Abstract Low-code frameworks strive to simplify and speed up application development. An essential mechanism to achieve these goals is to have native support for the safe reuse and usage of parameterized coarse-grain components, providing developers with strong guardrails and a rich software-building experience. —a rich template language for the OutSystems platform—was designed to simplify the use and creation of such components. Thus, the application developer can quickly reuse and assemble sophisticated and thoroughly tested application blocks. However, without a built-in composition and evolution mechanism, templates are still hard to create and maintain. This sometimes requires the repetition of code across different templates and creates a conflict between the customizations of the instantiated application models, and the update and reapplication of a template definition. This paper presents a principled mechanism for using abstraction in the creation of templates and simultaneously supporting the evolution of templates in applications after use. First, we introduce a template composition mechanism, its typing discipline, and its instantiation algorithm for model-driven low-code development environments. We start by extending to support nested templates and allow the instantiation (hatching) of templates in the definition of other templates. Nesting promotes a significant increase in code reuse potential, leading to a safer evolution of applications. We then introduce the support for customizable template instances, which allows one to evolve templates’ code and then update a template instance without losing customizations performed in the generated code. The present definition seamlessly extends the existing OutSystems metamodel with template constructs expressed by model annotations that maintain backward compatibility with the existing language toolchain. We present the metamodel, a set of annotations to support the extensions, and the corresponding validation and instantiation algorithms. In particular, we introduce a type-based validation procedure for abstractions that ensures that using templates always produces valid models. This work also extends prior developments on Nested OSTRICH with the support for safe customizations of instantiated code. We validate Nested OSTRICH using the benchmark by identifying the degree of reusability that can be reached in the existing sample of real templates and template uses. Our prototype is an extension of the OutSystems IDE that allows the annotation of models and their use to produce new models. We also analyze which existing OutSystems sample screen templates can be improved by using and sharing nested templates .

Low-code frameworks strive to simplify and speed-up application development. Native support for the reuse and composition of parameterised coarse-grain components (templates) is essential to achieve these goals. OSTRICH --- a rich template language for the OutSystems platform --- was designed to simplify the use and creation of such templates. However, without a built-in composition mechanism, OSTRICH templates are hard to create and maintain.

Low-code platforms aim at allowing non-experts to develop complex systems and knowledgeable developers to improve their productivity in orders of magnitude. The greater gain comes from (re)using components developed by experts capturing common patterns across all layers of the application, from the user interface to the data layer and integration with external systems. Often, cloning sample code fragments is the only alternative in such scenarios, requiring extensive adaptation to reach the intended use. Such customization activities require deep knowledge outside of the comfort zone of low-code. To effectively speed up the reuse, composition, and adaptation of pre-defined components, low-code platforms need to provide safe and easy-to-use language mechanisms. This paper introduces OSTRICH, a strongly-typed rich templating language for a low-code platform (OutSystems) that builds on metamodel annotations and allows the correct instantiation of templates. We conservatively extend the existing metamodel and ensure that the resulting code is always well-formed. The results we present include a novel type safety verification of template definitions, and template arguments, providing model consistency across application layers. We implemented this template language in a prototype of the OutSystems platform and ported nine of the top ten most used sample code fragments, thus improving the reuse of professionally designed components.

Context: The OutSystems Platform is a development environment composed of several DSLs, used to specify, quickly build and validate web and mobile applications. The DSLs allow users to model different perspectives such as interfaces and data models, define custom business logic and construct process models. Problem: The DSL for process modelling (Business Process Technology (BPT)), has a low adoption rate and is perceived as having usability problems hampering its adoption. This is problematic given the language maintenance costs. Method: We used a combination of interviews, a critical review of BPT using the "Physics of Notation" and empirical evaluations of BPT using the System Usability Scale (SUS) and the NASA Task Load indeX (TLX), to develop a new version of BPT, taking these inputs and Outsystems' engineers culture into account. Results: Evaluations conducted with 25 professional software engineers showed an increase of the semantic transparency on the new version, from 31% to 69%, an increase in the correctness of responses, from 51% to 89%, an increase in the SUS score, from 42.25 to 64.78, and a decrease of the TLX score, from 36.50 to 20.78. These differences were statistically significant. Conclusions: These results suggest the new version of BPT significantly improved the developer experience of the previous version. The end users background with OutSystems had a relevant impact on the final concrete syntax choices and achieved usability indicators.

Abstract One key aspect of data-centric applications is the manipulation of data stored in persistent repositories, which is moving fast from querying a centralized relational database to the ad-hoc combination of constellations of data sources. The extension of general purpose languages with query operations is increasingly popular, as a tool to improve reasoning and optimizing capabilities of interpreters and compilers. However, not much is being done to integrate and orchestrate different and separate sources of data. We present a data manipulation language that abstracts the nature and location of data-sources. We define its semantics and a type directed query localization mechanism to be used in development tools for heterogeneous environments to efficiently compile them into native queries. We introduce a localization procedure based on rewriting of query expressions that is confluent, terminating and provides the maximum mapping between site capabilities and the structure of the query. We provide formal type safety results that support the sound distribution of query fragments over remote sites. Our approach is also suitable for an interactive query construction environment by rich user interfaces that provide immediate feedback on data manipulation operations. This approach is currently the base for the data layer of a development platform for mobile and web applications.