# Composite term not checked **URL:** \ **Category:** Help!\ **Created:** [October 23, 2023, 3:40pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912 "2023-10-23T15:40:22Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![marceloslacerda](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/marceloslacerda/32/5769_2.png) [@marceloslacerda](https://swi-prolog.discourse.group/u/marceloslacerda)\ **Post date:** [October 23, 2023, 3:40pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/1 "2023-10-23T15:40:22Z")
I’m using: SWI-Prolog version 9.0.4 Why does this prolog program: ```prolog mycomp(a, b). mypred(X):- X=mycomp(_, _). ``` Unify with `?- mypred(mycomp(z, w)).` ? I would have guessed that prolog would fail because `z` and `w` are not valid terms for `mycomp`. How would I have to rewrite my program for that to happen? ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [October 23, 2023, 6:08pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/2 "2023-10-23T18:08:34Z")
> [@marceloslacerda](#): > > ```prolog > mypred(X):- > X=mycomp(_, _). > > ``` As discussed earlier, this is equivalent to: ```prolog mypred(mycomp(_, _)). ``` Since the underscore is an anonymous variable, it will match anything. In particular: ```prolog ?- mycomp(z, w) = mycomp(_, _). true. ``` > [@marceloslacerda](#): > > I would have guessed that prolog would fail because `z` and `w` are not valid terms for `mycomp`. How would I have to rewrite my program for that to happen? I don’t quite get your question. ---
**Author:** ![marceloslacerda](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/marceloslacerda/32/5769_2.png) [@marceloslacerda](https://swi-prolog.discourse.group/u/marceloslacerda)\ **Post date:** [October 23, 2023, 6:44pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/3 "2023-10-23T18:44:47Z")
> mypred(mycomp(\_, \_)). Seems pretty obvious now that you wrote it 😁 > I don’t quite get your question. What’s the elegant way of defining a predicate mypred(X) that only holds if X is a compound term mycomp(Z,W)? The best I can think of now is something like: ```prolog mycomp(a, b). mypred(mycomp(X, Y)):- mycomp(X, Y). ``` But it seems redundant to mention mycomp twice in the same predicate. More generally, what’s the usual way that data encapsulation is done in prolog? ---
**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:** [October 23, 2023, 6:58pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/4 "2023-10-23T18:58:25Z")
This does the equivalent of what your “best I can think of” code does: ```prolog mypred(mycomp(a,b)). ``` But, from your question, you seem to be asking something different. Did you mean this? ```prolog mypred(mycomp(_Z,_W)). ``` ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [October 23, 2023, 6:58pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/5 "2023-10-23T18:58:36Z")
Hmmm… In the head of the predicate, `mycomp(X,Y)` is a Prolog term. In the body, it is a goal that you evaluate (or query…). > [@marceloslacerda](#): > > What’s the elegant way of defining a predicate mypred(X) that only holds if X is a compound term mycomp(Z,W)? This is it: ```prolog mypred(mycomp(Z, W)). ``` But now both are “singleton variables” without any further constraints on them, so that will succeed for any Z and any W. What kind of data encapsulation do you mean? This sounds like a technique, not an aim… When you say “compound term”, you do realize this is really just a data structure and not a predicate? You can evaluate it as a predicate though, this is what happens if you put it in the place of a goal. I still have a feeling this is a terminology confusion 😅 EDIT: if you want to pass a predicate to a predicate, then you have define a meta-predicate. There is nothing special about it, really. For example, maplist is a meta-predicate, so you can do something like this: ```prolog ?- maplist(plus(1), [2,3], Result). Result = [3, 4]. ``` This is equivalent to: ```prolog ?- plus(1, 2, X), plus(1, 3, Y), Result = [X, Y]. ``` Is that at all relevant for your question?? ---
