How to convert a Seq[Reader[E, A]]) into a Reader[E, Seq[A]]

Question

Here is my actual solution

  private def transpose[E, A](readers : Seq[Reader[E, A]]) : Reader[E, Seq[A]] =
    Reader { e: E => readers.map { r => r(e) } }

Is there a more straightforward solution with scalaz (maybe with an existing combinator) ?

score 5 · Accepted Answer · edited May 23 '17 at 10:28

5

This operation is essentially sequence:

import scalaz.Reader, scalaz.std.list._, scalaz.syntax.traverse._

def transpose[E, A](readers: List[Reader[E, A]]): Reader[E, List[A]] =
  readers.sequenceU

In general if M has a Traverse instance and N is a monad, you can transform M[N[A]] into an N[M[A]] in this way (see my answer here for some additional details).

Note that sequence won't work with Seq, since Scalaz doesn't provide a Traverse instance for Seq (although it does for List, Vector, etc.). You can write your own, but I'd suggest not doing that.

(As a footnote, the U at the end of sequenceU here is just part of a hack to help out Scala's type inference—see my blog post here for some background.)

edited May 23 '17 at 10:28

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answered Nov 30 '15 at 21:35

Travis Brown

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I didn't know that I can use `sequence` with a monad that has a type constructor with 2 types. Thanks ! – Yann Moisan Nov 30 '15 at 21:56
`sequenceU` doesn't have the expected return type : `G.M[List[G.A]]` – Yann Moisan Nov 30 '15 at 22:08
@YannMoisan What do you mean? In this case `List` _is_ the `G.A`. – Travis Brown Nov 30 '15 at 23:30
My bad, it is just a false negative in my IDE (InteliiJ). – Yann Moisan Dec 01 '15 at 05:51

How to convert a Seq[Reader[E, A]]) into a Reader[E, Seq[A]]

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