fairstream

Simple fair and terminating backtracking Monad Transformer

Backtracking, Interleaving, and Terminating Monad Transformers

Motivation

The problem with depth-first search flatMap of fs2.Stream and standard library's collection is that when you nest infinite streams, it gets stuck exploring the first branch forever:

valnumber= fs2.Stream.iterate(1)(_ +1) // 1, 2, 3, ...valtriples=for {
i <- number
j <- number // stuck here: tries j=1,2,3,... for i=1 forever
k <- number
if i * i + j * j == k * k
} yield (i, j, k)
triples.take(1).toList // never terminates

Fair uses fair disjunction (mplus) instead of simple concatenation. It interleaves branches so that every candidate is eventually reached:

importcom.codiff.fairstream.Fairimportcom.codiff.fairstream.Fair._lazyvalnumber:Fair[Int] = mplus(unit(0), number.map(_ +1))
valtriples=for {
i <- number
_ <- guard(i >0)
j <- number
_ <- guard(j >0)
k <- number
_ <- guard(k >0)
_ <- guard(i * i + j * j == k * k)
} yield (i, j, k)
Fair.runM(None, Some(5), triples)
// List((3,4,5), (4,3,5), (6,8,10), (8,6,10), (5,12,13))

The fairstream-fs2 allows converting Fair/FairT to fs2.Stream:

importcom.codiff.fairstream.fs2.syntax._// Fair[A] => fs2.Stream[Pure, A]valstream= triples.toFs2.take(10).toList
// FairT[IO, A] => fs2.Stream[IO, A]valioStream= compatible.toFs2.take(5).compile.toList