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defs-manager.hpp File Reference

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Description

Management of defaults and default rules.

It is one of the central ideas to shape the behaviour of Lumiera not through hard wired procedures, but rather by the combination of pattern elements driven by rules. A key element within this approach is the notion of an default: Whenever some concrete calculation or behaviour needs an additional element as missing link, in order to reach its intrinsic goals, we (the developers) ask ourselves if the for of this additional element can be derived from first principles or if it is already determined by the input set driving the operation. If this is not the case (and the expectation is that most elements are not fully determined), then we query for an default. This pattern creates a lot of implicit extension points, which are safe to use, since they are defined on a logical level. We ask for something to fulfil a given contract.

Contrast this to a system, which just has arbitrarily built-in flexibility by parametrisation variables: such systems are known to be brittle, since the parameter values are implicitly tied to the behaviour of the operation through shared knowledge of implementation details. It requires both "Druid knowledge" and an overall focus on the implementation mechanics to adjust such parameters. In this respect, a logical query for defaults is quite different, insofar it forces the implementation to respect abstractions on a very fine grained level, and it allows to express the actual defaults by rules which talk the language of the usage context, not the implementation context. The user shall be able to define generic (logical) rules how the Session should behave for the given editing project. And by combination with an additional set of logical consistency rules, a resolution engine can figure out what would be the right default element to use at a given point in the implementation.

Note
as of 2017, we are far from such a system, but it is of uttermost importance that we build our implementations with this goal in mind

Configuration Query Interface

The Defaults Manager exposes an interface similar to a database. The intended audience of this interface is the writer of low-level implementation code. This facade interface is meant to create a more familiar front-end to an essentially logic and rules driven configuration system. And, as an additional benefit, it can be implemented just by a glorified table lookup. Which indeed is what we do until the more elaborate rules based system is in place.

An implementation process in need for some missing link typically knows the type of that missing element. Which means, this type is possibly an abstract type and defines the contract any solution has to fulfil. Thus, the usage context can just demand "give me a suitable XYZ!". In practice, there are several flavours to this interaction, each of which is expressed by a dedicated method on the DefsManager facade interface:

  • we can just retrieve a suitable solution element, no questions asked.
  • we can query for a suitable solution, while limiting the search to what is already known and defined. This entails the possibility that there is no known solution yet.
  • we can demand to fabricate a suitable solution element. Such a call might still deliver something already fabricated, but the emphasis is on the "make it so" demand.
  • we can provide and associate a solution element we created ourselves, to be available from now on, and associated with a given query. Even this call implicates the necessity for the query to be fulfilled by the given element, which, as a consequence includes the possibility of failure.
  • for sake of completeness, we can also require a specific element to be purged from knowledge

Fake implementation

As of 2018, the Lumiera project still has to reach the goal of a complete running engine; we are proceeding with partial integrations for the time being. And we postpone advanced topics, like integration of an actual rules solver to future milestones. Meanwhile, we use a fake implementation with preconfigured, hard-wired "answers" to some frequently encountered standard queries. This Fake implementation is configured and instantiated by the Defaults Manager

See also
DefsManager_test
DefsManagerImpl_test

Definition in file defs-manager.hpp.

#include "lib/p.hpp"
#include "lib/nocopy.hpp"
#include "common/query.hpp"
#include <memory>

Classes

class  DefsManager
 Organise a collection of preconfigured default objects. More...
 

Namespaces

 lumiera
 Lumiera public interface.
 
 lumiera::query
 
 lumiera::query::impl