Handling errors

Every operation can fail in a few specific, named ways. Each error has a _tag, so you can handle the ones you care about and let the rest through.

import { Effect } from "effect";
import { fit, prophetFittingBackendLayer } from "effect-prophet";

// A typo: the timestamp is not a full UTC ISO string.
const history = [
  { timestamp: "2025-01-01", value: 10 },
  { timestamp: "2025-01-02T00:00:00.000Z", value: 12 },
];

const message = await Effect.runPromise(
  fit(history).pipe(
    Effect.as("fitted"),
    // Every failure has a `_tag`, so you can handle each kind separately.
    Effect.catchTag("InputValidationError", (error) =>
      Effect.succeed(`Bad input (${error.input}): ${error.message}`),
    ),
    Effect.catchTag("FittingError", (error) =>
      Effect.succeed(`Could not fit (${error.reason}): ${error.message}`),
    ),
    Effect.provide(prophetFittingBackendLayer),
  ),
);

console.log(message);

Bad input (observations): Expected a valid canonical UTC timestamp (for example, 2024-01-01T00:00:00.000Z)
  at [0]["timestamp"]

The errors you may see

Error	Meaning	Typical fix
InputValidationError	The data or options are malformed: a bad timestamp, a missing field, an unknown option	The message and issues point to the exact field
FittingError	The model couldn't be fitted to this data. reason says why, e.g. insufficient-observations or memory-limit.	Check for too few rows or all-identical timestamps; see limits
PredictionError	A forecast couldn't be computed for a row	Check that future rows have every field the model needs
CountryHolidayError	A country calendar couldn't be used. reason says why, e.g. known-coverage-restriction.	See calendar errors
ModelSerializationError	Saved model data couldn't be read	Make sure it was saved by the same version
EvaluationError and friends	Cross-validation, search or holdout checks failed	reason and the wrapped cause explain which step

Without Effect

If you just await Effect.runPromise(...) and an error happens, the promise rejects with that error, so a normal try/catch works:

try {
  const model = await Effect.runPromise(
    fit(history).pipe(Effect.provide(prophetFittingBackendLayer)),
  );
} catch (error) {
  console.error(error);
}