Constraints.md

Constraints

This page describes two languages which can be used to write constraints in YAFMT. See constraints editor how to add constraints to feature model. See also how to extend YAFMT by adding new constraint language.

SamCL (Sample Constraint Language)

SamCL is a minimalistic constraint language which main purpose is to show how to use the cz.zcu.yafmt.clang extension point. It only allows 2 kinds of constraints:

A mute-with B - A and B cannot be selected at the same time.
A requires B - Selection of A requires B to be also selected.

**Note: ** Feature is always referenced by its ID.

BoolCL (Boolean Constraint Language)

BoolCL is a constraint language based on rules of the Boolean algeabra. The following table shows list of operators and their example usage. The operators are sorted from the highest to the lowest prioryty.

Name	Example	Description
Identifier	`A`	Evaluates to `true` when a feature which ID is `A` is present in the feature configuration. We shortly refer to this as: `A` must be selected.
Negation	`not A`	`A` must not be selected.
Disjunction	`A or B`	One of `A` or `B` must be selected.
Conjunction	`A and B`	Both `A` and `B` must be selected.
Implication	`A implies B`	When `A` is selected, `B` must be also selected.
Equivalence	`A equals B`	`A` must be selected exactly when `B` is selected and vice versa.

You can use paranthenses to make more complicated expressions like this: (A or B) and C. There are also two kinds of contextual expressions which usage we will show in the following example of a feature model:

We can see that the B is cloneable so there can be more than one copy of B in feature configuration. We want to make sure that D is always present in the configuration when C is. We could simply write this as C implies D, but such expression is always evaluated in global scope. To limit scope of the evaluation, we can use the following contextual expressions:

Example	Description
`forall B: C implies D`	It is evaluated in context of each copy of `B`. Its result is `true` if evaluation for each copy of `B` was `true`.
`exists B: C implies D`	It is evaluated in context of each copy of `B`. Its result is `true` if at least one evaluation for copy of `B` was `true`.

The next table shows results of evaluation of these expresions against the feature configurations below.

Expression	Evaluation Result for #1	Evaluation Result for #2	Evaluation Result for #3
`C implies D`	`true`	`true`	`true`
`exists B: C implies D`	`false`	`true`	`true`
`forall B: C implies D`	`false`	`false`	`true`

Adding New Constraint Language

New constraint language can be added by extending the cz.zcu.yafmt.clang extension point:

<extension point="cz.zcu.yafmt.clang">
  <language id="com.example.myclang"
            name="My Constraint Language"
            shortName="MyCL"
            class="com.example.myclang.MyConstraintLanguage"/>
</extension>

Plug-ins that want to use this extension point must implement the cz.zcu.yafmt.clang.IConstraintLanguage interface or extend the cz.zcu.yafmt.clang.ConstraintLanguage abstract class. See cz.zcu.yafmt.clang.sample package from the cz.zcu.yafmt.clang plug-in (SamCL) for an example implementation. You can also check the cz.zcu.yafmt.clang.bcl plug-in (BoolCL) for more complex example.

Adding Custom Editor for an Existing Constraint Language

Custom editor for an existing constrain language can be added by extending the cz.zcu.yafmt.clang.ui extension point:

<extension point="cz.zcu.yafmt.clang.ui">
  <editingSupport languageId="com.example.myclang"
                  class="com.example.myclang.ui.MyConstraintLanguageEditingSupport"/>
</extension>

Plug-ins that want to use this extension point must implement the cz.zcu.yafmt.clang.ui.IEditingSupport interface or extend the cz.zcu.yafmt.clang.ui.EditingSupport abstract class. See the cz.zcu.yafmt.clang.bcl.ui plug-in (BoolCL UI) for an example implementation.

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