Effect in five minutes

Effect Prophet is built on Effect. Here is everything you need to know to use it, even if you have never used Effect before.

An Effect is a description of work

Calling fit(history) does not fit anything yet. It returns an Effect: a value that describes the work to be done. Nothing happens until you run it.

const fitting = fit(history); // nothing has happened yet

Running an Effect

The simplest way to run an Effect is Effect.runPromise, which returns an ordinary Promise:

const model = await Effect.runPromise(fitting.pipe(Effect.provide(prophetFittingBackendLayer)));

If you only use plain async/await code, this is all you need: wrap each call in Effect.runPromise(...) and await it.

Providing the engine

fit needs a forecasting engine, the compiled WebAssembly code that does the math. You supply it with Effect.provide(prophetFittingBackendLayer). In Effect, a bundle of ready-made services like this is called a layer.

Functions that don't need the engine, such as predict and the save/load helpers, can be run without it.

Doing several steps at once

To fit and then predict in one go, use Effect.gen. Inside it, yield* waits for each step, like await does in an async function:

const program = Effect.gen(function* () {
  const model = yield* fit(history);
  const forecast = yield* predict(model, futureDates);

  return forecast;
});

const forecast = await Effect.runPromise(program.pipe(Effect.provide(prophetFittingBackendLayer)));

Most examples on this site use this pattern.

Errors are values

When something goes wrong, for example a malformed timestamp, Effect Prophet doesn't throw a surprise exception from deep inside. Each operation lists the errors it can fail with, and you can handle each kind by name. See Handling errors.

If you don't handle them, Effect.runPromise rejects the promise with the error, just like a normal async function would.

Want to learn more?

The Effect documentation is a good place to start. None of it is required to use this library.