Multi-stage programming (MSP) holds great promise, allowing the reliable generation of specialized, partially-evaluated
code with static type- and scope-safety guarantees. Yet, we
argue that MSP has not reached its full potential yet, as it has
been traditionally limited to generating expressions, and has
lacked principled facilities for generating modular programs
and data structures. In that sense, we argue that MSP has
been reserved for programming “in the small,” focused on
generating efficient kernels of computation on the scale of
single function bodies. We present a novel technique called
staged classes, which extends MSP with the ability to manipulate class definitions as first-class constructs. This lets
programmers use MSP “in the large,” on the level of applications, rather than mere functions. This way, applications
can be designed in an abstract and modular way without
runtime cost, as staged classes guarantee the removal of all
staging-time abstractions, resulting in the generation of efficient specialized modules and data structures. We describe
the design of a prototype relational database system in Scala,
which uses several stages of runtime compilation to maximize the efficiency of query execution and data storage. We
also show that staged classes can be used for defining type- and scope-safe implementations of type providers.