Prepending Transforms | Effect Http Client

HttpClient.mapRequestInput() is similar to mapRequest, but it prepends the transformation instead of appending it. The difference matters when you stack multiple transforms.

Consider this scenario: you have a client with a base URL transform (appended via mapRequest), and you want to add another transform that modifies the URL path. The order of execution determines the final URL.

1
import {
2
  FetchHttpClient,
3
  HttpClient,
4
  HttpClientRequest,
5
} from "effect/unstable/http";
6
import { Effect } from "effect";
7

8
function demonstrateOrder() {
9
  return Effect.gen(function* () {
10
    // mapRequest APPENDS — runs after existing transforms
11
    const client1 = (yield* HttpClient.HttpClient).pipe(
12
      HttpClient.mapRequest(
13
        HttpClientRequest.prependUrl("https://dummyjson.com"),
14
      ),
15
      // This runs AFTER prependUrl, so it sees the full URL
16
      HttpClient.mapRequest((request) => {
17
        console.log("[mapRequest] URL is:", request.url);
18
        return request;
19
      }),
20
    );
21

22
    // mapRequestInput PREPENDS — runs before existing transforms
23
    const client2 = (yield* HttpClient.HttpClient).pipe(
24
      HttpClient.mapRequest(
25
        HttpClientRequest.prependUrl("https://dummyjson.com"),
26
      ),
27
      // This runs BEFORE prependUrl, so it sees only the path
28
      HttpClient.mapRequestInput((request) => {
29
        console.log("[mapRequestInput] URL is:", request.url);
30
        return request;
31
      }),
32
    );
33

34
    console.log("--- Using mapRequest ---");
35
    yield* client1.get("/users/1").pipe(Effect.ignore);
36

37
    console.log("\n--- Using mapRequestInput ---");
38
    yield* client2.get("/users/1").pipe(Effect.ignore);
39
  }).pipe(Effect.provide(FetchHttpClient.layer));
40
}
41

42
// Test it
43
Effect.runPromise(demonstrateOrder());

Output:

1
--- Using mapRequest ---
2
[mapRequest] URL is: https://dummyjson.com/users/1
3

4
--- Using mapRequestInput ---
5
[mapRequestInput] URL is: /users/1

With mapRequest, the log sees the full URL because it runs after prependUrl. With mapRequestInput, the log sees only /users/1 because it runs before prependUrl.

Use mapRequestInput when you need a transform to run before all other registered transforms. There’s also a mapRequestInputEffect variant for effectful prepended transforms.

In most cases, mapRequest is what you want. Reach for mapRequestInput only when execution order matters.

Stay in the loop