Side Effect on Errors | Effect Http Client

Sometimes you don’t want to recover from an error. You just want to log it, send it to an error tracking service, increment a metric, then let it continue propagating.

HttpClient.tapError() runs a side effect when an error occurs, without changing the error or preventing it from propagating.

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import {
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  FetchHttpClient,
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  HttpClient,
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  HttpClientRequest,
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} from "effect/unstable/http";
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import { Effect } from "effect";
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function fetchUserWithErrorTracking(userId: number) {
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  return Effect.gen(function* () {
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    const client = (yield* HttpClient.HttpClient).pipe(
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      HttpClient.filterStatusOk,
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      HttpClient.tapError((error) =>
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        Effect.sync(() => {
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          // Simulate sending to an error tracking service
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          console.log("[ErrorTracker] Reported:", error._tag, error.message);
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        }),
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      ),
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    );
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    const request = HttpClientRequest.get(
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      `https://dummyjson.com/users/${userId}`,
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    );
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    const response = yield* client.execute(request);
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    const user = yield* response.json;
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    return user;
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  }).pipe(Effect.provide(FetchHttpClient.layer));
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}
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// Test with an invalid user — error is tracked, then still propagates
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Effect.runPromise(fetchUserWithErrorTracking(9999)).then(
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  (user) => console.log("User:", user.firstName),
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  (error) => console.log("[Main] Unhandled error:", error.message),
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);

Output:

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[ErrorTracker] Reported: HttpClientError StatusCode: non 2xx status code (404 GET https://dummyjson.com/users/9999)
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[Main] Unhandled error: StatusCode: non 2xx status code (404 GET https://dummyjson.com/users/9999)

Notice both lines print. The tapError callback runs first, then the error continues to propagate. It’s not swallowed.

tapError is particularly useful for observability. You can add it to a client once, and every request through that client will have its errors observed without affecting the error handling strategy of the calling code.

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