A Haskell library for the pure core of event sourcing, workflow engines, and durable execution, built on one formalism: the symbolic-register finite-state transducer.

keiki keeps the runtime boundary deliberately small. You describe an aggregate or workflow once as a typed transducer; the library provides forward decisions, replay, acceptors, projections, composition machinery, diagram renderers, and optional symbolic checks from that single declaration.

The name

継起 (keiki) is the Japanese word for "successive occurrence" — events that follow, one after another, each rising out of what came before.

The choice is literal. An event-sourced aggregate is a sequence of events in succession, each one rising out of the state the previous events left behind. keiki names that pattern directly: 継 (the prior event is succeeded by) 起 (the next one rising). The replay fold — reconstitute :: [Event] → State — is exactly the operation the word describes.

(Disambiguation, since the romanisation is ambiguous: this is 継起, not 景気 keiki "economic conditions" nor 計器 keiki "gauge / instrument". Only the 継起 reading carries the "succession of events" sense.)

What it is

A unified treatment of three problems often solved with separate runtime concepts:

1. Event sourcing — store the immutable log of events, replay to recover state.
2. Workflow engines — coordinate long-running multi-step processes.
3. Durable execution — resume cleanly after a crash, mid-process.

keiki models all three as finite-state transducers with a typed register file and predicate-labelled guards: a practical hybrid of Symbolic Finite Transducers and Streaming String Transducers. From one SymTransducer declaration, the library provides:

• strict forward-decision and replay operations with structured failure diagnostics (stepEither, reconstituteEither, and replayEvents),
• input- and output-side Acceptors,
• per-vertex projections (the "B-presentation" view),
• checked sequential composition (composeChecked) plus alternative and feedback1,
• profunctor / Category / Strong / Choice / Arrow instances,
• behavior-readable Mermaid and Markdown renderers, with explicit compact topology output,
• eager builder validation and default-on replay-safety checks, plus opt-in SBV + z3 checks for single-valuedness and output-dependent replay inversion,
• trusted Generic/TH wire schemas that align event fields structurally for replay proofs without treating diagnostic constructor names as evidence.

delta / omega / applyEvent use concrete predicate evaluation — no solver in the per-event hot path. Solver dispatch is reserved for build-time analysis.

Status

Pre-1.0 and released under the Haskell PVP. The current surface is validated against the in-tree suites plus the downstream jitsurei worked-example package; see CHANGELOG.md for release history and migrations.

The core package is intentionally codec-free. JSON support lives in keiki-codec-json, and downstream codec testing helpers live in keiki-codec-json-test. The JSON event codec supports pinned wire kinds, in-band schema versions, and explicit upcaster chains for persisted-event evolution.

Build

Requires GHC 9.12 and (for the symbolic analyses only) z3 on PATH.

nix develop          # provides ghc 9.12, cabal, z3
cabal build all
cabal test all

Pure evaluation (delta, omega, step, reconstitute) does not depend on z3. The symbolic checks in Keiki.Symbolic will fail loudly if z3 is missing when invoked.

A taste

The smallest useful aggregate, in full:

emailDelivery
  :: SymTransducer (HsPred EmailRegs EmailCmd)
                   EmailRegs EmailVertex EmailCmd EmailEvent
emailDelivery = B.buildTransducer EmailPending emptyRegFile
                  (\case EmailSentVertex -> True; _ -> False) do
  B.from EmailPending do
    B.onCmd inCtorSendEmail $ \d -> B.do
      B.slot @"emailRecipient" .= d.recipient
      B.slot @"emailSubject"   .= d.subject
      B.slot @"emailSentAt"    .= d.at
      B.emit wireEmailSent EmailSentTermFields
        { recipient = d.recipient
        , subject   = d.subject
        , at        = d.at
        }
      B.goto EmailSentVertex

Forward decisions, structured replay, Acceptors, and the per-vertex view all operate from this one declaration. See the downstream jitsurei package for the full worked aggregates the test suite drives.

The primary toMermaid emailDelivery diagram is derived from the same edge AST and shows its command, event constructor, complete register assignments, and guard. Use toTopologyMermaid emailDelivery when only shape is intended. Ordinary lit values appear in readable diagrams; model a value with opaqueLit when it lacks Show or must remain redacted.

Repository documentation

The repository includes user guides, design notes, and implementation history:

For a status snapshot — what's implemented, what's next, and what's intentionally out of scope — see ROADMAP.md.

Recommended shortest path:

1. docs/foundations/00-reading-guide.md — orientation.
2. docs/guide/user-guide.md — write your first aggregate.
3. docs/research/synthesis-c-foundation-b-presentation-with-worked-examples.md — the design baseline.

License

BSD-3-Clause. See keiki.cabal for package metadata.