# Clause/2 gives different results when library(clpfd) is loaded **URL:** \ **Category:** General\ **Created:** [October 6, 2020, 2:52pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012 "2020-10-06T14:52:23Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![meditans](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/meditans/32/1768_2.png) [@meditans](https://swi-prolog.discourse.group/u/meditans)\ **Post date:** [October 6, 2020, 2:52pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/1 "2020-10-06T14:52:23Z")
Consider this declaration, and the related query: ```prolog example(Lss) :- maplist(member(3), Lss). %?- Term = example(X), clause(Term, Body). %@ Term = example(X), %@ Body = maplist(member(3), X). ``` But now, if I also load `library(clpfd)` I get: ```prolog :- use_module(library(clpfd)). example(Lss) :- maplist(member(3), Lss). %?- Term = example(X), clause(Term, Body). %@ Term = example(X), %@ Body = '__aux_maplist/2_member+1'(X, 3). ``` As you can see, the result changes, and the clause is read as `__aux_maplist` instead. This is a problem for me because I’m writing meta-interpreters for typing some of my code using clause/2, and the fact that the result changes based on what modules are imported was totally inscrutable to me! How would you solve this problem? _Why_ is `library(clpfd)` influencing unrelated code? Can I isolate different concerns of my code in a better way? Is this documented anywhere? ---
**Author:** ![jamesnvc](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/jamesnvc/32/14_2.png) [@jamesnvc](https://swi-prolog.discourse.group/u/jamesnvc)\ **Post date:** [October 6, 2020, 3:40pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/2 "2020-10-06T15:40:33Z")
That looks to me like `library(apply_macros)` is being loaded, which does that sort of source transform. ---
**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:** [October 6, 2020, 3:42pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/3 "2020-10-06T15:42:20Z")
library(clpfd) (and several others) load library(apply\_macros), which optimizes certain meta predicates. I’m not really sure how we should handle this. It should not affect meta-interpreters as the clauses in the DB are still nice and correct Prolog that is even simpler than the original. Just making it work for the library that loads library(apply\_macros) would be an option. The disadvantage is that a common usage scenario is to load this library and be sure that all these simple meta calls are compiled away. Most Prolog systems won’t even allow to get the code for static predicates. This would suggest not to apply this expansion for dynamic code and have some option to load a file as _all dynamic_. I’m not sure what the value of that is either. ---
**Author:** ![meditans](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/meditans/32/1768_2.png) [@meditans](https://swi-prolog.discourse.group/u/meditans)\ **Post date:** [October 6, 2020, 4:15pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/4 "2020-10-06T16:15:28Z")
Thanks @jamesnvc and @jan for pointing out that the rewrite is introduced in `library(apply_macros)`. I guess the documentation is [here](https://www.swi-prolog.org/pldoc/doc/_SWI_/library/apply_macros.pl), but there’s no mention of `library(yall)` and yet: ```prolog :- use_module(library(apply_macros)). example2(As) :- L = [1,2,3], maplist({L}/[X]>>member(X,L), As). %?- clause(example2(_), Body). %@ Body = (_13262=[1, 2, 3], ' __aux_maplist/2___ aux_yall_0acee418088eabf60688ed3721777c91323f1885+1'(_12856, _13262)). ``` And this introduces another problem: while in my original question I could maybe parse the functor of `'__aux_maplist/2_member+1'(X, 3)`, now I’m unsure what to do with the `0acee...` part of the `yall`. Notice also that this is becoming the implementation of a grammar for functor names for which I don’t have a spec (`yall` doesn’t seem even mentioned in `apply_macros`!). @jan could you elaborate on your suggestion in this case? I’m not sure I get it! Ideally, I shouldn’t care about these optimization for this meta-interpreter, and I would like to just access the code I actually wrote. But that’s not what you’re suggesting, right? ---
**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 6, 2020, 4:52pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/5 "2020-10-06T16:52:42Z")
> [@meditans](#): > > there’s no mention of `library(yall)` `library(yall)` doesn’t use `library(apply_macros)` … it works by creating new predicates that contain the “lambda” definitions. This is separate from `library(apply_macros)`, which expands out a specific set of predicate goals – in “apply macros”, “apply” isn’t a verb; the name means “expand (macros) from library(apply)”. ---
**Author:** ![meditans](https://yyz2.discourse-cdn.com/free1/user_avatar/swi-prolog.discourse.group/meditans/32/1768_2.png) [@meditans](https://swi-prolog.discourse.group/u/meditans)\ **Post date:** [October 6, 2020, 4:57pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/6 "2020-10-06T16:57:37Z")
@peter.ludemann thanks! But if `library(yall)` doesn’t use `library(apply_macros)` why do I have this different behaviors: Without `library(apply_macros)`: ```prolog example2(As) :- L = [1,2,3], maplist({L}/[X]>>member(X,L), As). %?- clause(example2(_), Body). %@ Body = (_6040=[1, 2, 3], maplist({_6040}/[_6062]>>member(_6062, _6040), _5634)). ``` With `library(apply_macros)`: ```prolog :- use_module(library(apply_macros)). example2(As) :- L = [1,2,3], maplist({L}/[X]>>member(X,L), As). %?- clause(example2(_), Body). %@ Body = (_13262=[1, 2, 3], ' __aux_maplist/2___ aux_yall_0acee418088eabf60688ed3721777c91323f1885+1'(_12856, _13262)). ``` Also, I’m still not sure on which is the advice I should follow when I want to interpret my code. ---
**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 6, 2020, 5:15pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/7 "2020-10-06T17:15:46Z")
> [@meditans](#): > > But if `library(yall)` doesn’t use `library(apply_macros)` why do I have this different behaviors: I think that `library(yall)` provides an “interpreted” version of \>\>/2 for situations where its term expansion misses something – for example, as a closure passed to a meta-predicate. When library(apply\_macros) is used, it expands the maplist/2 call and that allows `library(yall)`'s term expansion to work. (This is an educated guess – I’m too lazy to look inside the two libraries to see the details of how they work.) ---
**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:** [October 6, 2020, 6:40pm UTC](https://swi-prolog.discourse.group/t/clause-2-gives-different-results-when-library-clpfd-is-loaded/3012/8 "2020-10-06T18:40:29Z")
The maplist/2 expansion is a requirement to do the yall code expansion. Note that the yall code expansion is a complicated thing. Earlier messages here showed the scoping difference between interpreted and compiled yall lambda expressions. Still, the problem remains: “why do you care”?