Helpful editor support for textual languages including programming languages and domain-specific languages requires, among others, validation hints to visualize violated constraints during editing. In contrast to syntactic checks, type checking validates some static semantics of the written text, e.g. that expressions used in if-conditions are evaluated to boolean values and not to integer values. While the syntactic constraints are often defined with grammars, type checking requires type systems in order to annotate parts of the written text with type information on which constraints are evaluated. Complex type systems often occur in textual languages, but graphical languages benefit from type checking of the underlying data model as well. In order to simplify the development of such type systems, Typir is introduced as the new open-source framework for type checking in the web. Implemented in TypeScript, Typir is prepared for integration in the web stack, including web-based frameworks for graphical notations (Eclipse Sprotty, Eclipse GLSP) and textual languages (Eclipse Langium), while running in web-based IDEs like Eclipse Theia or VS Code. After motivating the needs for type checking and the challenges for implementing type systems in general, this talks shows examples, how to apply Typir in practice to provide useful validation hints in editors without developing type systems from scratch for each new language. The aim of this presentation is to share the first results of Typir with the community and to discuss the needs of the community for type checking in order to improve Typir and the whole language engineering ecosystem.
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