A code fragment in the context of https://github.com/ps-mr/LinqOnSteroids/

Optimize.scala

import ivm._
import expressiontree._
import Lifting._

import collection.generic.CanBuildFrom
import collection.TraversableLike

object OptimizationV1 {
  val mergeMaps: PartialFunction[Exp[_], Exp[_]] = {
    case MapNode(m@MapNode(coll, f), g) =>
      (coll map (f andThen g))(m.cbf)  //Here, g andThen f would be accepted by the compiler.
  }
}

object OptimizationV2 {
  //Some typechecking:
  private def buildMergedMaps[T, U, V](coll: Exp[Traversable[T]], f: Fun[T, U], g: Fun[U, V]): Exp[Traversable[V]] =
    (coll map (f andThen g)) //Here, g andThen f would be rejected.

  val mergeMaps: PartialFunction[Exp[_], Exp[_]] = {
    case MapNode(m @ MapNode(coll, f), g) =>
      buildMergedMaps(coll, f, g)
  }
}

object OptimizationV3 {
  private def buildMergedMaps[
    T, U, V, To <: Traversable[V] with TraversableLike[V, To]](coll: Exp[Traversable[T]],
                                                               f: Fun[T, U], g: Fun[U, V],
                                                               cbf: CanBuildFrom[Nothing, V, To with TraversableLike[V, To]]): Exp[To] =
    coll map(f andThen g)(collection.breakOut(cbf))

  val mergeMaps: PartialFunction[Exp[_], Exp[_]] = {
    case m @ MapNode(MapNode(coll, f), g) =>
      buildMergedMaps(coll, f, g, m.c)
  }
}