We present a template composition mechanism and its typing and instantiation algorithms for model-driven low-code development environments. We evolve OSTRICH a rich template language for the OutSystems platform to support nested templates. We allow the instantiation (hatching) of templates in the definition of other templates, thus significantly increasing the potential for code reuse leading to a safer evolution of applications. The present definition seamlessly extends the existing OutSystems metamodel with template constructs expressed by (innocuous) model annotations, thus maintaining backward compatibility with the existing language toolchain. We present the metamodel, its annotations, and the corresponding validation and instantiation algorithms. In particular, we introduce a type-based validation procedure that ensures that using a template inside a template produces valid models.

Our presentation is driven by a running example that starts with an application illustrating a common code pattern, a screen displaying a database table. We progressively use compositions of focused templates that generalise the pattern making the code modular and reusable. The initial application, and its variants, can be readily instantiated by using the presented template composition mechanism.

The work is validated and compared to the state of the art using the OSTRICH benchmark. Our prototype is an extension of the OutSystems IDE allowing the annotation of models and their use to produce new models. We also analyse which existing OutSystems sample screens templates can be improved by using and sharing nested templates.