Generator

Creating a New Generator

A new generator is created by using the New -> Generator command in the language's popup menu.

It is possible to create more than one generator for a language. When a language has only a single generator, using that language implies using its one and only generator to generate code for the model. However, when using languages with multiple generators an extra step is needed by the user to select the generator to trigger. Currently this is done through generator plans.

When creating a new generator module, MPS will also create the generator model 'main@generator' containing an empty mapping configuration node.

Generator Properties

As a module, generator can depend on other modules, have used languages and used devkits (see Module meta-information ).

The generator properties dialog also has two additional properties:

• depends on generators- specifies the dependencies on other generators; this allows the dependent generator to make references to templates defined in another generator;

• mapping constraints- priority relationships between mapping rules can be specified. If such a relationship involves other generator rules, then declaring a dependency on that generator is also required.

Generating Generator

MPS generator engine (or the Generator language runtime) uses mixed compilation/interpretation mode for transformation execution.

Templates are interpreted and filled at runtime, but all functions in rules, macros, and scripts must be pre-compiled.

To avoid any confusion, always follow this rule: after any changes made to the generator model, the model must be re-generated (Shift+F9 ). Even better is to use Ctrl+F9, which will re-generate all modified models in the generator module.

Mapping Configurations

A Mapping Configuration is a minimal unit, which can form a single generation step. It contains #Generator Rules, defines mapping labels and may include pre- and post-processing scripts.

Generator Rules

Applicability of each transformation is defined by generator rules.
There are six types of generator rules:

• conditional root rule

• root mapping rule

• weaving rule

• reduction rule

• pattern rule

• abandon root rule

• drop attribute rule (new in 3.3)

Each generator rule consists of a premise and a consequence (except for the abandon root rule and drop attribute rule, with predefined consequence that cannot be specified by the user).

All rules except for the conditional root rule contain a reference to the concept of the  input node (or just input concept) in its premises. All rule premises also contain an optional condition function.

Rule consequence commonly contains a reference to an external template (i.e. a template declared as a root node in the same or different model) or so-called in-line template (conditional root rule and root mapping rule can only have reference to an external template). There are also several other versions of consequences.

The following screenshot shows the contents of a generator model and a mapping configuration example.

Macros

The code in templates can be parameterized through macros. The generator language defines three kinds of macros:

• property macro- computes a property value;

• reference macro- computes the target (node) of a reference;

• node macro- is used to control template filling at generation time. There are several versions of node macro - LOOP-macro is an example.

Macros implement a special kind of so-called annotation concept and can wrap property, reference or node cells (depending on the kind of macro) in the template code.

Code wrapping (i.e. the creation of a new macro) is done by pressing Ctrl+Shift+M or by applying the 'Create macro' intention.

The following screenshot shows an example of a property macro.

Macro functions and other parameterization options are edited in the inspector view. Property macro, for instance, requires specifying the value function, which will provide the value of the property at generation time. In the example above, output class node will get the same name that the input node has.

The node parameter in all functions of the generator language always represents the context node to which the transformation is currently being applied (the input node ).

Some macros (such as LOOP and SWITCH-macro) can replace the input node with a new one, so that subsequent template code (i.e. code that is wrapped by those macros) will be applied to the new input node.

External Templates

External templates are created as a root node in the generator model.

There are two kinds of external templates in MPS.

One of them is root template. Any root node created in generator model is treated as a root template unless this node is a part of the generator language (i.e. mapping configuration is not a root template ). Root template is created as a normal root node (via Create Root Node menu in the model's popup).

The following screenshot shows an example of a root template.

This root template will transform input node (a Document) into a class (baseLanguage ). The root template header is added automatically upon creation, but the concept of the input node is specified by the user.

It is a good practice to specify the input concept, because this allows MPS to perform static type checking in the code of the macro function.

A Root template (reference) can be used as a consequence in conditional root rules and root mapping rules. ( When used in a conditional root rule, the input node is not available).

The second kind of template is defined in the generator language and its concept name is 'TemplateDeclaration'. It is created via the 'template declaration' action in the Create Root Node menu.

The following screenshot shows an example of template declaration.

The actual template code is 'wrapped' in a template fragment. Any code outside template fragment is not used in transformation and serves as a context (for example you can have a Java class, but export only one of its method as a template).

Template declaration can have parameters, declared in the header. Parameters are accessible through the #generation context.

Template declaration is used in consequence of weaving, reduction and pattern rules. It is also used as an included template in INCLUDE-macro (only for templates without parameters) or as a callee in CALL-macro.

Template Switches

A template switch is used when two or more alternative transformations are possible in a certain place in template code. In that case, the template code that allows alternatives is wrapped in a SWITCH-macro, which has reference to a Template Switch. Template Switch is created as a root node in the generator model via the Create Root Node menu (this command can be seen in the 'menu' screenshot above).

The following screenshot shows an example of a template switch.