When you're working with APIs in the Effect ecosystem, you may come across two different ways to use the same API. These two ways are called the "data-last" and "data-first" variants.

When an API supports both variants, we call them "dual" APIs.

Let's explore these two variants using a concrete example of a dual API: Effect.map.

The Effect.map function is defined with two TypeScript overloads. The terms "data-last" and "data-first" refer to the position of the self argument (also known as the "data") in the signatures of the two overloads:

ts
export declare const map: {
  // data-last
  <A, B>(f: (a: A) => B): <E, R>(self: Effect<A, E, R>) => Effect<B, E, R>
  // data-first
  <A, E, R, B>(self: Effect<A, E, R>, f: (a: A) => B): Effect<B, E, R>
}

ts
export declare const map: {
  // data-last
  <A, B>(f: (a: A) => B): <E, R>(self: Effect<A, E, R>) => Effect<B, E, R>
  // data-first
  <A, E, R, B>(self: Effect<A, E, R>, f: (a: A) => B): Effect<B, E, R>
}

#data-last

In the first overload, the self argument comes in the last position:

ts
<A, B>(f: (a: A) => B): <E, R>(self: Effect<A, E, R>) => Effect<B, E, R>

ts
<A, B>(f: (a: A) => B): <E, R>(self: Effect<A, E, R>) => Effect<B, E, R>

This is the variant we have been using with pipe. You pass the Effect as the first argument to the pipe function, followed by a call to Effect.andThen:

ts
const mappedEffect = pipe(effect, Effect.andThen(func))

ts
const mappedEffect = pipe(effect, Effect.andThen(func))

This variant is useful when you need to chain multiple computations in a long pipeline. You can continue the pipeline by adding more computations after the initial transformation:

ts
pipe(effect, Effect.andThen(func1), Effect.andThen(func2), ...)

ts
pipe(effect, Effect.andThen(func1), Effect.andThen(func2), ...)

#data-first

In the second overload, the self argument comes in the first position:

ts
<A, E, R, B>(self: Effect<A, E, R>, f: (a: A) => B): Effect<B, E, R>

ts
<A, E, R, B>(self: Effect<A, E, R>, f: (a: A) => B): Effect<B, E, R>

This variant doesn't require the pipe function. Instead, you can directly pass the Effect as the first argument to the Effect.andThen function:

ts
const mappedEffect = Effect.andThen(effect, func)

ts
const mappedEffect = Effect.andThen(effect, func)

This variant is convenient when you only need to perform a single operation on the Effect.