ServerFn

EFFRONT.ServerFn.make creates a React Server Function with Schema-decoded arguments and an Effect handler.

ServerFn.make

EFFRONT.ServerFn.make({ input, handler }) returns a function with the caller signature (...encodedArgs) => Promise<Output>. Export it from a "use server" module:

"use server";

import { Effect, Schema } from "effect";
import { EFFRONT } from "./effront";

export const describe = EFFRONT.ServerFn.make({
  input: [Schema.FiniteFromString, Schema.String],
  handler: (count, label) => Effect.succeed({ count, label }),
});

./effront exports the application's shared factory. A client call with "2" and "items" resolves to { count: 2, label: "items" }.

AvailableServices includes application services and services from the factory's middleware scope. The successful handler value determines Output. It is not re-encoded with the input Schema. Inputs and outputs must satisfy React's serialization contract.

For React components that call the function, see Mutation Server Functions.

Argument shapes

An array of Schemas describes positional arguments. An array or tuple Schema describes one argument. A single decoder ignores extra native arguments and decodes undefined when its argument is omitted. A zero-argument function instead rejects extra native arguments, including an explicit undefined, before running its handler.

useActionState supplies previous state followed by FormData:

"use server";

import { Effect, Schema } from "effect";
import { EFFRONT } from "./effront";

export const update = EFFRONT.ServerFn.make({
  input: [
    Schema.Struct({ count: Schema.Finite }),
    Schema.fromFormData(Schema.Struct({ name: Schema.NonEmptyString })),
  ],
  handler: (previousState, form) =>
    Effect.succeed({ count: previousState.count + 1, name: form.name }),
});

All positional arguments are decoded before the handler runs. Input decoding failures and uncaught handler failures reject the invocation, rather than becoming Output or the next action state. Return a serializable result for expected domain failures that should appear as form state.

Schema decoding does not establish identity or permissions. Record IDs, hidden fields, and previousState remain client-supplied values.

Execution constraints

A function created from EFFRONT.withMiddleware(...) retains that application's identity and middleware chain. The function's middleware wraps execution and the refreshed response. See Authentication and authorization for independent page and function access checks.

Direct invocation with await in RSC or other server code rejects with TypeError. Extract work needed by other server code into a regular Effect and call it from both places.

Request protocol

Server Function POST requests use the shared HTTP body-size and Content-Length limits, also documented for the Fetch handler. For browser calls, Effront passes arraySizeLimit: 10_000 to React's argument decoder.

The Origin check compares URL host, including any port, rather than the full origin or scheme. The HTTP Content-Length check runs before the Server Function checks. A body that passes the header check can still exceed the received-byte limit and produce 400.

Application input Schema failures are distinct from React protocol decoding failures. For browser calls, Effront includes input Schema and handler failures in the function result. The Flight response has status 200 if the refreshed page renders successfully. The failed result rejects the invocation before Effront applies the refreshed page.