[英]Spurious attribute_goal/2 in SICStus Prolog 4.7.1
Given the following minimalistic "solver" in SICStus Prolog...鉴于 SICStus Prolog 中的以下简约“求解器”...
:- module(attach, [attach/1]). :- use_module(library(atts)). :- attribute a/0. attach(X) :- put_atts(X,a). verify_attribute(_,_,[]). attribute_goal(V,attach(V)) :- get_atts(V,a), put_atts(V,-a).
... I observe: ...我观察到:
| ?- attach(X), copy_term(X,Xc,Xcc), copy_term(X,Yc,Ycc). Xcc = attach:attach(Xc),Ycc = true ?
Where has the residual goal gone?!残留的目标去哪了?!
Wasn't the put_atts/2
in attribute_goal/2
undone ? attribute_goal/2
put_atts/2
不是撤消了吗?
Edit.编辑。 This previous answer (of a somewhat related question) hints at a similar issues when using the builtin predicate
when/2
. This previous answer (of a somewhat related question) hints at a similar issues when using the builtin predicate
when/2
。
No, the side effects (the put_atts(V, -a)
that removes the attribute) of your attribute_goal/2
will not be undone by copy_term/3
(or frozen/2
et al.) and I do not think there is anything in the SICStus documentation that suggests that it would be.不,您的
attribute_goal/2
的副作用(删除属性的put_atts(V, -a)
)不会被copy_term/3
(或frozen/2
等)取消,我认为没有任何东西SICStus 文档表明它会是。
A simplified version of copy_term/3
, that only handles variables, would look something like: copy_term/3
的简化版本,只处理变量,看起来像:
my_copy_term(Var, Copy, Goal) :-
( attribute_goal(Var, VarGoal) ->
true
; VarGoal = true
),
copy_term(Var-VarGoal, VarCopy-VarGoalCopy),
Copy = VarCopy,
Goal = VarGoalCopy.
So, if attribute_goal/2
removes an attribute, like in your example, the attribute will still be absent when ( my_
) copy_term/3
succeeds.因此,如果
attribute_goal/2
删除了一个属性,就像在您的示例中一样,当 ( my_
) copy_term/3
成功时,该属性仍然不存在。
The effect of put_atts/2
will be undone on backtracking, as can be seen in the following two examples, where the first is your example, the second is the same but backtracking over the copy_term/3
before the second copy_term/
3 is invoked. put_atts/2
的效果将在回溯时取消,如以下两个示例所示,第一个是您的示例,第二个是相同的,但在调用第二个copy_term/
copy_term/3
之前回溯 copy_term/3 。
| ?- attach(X), copy_term(X,Xc,Xcc), copy_term(X,Yc,Ycc).
Xcc = attach:attach(Xc),
Ycc = true ?
yes
% zip,source_info
| ?- attach(X), (copy_term(X,Xc,Xcc); copy_term(X,Yc,Ycc)).
Xcc = attach:attach(Xc) ? ;
Ycc = attach:attach(Yc) ? ;
no
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.