# How does one check a list for \_? **URL:** \ **Category:** Help!\ **Tags:** on-topic-only\ **Created:** [May 19, 2022, 10:53am UTC](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408 "2022-05-19T10:53:29Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![EricGT](https://avatars.discourse-cdn.com/v4/letter/e/f1d935/32.png) [@EricGT](https://swi-prolog.discourse.group/u/EricGT)\ **Post date:** [May 19, 2022, 10:53am UTC](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408/1 "2022-05-19T10:53:29Z")
In needing a set type (think has\_type/2) that is more rigours than ordered list, e.g. list\_to\_set/2, the list must not allow the use of `_`. How does one check a list for `_`? * * * > **Details (Click triangle to expand)** > > In other words with creating a set from a list (list\_to\_set/2) this is acceptable > > ```prolog > ?- list_to_set([a],Set). > Set = [a]. > > ?- list_to_set([a,a],Set). > Set = [a]. > > ?- list_to_set([a,A],Set). > Set = [a, A]. > > ?- list_to_set([A,A],Set). > Set = [A]. > > ``` > > but this is not > > ```prolog > ?- list_to_set([_,_],Set). > Set = [_, _]. > > ``` > > Thus an enhanced form of list\_to\_set/2 is needed for my case. > > I am not asking for or abdicating that list\_to\_set/2 be modified in the SWI-Prolog system, my case just needs an additional guard for the type check to fail when a list has any `_` in it. * * * > **Personal notes (Click triangle to expand)** > > Prolog has some predicates for revealing properties of an entity. These predicates usually have the name property in them. There is one for variables, i.e. var\_property/2. > > Thus using var\_property/2 was tried. > > ```prolog > ?- var_property(_,P). > P = fresh(true) ; > false. > > ?- var_property(A,P). > P = fresh(true) ; > false. > > ``` > > however it did not uniquely identify a `_` variable. > > * * * > > After reading [reply](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408/2) from Jan W. tired. > > ```prolog > ?- ?=(_,_). > false. > > ?- ?=(A,A). > true. > > ?- ?=(_a,_a). > true. > > ?- ?=( __a,__ a). > true. > > ``` > > which reveals a means of checking for a difference in the kinds of variables. > > * * * > > To see the internal representation of a list with `_` write\_term/2 can be used. > > ```prolog > ?- L = [_,_,a,A,_a,_A, __a,__ A], write_term(L,[]). > [_2708,_2714,a,_2726,_2732,_2738,_2744,_2750] > L = [_, _, a, A, _a, _A, __a,__ A]. > > ``` > > * * * * * * After seeing [reply](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408/2) from Jan W. My concern for two `_` in a list converted to a set is unwarranted, the variables are different, e.g. ```prolog ?- list_to_set([_,_],Set),write_term(Set,[]). [_5734,_5740] Set = [_, _] ``` as it is the internal representation that matters not the user friendly representation. ---
**Author:** ![jan](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jan/32/4_2.png) [@jan](https://swi-prolog.discourse.group/u/jan)\ **Post date:** [May 19, 2022, 11:19am UTC](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408/2 "2022-05-19T11:19:35Z")
In Prolog you only have variables that maybe different or aliased. `_` is just a concept of read\_term/2 which makes variables that have the same name be the same variable except for `_`, which produces a new variable for every occurrence. So, `[_|_]` is exactly the same as `[_1, _2]` (using numeric syntax to avoid the discussion about singleton/multiton). list\_to\_set/2, like sort/2 and the ord\_\* predicates all compare terms based on the _standard order of terms_. This means that only terms that compare equal under `==/2` are removed. Further instantiation of elements may cause more elements to become equal under `==/2` _unless_ [?=/2](https://www.swi-prolog.org/pldoc/doc_for?object=?%3D%20/%202) is true for all pairs of the set. [?=/2](https://www.swi-prolog.org/pldoc/doc_for?object=?%3D%20/%202) is true if the two arguments are equal or can never become equal. ---
**Author:** ![peter.ludemann](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/peter.ludemann/32/48_2.png) [@peter.ludemann](https://swi-prolog.discourse.group/u/peter.ludemann)\ **Post date:** [May 20, 2022, 12:25am UTC](https://swi-prolog.discourse.group/t/how-does-one-check-a-list-for/5408/3 "2022-05-20T00:25:24Z")
I think what you’re looking for is a verification that no two items in the list can ever be instantiated in such a way that their ordering changes (e.g., they currently do not compare equal under ==/2 but could unify with =/2). You might be interestested in the [Special unification and comparison predicates](https://www.swi-prolog.org/pldoc/man?section=unifyspecial) and also in `\+ \+ X=Y`.