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Pressline

Effect primer

Pressline’s server is Effect throughout. What you meet in the code:

  • Effect<A, E, R>: a description of a computation producing A, failing with typed E, needing services R. Nothing runs until a runtime runs it. Effect.gen(function* () { const x = yield* step; … }) sequences steps.
  • Services are Context.Tags (Db, Psp, FulfillmentProvider, Mailer, DesignSource, Config); Layers build them. Tests provide in-memory layers; production provides adapters. The web handler is built once from a Layer of services. The service-layer view draws the five services and their adapters.
  • Errors are Schema.TaggedError classes; the HTTP layer maps each to a status. Effect.either when you want to inspect a failure; Effect.orDie for infrastructure faults that are bugs, not outcomes.
  • Schema validates every boundary: config, env, API bodies, database rows, provider wire shapes.
  • HttpApi: endpoints are declared once (api.ts), implemented by groups, served by HttpApiBuilder.toWebHandler, and the same declaration produces the OpenAPI document. The Engine-facing contract is its own HttpApi in @pressline/contract, from which the Engine client is derived.
  • Clock: time comes from Effect’s Clock, so tests advance it.

Read docs/adr/0011-effect-throughout.md for why.