Amodel visitor can cope reasonably well for simple skeleton code generation from
modeling languages like UML into the object-oriented programming languages it
represents. For DSM languages, single-stage model visitors normally fall short,
unless the output itself is a DSL with a similar semantic structuring to the
TYPES OF GENERATOR FACILITIES 271
models. The basic idea of a model visitor is however useful in all types of generation
facilities.
Model-to-Model Transformations Rather than aiming to produce textual
output, a generator can also create or alter a model. This still leaves us with a model,
which will need a further generator to produce the textual output required by
compilers and other existing implementation tools. It is generally a particularly poor
idea to create model transformations that produce models that users are still expected
to edit, for the same reasons as generated code should not be edited. Some of the issues
with editing generated models in the context of MDAwere covered in Section 3.3.3.
The value of model-to-model transformations is thus better realized when they
form one part of a chain of transformations, resulting eventually in textual output,
and whose intermediate stages are invisible to themodeler. The decision of whether
to generate in one step from models to text, or in several steps with intermediate
transient models, is thus one that can be made by the metamodeler based on the tools
available.
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