Semantic specifications of programming languages typically have poor modularity.
This hinders reuse of parts of the semantics of one language when specifying a
different language --- even when the two languages have many constructs in common
--- and evolution of a language may require major reformulation of its semantics.
Such drawbacks have discouraged language developers from using formal semantics to
document their designs.

In the PLanCompS project, we are developing a component-based approach to semantics.
Our approach is based on reusable fundamental constructs (funcons), which provide
good modularity and support co-evolution of languages and their formal semantics.
In this talk I will overview the funcon approach, and present some exemplary
funcons and semantic definitions.