# fn.Gen

Contract for functions whose body yields Effects and returns an `Fx`.

## Signatures

```ts
type Gen = {
    <Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>>(body: (this: unassigned, ...args: Args) => Generator<Eff, ReturnFx>): (...args: Args) => Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>;
    <Self, Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>>(body: (this: Self, ...args: Args) => Generator<Eff, ReturnFx>): (this: Self, ...args: Args) => Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>;
    <Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A>(body: (this: unassigned, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A): (...args: Args) => A;
    <Self, Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A>(body: (this: Self, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A): (this: Self, ...args: Args) => A;
    <Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A, B>(body: (this: unassigned, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A, b: (_: A, ...args: Args) => B): (...args: Args) => B;
    <Self, Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A, B>(body: (this: Self, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A, b: (_: A, ...args: Args) => B): (this: Self, ...args: Args) => B;
    <Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A, B, C>(body: (this: unassigned, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A, b: (_: A, ...args: Args) => B, c: (_: B, ...args: Args) => C): (...args: Args) => C;
    <Self, Eff extends Effect.Effect<any, any, any>, ReturnFx extends Fx.Any, Args extends Array<any>, A, B, C>(body: (this: Self, ...args: Args) => Generator<Eff, ReturnFx>, a: (_: Fx<Fx.Success<ReturnFx>, Fx.Error<ReturnFx> | Effect.Error<Eff>, Fx.Services<ReturnFx> | Effect.Services<Eff>>, ...args: Args) => A, b: (_: A, ...args: Args) => B, c: (_: B, ...args: Args) => C): (this: Self, ...args: Args) => C;
};
```

## Why

`Gen` combines services and errors from yielded Effects with those of the returned
`Fx`, while retaining the original arguments and `this` type. Optional pipeable
arguments transform the resulting `Fx` without hiding those channels.

## Ownership and lifetime

Defining the function is inert. The body and yielded Effects execute once per run
of the `Fx` returned by an invocation; that run owns acquisition and interruption.

## Examples

```ts
import { Effect } from "effect"
import { Fx } from "@typed/fx"

const item = Fx.fn(function* (id: number) {
  const prefix = yield* Effect.succeed("item")
  return Fx.succeed(`${prefix}:${id}`)
})
const program = Fx.collectAll(item(1))
```