`bindTo` messes up the monadic index

🐛 Bug report

bindTo messes up the monadic index. For example, consider a broader monadic index that takes you through AuthenticationOpen to AuthorizationOpen and then StatusOpen and so forth.

authentication middleware:

declare const authenticate: Middleware<AuthenticationOpen, AuthorizationOpen, unknown, AuthenticatedUser>

one of many authorization middlewares

declare const userHasReadAccess: (u:AuthenticatedUser) => Middleware<AuthorizationOpen, StatusOpen, unknown, AuthorizedUser>

const myController = pipe(
  authenticate,
  H.ichain(userHasReadAccess),
  H.bindTo('user'), // <-- property AuthenticationOpen is missing in type StatusOpen . . .
  H.bind('query', () => H.decodeQuery(/* . . . */)),
  /* . . . */

bindTo takes Middleware<I,I,E,A> and returns Middleware<I,I,E,{ ... }> but it should take Middleware<I,O,E,A> because not every middleware passed into bindTo will have the same "in’ and “out” index.

Your environment

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

Software	Version(s)
fp-ts	2.10.5
hyper-ts	0.6.5
TypeScript	3.9.2

[DenisFrezzato]

can you clarify the difference between bind and ibind then?

It’s the same difference between chain and ichain (since bind is just a utility around chain). ichain is the indexed version of chain, which additionally updates the indexes.

honestly I only ever use ichain and never chain

You should use chain when the operation doesn’t change the indexes.

import * as H from 'hyper-ts'
import * as M from 'hyper-ts/lib/Middleware'
import { pipe } from 'fp-ts/function'
import * as t from 'io-ts'


declare function sendStatus<E>(
  status: H.Status,
): M.Middleware<H.StatusOpen, H.ResponseEnded, E, void>

const ReqBody = t.type({ name: t.string })

const userHandler = pipe(
  M.decodeBody(ReqBody.decode),
  // Chain a new Middleware, same indexes.
  M.chain((user) => user.name === 'Denis' ? M.right(user) : M.left('Wrong name')),
  // Chain a new Middleware with different indexes.
  M.ichain(() => sendStatus(H.Status.NoContent)),
  M.orElse(() => sendStatus(H.Status.BadRequest))
)

Also, I’ll do a PR for map.

map is fine, we need an indexed version imap for that too, see https://hackage.haskell.org/package/indexed-0.1.3/docs/Data-Functor-Indexed.html or https://pursuit.purescript.org/packages/purescript-indexed-monad.

[kylegoetz]

@DenisFrezzato I’m closing my issue since it appears you’ve implemented this in a forthcoming version.