Question: Types on interface vs. types on instance

[jloleysens]

Guidance here would be appreciated :)

Current Behavior

Looking at the type declaration of Either in the .d.ts shipped with the npm package I am seeing:

/**
 * @since 2.0.0
 */
export interface Left<E> {
    readonly _tag: 'Left';
    readonly left: E;
}
/**
 * @since 2.0.0
 */
export interface Right<A> {
    readonly _tag: 'Right';
    readonly right: A;
}
/**
 * @since 2.0.0
 */
export declare type Either<E, A> = Left<E> | Right<A>;

But either instance is defined as:

export declare const either: Monad2<URI> & Foldable2<URI> & Traversable2<URI> & Bifunctor2<URI> & Alt2<URI> & Extend2<URI> & ChainRec2<URI> & MonadThrow2<URI>;

Which contains Applicative and so Functor meaning it can map but this code currently contains a type error:

import { Either } from 'fp-ts/lib/Either';
const myValidationHandler = (myEither: Either<any, any>) => {
  myEither./* error --> */map(() => {});
};

Message:

Property 'map' does not exist on type 'Either<any, any>'.
  Property 'map' does not exist on type 'Right<any>'.

Expected behaviour

I would have expected the types as defined on the instance to be shared with the types as defined on the interface (per the example above). Or am I using the interfaces incorrectly? Should I rather be using Functor directly? The map functionality seems to be present at runtime though.

Your environment

Which versions of fp-ts are affected by this issue? Did this work in previous versions of fp-ts?

Software	Version(s)
fp-ts	2.3.1
TypeScript	3.7.2

[mlegenhausen]

const either is defined as Functor not type Either.

Your corrected code is

import { Either, either } from 'fp-ts/lib/Either';
const myValidationHandler = (myEither: Either<any, any>) => {
  either.map(myEither, () => {});
};

[gcanti]

Or

import { Either, map } from 'fp-ts/lib/Either'
import { pipe } from 'fp-ts/lib/pipeable'

const myValidationHandler = (myEither: Either<any, any>) =>
  pipe(
    myEither,
    map(() => {})
  )

if you prefer data-last functions

[jloleysens]

Thanks for the quick responses! My question is more about why the type Either doesn’t have the behavior types attached to it like either instance? Why is type either != type Either?

…

On Thu, 16 Jan 2020 at 12:58, Giulio Canti ***@***.***> wrote: Or import { Either, map } from 'fp-ts/lib/Either'import { pipe } from 'fp-ts/lib/pipeable' const myValidationHandler = (myEither: Either<any, any>) => pipe( myEither, map(() => {}) ) if you prefer data-last functions — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#1092?email_source=notifications&email_token=AB6G67HNJHDJQ57VEGDM3IDQ6BDVLA5CNFSM4KHQ5BTKYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEJD2BUA#issuecomment-575119568>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AB6G67C5PDJV6GU4W3DKUCTQ6BDVLANCNFSM4KHQ5BTA> .

[mlegenhausen]

This was the version 1 behavior of the library where every type was a class with all methods attached. The change was made cause of disadvantages when using classes like serialization, extendibility and tree shaking problems. You can find the full discussion in #823