**Author:** ![Ian](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/ian/32/1026_2.png) [@Ian](https://swi-prolog.discourse.group/u/Ian)\ **Post date:** [October 23, 2023, 8:38pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/6 "2023-10-23T20:38:38Z")
I suspect the OP is looking for: ```prolog mypred(X):- X=mycomp(_,_), X. ``` Possibly with a `ground(X)` in the middle if only explicit facts are required. ---
**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\ **Post date:** [October 24, 2023, 6:01am UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/7 "2023-10-24T06:01:06Z")
I think you should try the good old clause/2: ```prolog ?- [user]. |: mypred(X) :- clause(X, true). |: % user://1 compiled 0.00 sec, 1 clauses true. ?- [user]. |: mycomp(a, b). |: % user://2 compiled 0.00 sec, 1 clauses true. ?- mypred(mycomp(z, w)). false. ?- mypred(mycomp(A, B)). A = a, B = b. ``` (PS: where you see ```prolog % user://1 compiled 0.00 sec, 1 clauses ``` it means I’ve closed the input stream, hitting Ctrl+Z) ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [October 24, 2023, 8:31am UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/8 "2023-10-24T08:31:10Z")
> [@CapelliC](#): > > I’ve closed the input stream, hitting Ctrl+Z This is of course on Windows, on \*nix it is Ctrl+D. Ctrl+Z will stop your process. ---
**Author:** ![marceloslacerda](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/marceloslacerda/32/5769_2.png) [@marceloslacerda](https://swi-prolog.discourse.group/u/marceloslacerda)\ **Post date:** [October 24, 2023, 11:07am UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/9 "2023-10-24T11:07:46Z")
Is there any difference between ```prolog my_pred(X):- X. ``` and ```prolog my_pred(X):- clause(X, true). ``` ? ---
**Author:** ![Boris](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/boris/32/7486_2.png) [@Boris](https://swi-prolog.discourse.group/u/Boris)\ **Post date:** [October 24, 2023, 11:25am UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/10 "2023-10-24T11:25:11Z")
One obvious difference is that this: > [@marceloslacerda](#): > > `my_pred(X):- X.` really just evaluates X. You can give it anything: ```prolog ?- [user]. |: my_pred(X) :- X. |: ^D% user://1 compiled 0.00 sec, 1 clauses true. ?- my_pred(format("hello!~n", [])). hello! true. ``` If you use clause/2 with `true` as the second argument then this will certainly fail. By the way, this definition: > [@marceloslacerda](#): > > `my_pred(X):- X.` looks a lot like [call](https://www.swi-prolog.org/pldoc/doc_for?object=call/1), doesn’t it? Even the docs say: > Note that a plain variable as a body term acts as [call/1](https://www.swi-prolog.org/pldoc/man?predicate=call/1) … ---
**Author:** ![CapelliC](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/capellic/32/17_2.png) [@CapelliC](https://swi-prolog.discourse.group/u/CapelliC)\ **Post date:** [October 24, 2023, 6:28pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/12 "2023-10-24T18:28:36Z")
They are so much different, like day and night… The first form is using call/1, the second instead is using reflection, and this means that the first one **executes** the code, while the second one **lists** (kind of) it. ---
**Author:** ![marceloslacerda](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/marceloslacerda/32/5769_2.png) [@marceloslacerda](https://swi-prolog.discourse.group/u/marceloslacerda)\ **Post date:** [October 25, 2023, 3:25pm UTC](https://swi-prolog.discourse.group/t/composite-term-not-checked/6912/13 "2023-10-25T15:25:19Z")
I was so happy with Ian’s solution (and I’ve used it so much) that I made my own predicate to encapsulate it: ```prolog of_type(Variable, CompoundTerm):- Variable=CompoundTerm, CompoundTerm. ``` That I test with: ```prolog test_compound(a, b). test_of_type(X, Y, Z):- of_type(X, test_compound(Y, Z)). ``` ```prolog ?- test_of_type(a, b, c). false. ?- test_of_type(a, X, Y). false. ?- test_of_type(test_compound(X, Y), X, Y). X = a, Y = b. ?- test_of_type(test_compound(_, _), X, Y). X = a, Y = b. ?- test_of_type(test_compound(z, w), X, Y). false. ```