pqi

A driver-agnostic interface to the PostgreSQL libpq API.

Ecosystem

Motivation

Every major Haskell PostgreSQL driver today depends on postgresql-libpq, a binding to the C libpq library. This means every user of every driver needs libpq installed - on their development machine, in CI, in production containers, on cross-compilation targets. There is no way to opt out.

pqi solves this by separating the interface from the implementation. It defines a driver-agnostic type class (IsConnection) that mirrors the libpq API surface, then ships two adapters:

• pqi-ffi - a thin wrapper around postgresql-libpq. Battle-tested, production-safe. The default choice.
• pqi-native - a from-scratch pure-Haskell implementation of the PostgreSQL wire protocol. Alpha. It produces identical output to postgresql-libpq for all protocol-derived values (verified by differential testing), but it has not yet been exercised in production at scale. Because it implements the same pqi interface as pqi-ffi, the two are fully interchangeable - switching between them is a one-line change, so adopting pqi-native now carries no lock-in. If you adopt it, we want to hear from you.

A driver built against pqi gives its users transport choice without any changes to the driver itself. Each adapter package exports a single top-level Adapter value bundling its connection-establishing functions; the user picks one at connection time:

-- C-backed (safe, requires libpq)
connection <- connectdb Pqi.Ffi.adapter settings

-- Pure Haskell (alpha, no C dependency)
connection <- connectdb Pqi.Native.adapter settings

Testing model

pqi comes accompanied by a conformance suite isolated into an implementation-agnostic pqi-conformance package that covers various edge-cases and error conditions and covers most operations with a precondition that they must behave in exactly the same way that postgresql-libpq does.

Interface

pqi reproduces the API surface of the postgresql-libpq package, but reifies the connection - and the results and cancellation handles it produces - as plain records of closures instead of a single concrete type tied to libpq. There is exactly one Connection, one Result, and one Cancel type in the whole package; each field is an IO action that an adapter has already closed over its own underlying handle (a C PGconn pointer, a native socket, etc.). Code written against this interface runs unchanged on any adapter:

• pqi-ffi - a thin adapter backed by the C libpq library via postgresql-libpq.
• pqi-native - a pure-Haskell adapter that speaks the PostgreSQL wire protocol directly.

The interface mirrors libpq in semantics, not just shape: every compliant adapter must produce identical output to libpq for all protocol-derived values. This contract is enforced by pqi-conformance, which runs every operation differentially against postgresql-libpq and asserts equality.

This package ships only the interface: the Connection, Result, and Cancel records, the Adapter type that adapter packages bundle their connection-establishing functions under, and the shared type vocabulary (statuses, field codes, formats, OIDs). It does not itself construct any connections - that’s each adapter package’s job, exposed as a single top-level adapter :: Adapter value.

Relationship to postgresql-libpq

The function names, argument order, and semantics mirror Database.PostgreSQL.LibPQ. The deliberate departures are:

• Connection, Result, and Cancel are plain records of closures rather than a class-parameterised type and its associated types.
• OIDs are a plain Word32 and row/column/parameter indices are a plain Int32, instead of the C-specific newtypes of the original.
• There’s no invalidOid constant. It’s just 0.
• libpqVersion is omitted.
• unescapeBytea and resStatus are fields of Adapter rather than connection-independent top-level functions, since their implementations are adapter-specific.