Session types describe structured communication on heterogeneously
typed channels at a high level.
Their tail-recursive structure imposes a protocol that can be
described by a regular language.
The types of transmitted values are drawn from the underlying
functional language, abstracting from the
details of serializing values of structured data types.

Context-free session types extend session types by allowing nested
protocols that are not restricted to tail recursion. Nested protocols
correspond to deterministic context-free languages. Such protocols are
interesting in their own right, but they are particularly suited to
describe the low-level serialization of tree-structured data in a
type-safe way.

We establish the metatheory of context-free session types, prove that
they properly generalize standard (two-party) session
types, and take first steps towards type checking by showing
that type equivalence is decidable.