Read the ownership boundary before upgrading an RPC binding
Before upgrading a Haskell Cap’n Proto binding, separate schema-generated types, message data ownership, serialization, and RPC behavior. Then check project status, protocol assumptions, and the API changes an upgrade may introduce.
Start at the project’s own entry point
A binding’s public API is only part of the upgrade question. First find out which repository is authoritative, what functionality is supported, and what caveats the maintainers currently state. The Haskell Cap’n Proto project identifies Codeberg as its official home and describes the GitHub copy as a mirror; it recommends the tutorial as the introduction before consulting reference documentation. Its README also describes the API as unstable, so an upgrade should be treated as a source migration rather than assumed to be a drop-in replacement. [1]
That project note is not a promise about a particular future release. It is a reason to inspect the version you intend to adopt, read the relevant change history, and compile a small representative caller against it. Pinning a dependency version gives a build a defined API surface; it does not remove the need to review changes when deliberately moving that pin.
Generated schema types are one boundary
A schema defines fields, interfaces, and relationships that generated code exposes to the application. That generated layer answers questions such as which accessor represents a field and how a value is built. It does not, by itself, tell you the lifetime of the underlying message storage, whether an object can be retained after a callback, or how remote calls behave. Those questions belong to different boundaries and should be checked separately in the library’s tutorial and API documentation.
A useful migration exercise is to trace one value from construction to use: identify the schema declaration, the generated builder or accessor, the message that owns the bytes, and the point where those bytes are serialized. If a refactor changes a builder type or a message handle, ask whether it changes only syntax or also changes which scope must keep the message alive. This trace keeps a generated API change from being confused with a wire-format change.
Message ownership is not the wire protocol
Serialization turns a message into an encoded representation; RPC adds calls, references, and asynchronous interactions on top. Cap’n Proto’s encoding specification describes a message as a tree of objects and its physical representation as one or more segments. It defines primitive integer encodings as little-endian and places object layout rules in the wire format. Those facts describe serialized bytes, not how a particular Haskell binding manages memory or when a message may be discarded. [3]
Ownership and lifetime questions need to be answered from the binding’s API. For example, if a reader borrows data from a message, retaining the reader beyond the message’s lifetime would be a different hazard from serializing the same data to a byte buffer with independent ownership. Do not infer either behavior from the schema or protocol description alone. In an upgrade, inspect changed types and functions that create, read, or retain messages, then adapt the call sites where the ownership contract actually changed.
RPC adds latency and trust assumptions
Cap’n Proto RPC supports promise pipelining: a client can issue a call using a promised result of an earlier call, allowing dependent requests to travel without waiting for each prior response. The protocol documentation’s filesystem example describes a chain that would ordinarily require several round trips but can be pipelined into one. This is a protocol behavior with performance and interface-design consequences; it is not equivalent to ordinary serialization. [2]
The Haskell project’s README says its Level 1 RPC implementation is alpha quality and warns that it is not robust against resource-exhaustion attacks, discouraging use with untrusted peers. That warning should shape deployment decisions independently of whether a program typechecks after an upgrade. Review both the project’s current status and the protocol behavior your application relies on: identify whether the peer is trusted, whether remote calls can consume unbounded resources, and whether your client handles disconnects and asynchronous failures.
Make the upgrade a boundary-by-boundary review
A practical review separates four questions. Did the generated types change? Did message construction or ownership change? Did encoded schema data or compatibility assumptions change? Did RPC calls, promises, or failure handling change? A source-level fix in one category does not prove the others remain compatible. The protocol specification defines wire behavior, while the binding’s README describes project status and implementation limits; both are needed to reason about an upgrade.
For a pinned version change, make a small inventory of the application’s actual use: schemas, message-building code, readers held across scopes, serialized data persisted or sent elsewhere, and RPC calls to remote peers. Compare those points to the new API and to the project’s status notes. The result is a focused migration plan: update only the affected interfaces, preserve wire assumptions intentionally, and decide whether the RPC risk profile fits the peers involved.