. See [this](https://stackoverflow.com/a/14606600/772868). — false, May 30 '18 at 13:42
And [this](https://stackoverflow.com/a/20238931/772868) should your Prolog not have `dif/2` defined — false, May 30 '18 at 13:45
A couple of typographical issues: `unique([_,[]]).` ==> `unique([_]).` and `unique([T,Q]) :- ...` ==> `unique([T|Q]) :- ...`. — lurker, May 30 '18 at 14:52

score 2 · Accepted Answer · answered May 31 '18 at 11:59

2

Problem solved thanks to @false:

For those who wonder how, here's the "unique" function :

dif(X,Y) :- X \== Y.

unique([]).
unique([T|Q]) :- maplist(dif(T), Q), unique(Q).

answered May 31 '18 at 11:59

bastien-r

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Prolog - All different list not counting "_" variable

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