We present the concept of typelets, a specification technique for
dynamic graphical user interfaces (GUIs) based on types. The
technique is implemented in a dialect of ML, called MLFi (MLFi is
a derivative of OCaml, extended by LexiFi with extensions
targeted at the financial industry), which supports dynamic
types, for migrating type-level information into the object
level, so-called type properties, allowing easy specification of,
for instance, GUI control attributes, and type paths, which
allows for type-safe access to type components at runtime.
Through the use of Hindley-Milner style type-inference in MLFi,
the features allow for type-level programming of user
interfaces. The dynamic behavior of typelets are specified using
declarative rules. The technique extends the flat spreadsheet
programming model with higher-order rule composition techniques,
extensive reuse, and type safety. A layout specification language
allows layout programmers (e.g., end-users) to reorganize layouts
in a type-safe way without being allowed to alter the rule
machinery. The resulting framework is highly flexible and allows
for creating highly maintainable modules. It is used with success
in the context of SimCorp's high-end performance-critical
financial asset-management system with screens containing several
hundreds of GUI controls located in group-boxes, sub-tabs, and
menu structures and with very complex dependency structures
defined using declarative rule composition.