Scala: SeqT monad transformer?

Question

If we have such two functions...

def findUserById(id: Long): Future[Option[User]] = ???
def findAddressByUser(user: User): Future[Option[Address]] = ???

...then we are able to use cats OptionT monad transformer to write for-comprehension with them easily:

for {
  user    <- OptionT(findUserById(id))
  address <- OptionT(findAddressByUser(user))
} ...

I'd like to compose future of sequences this way, like this:

def findUsersBySomeField(value: FieldValue): Future[Seq[User]] = ???
def findAddressesByUser(user: User): Future[Seq[Address]] = ???

for {
  user    <- SeqT(findUsersBySomeField(value))
  address <- SeqT(findAddressesByUser(user))
} ...

But I can't find any SeqT implementation in Cats or Scalaz. Does some implementation of such monad transformer exist or I need to write monad transformer myself? Not that it too hard, just don't want to reinvent the wheel.

(example at the beginning of my question is brought from this article)

Scalaz has `ListT`. Cats had nothing like that last time I checked (which was 0.9.0) — Oleg Pyzhcov, Aug 29 '17 at 13:20
See https://stackoverflow.com/questions/12617916/why-is-listt-monad-transformer-considered-buggy-what-monad-laws-it-breaks — Luka Jacobowitz, Aug 29 '17 at 13:30
@LukaJacobowitz, linked question is about monad transformer from Haskell standard library. My question is about Scala. How can this link be helpful for me here? — Evgeny Veretennikov, Aug 29 '17 at 13:41
@OlegPyzhcov thanks, it helped! You could post it as answer. — Evgeny Veretennikov, Aug 29 '17 at 13:57
The same problems in Haskell also apply to Scala. Here's a link to the issue in the scalaz repo: https://github.com/scalaz/scalaz/issues/921 — Luka Jacobowitz, Aug 29 '17 at 14:22
@LukaJacobowitz they write that ListT works only if argument monad is commutative. But Future IS commutative monad, am I right? So, I can use ListT in my case. Thanks for the tip, anyway. — Evgeny Veretennikov, Aug 29 '17 at 14:37

score 3 · Accepted Answer · answered Aug 29 '17 at 15:05

Cats, as of 1.0.0-MF have nothing of sorts. That is explained at their FAQ:

A naive implementation of ListT suffers from associativity issues; see this gist for an example. It's possible to create a ListT that doesn't have these issues, but it tends to be pretty inefficient. For many use-cases, Nested can be used to achieve the desired results.

Scalaz, as of 7.2.15 has StreamT and ListT, although the latter has associativity issues.

Scala: SeqT monad transformer?

1 Answers1

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