Attributed variables: library interfaces / implementations / portability

Question

When I was skimming some prolog related questions recently, I stumbled upon this answer by @mat to question How to represent directed cyclic graph in Prolog with direct access t

Answer 1

I would like to focus on one important general point I noticed when working with different interfaces for attributes variables: When designing an interface for attributed variables, an implementor should also keep in mind the following:

• Is it possible to take attributes into account when reasoning about simultaneous unifications, as in [X,Y] = [0,1]?

This is possible for example in SICStus Prolog, because such bindings are undone before verify_attributes/3 is called. In the interface provided by hProlog (attr_unify_hook/2, called after the unification and with all bindings already in place) it is hard to take into account the (previous) attributes of Y when reasoning about the unification of X in attr_unify_hook/2, because Y is no longer a variable at this point! This may be sufficient for solvers that can make decisions based on ground values alone, but it is a serious limitation for solvers that need additional data, typically stored in attributes, to see whether a unification should succeed, and which are then no longer easily available. One obvious example: Boolean unification with decision diagrams.

As of 2016, the verify-attributes branch of SWI-Prolog also supports verify_attributes/3, thanks to great implementation work by Douglas Miles. The branch is ready for testing and intended to be merged into master as soon as it works correctly and efficiently. For compatibility with hProlog, the branch also supports attr_unify_hook/2: It does so by rewriting such definitions to the more general verify_attributes/3 at compilation time.

Performance-wise, it is clear that there may be a downside to verify_attributes/3, because making several variables ground at the same time may let you sooner see (in attr_unify_hook/2) that a unification cannot succeed. However, I will gladly and any time exchange this typically negligible advantage for the improved reliability, ease of use, and increased functionality that the more general interface gives you, and which is in any case already the standard behaviour in SICStus Prolog which is on top of its generality also one of the faster Prolog systems around.

SICStus Prolog also features an important predicate called project_attributes/2: It is used by the toplevel to project constraints to query variables. SWI-Prolog also supports this in recent versions.

There is also one huge advantage of the SWI interface: The residual goals that attribute_goals//1 and hence copy_term/3 give you are always a list. This helps users to avoid defaultyness in their code, and encourages a more declarative interface, because a list of pure constraint goals cannot contain control structures.

Interestingly, neither interface lets you interpret unifications other than syntactically. Personally, I think there are cases where you may want to interpret unifications differently than syntactically, however, there may also be good arguments against that.

The other interface predicates for attributed variables are mostly easily interchangable with simple wrapper predicates for different systems.