Program logics have proven a successful strategy for verification of complex programs. By providing local reasoning and means of abstraction and composition, they allow reasoning principles for individual components of a program to be combined to prove guarantees about a whole program. Crucially, these components and their proofs can be \emph{reused}. However, this reuse is only available once the program logic has been defined. It is a frustrating fact of the status quo that whoever defines a new program logic must establish every part, both semantics and proof rules, from scratch. In spite of programming languages and program logics typically sharing many core features, reuse is generally not available across languages. Even inside one language, if the same underlying operation appears in multiple language primitives, reuse is typically not possible when establishing proof rules for the program logic.

To enable reuse across and inside languages when defining complex program logics (and proving them sound), we serve program logics \emph{à la carte} by combining program logic fragments for the various effects of the language. Among other language features, the menu includes shared state, concurrency, and non-determinism as reusable, composable blocks that can be combined to define a program logic modularly. Our theory builds on ITrees as a framework to express language semantics and Iris as the underlying separation logic; the work has been mechanized in the Coq proof assistant.