scala - Combinatorial explosion of implicit objects -

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i have case class, this:

case class container[t, m](value: t, modifier: m)

i want make possible have ordering on container t have ordering too. don't want make t <: ordered[t], because container should able contain non-orderable values well.

my first try implement ordered, this:

case class container[t, m](value: t, modifier: m) extends ordered[container[t, m]] {   override def compare(that: container[t, m])(implicit ev: ordering[t]): int = ev.compare(value, that.value) }

but doesn't work: because of implicit parameter, compare no longer implements trait.

so decided try type class approach:

class containerordering[t, m](implicit ev: ordering[t]) extends ordering[container[t, m]] {   override def compare(x: container[t, m], y: container[t, m]): int = ev.compare(x.value, y.value) } implicit object containerorderingint extends containerordering[int, int]

this works (if import ordering.implicits._), have new problem: every type m, need separate implicit object. there aren't many types want use in m position (in fact, derive sealed trait define elsewhere), still means combinatorial explosion of implicit objects need define. i'm ok having separate implicit object each t, m should orthogonal this.

there must better way this. maybe can leverage fact m extends sealed trait. however, haven't been able so. advice?

put ordering companion object!

object container {     implicit def ordering[t: ordering, m] = new ordering[container[t, m]] {         override def compare(x: container[t, m], y: container[t, m]): int =             implicitly[ordering[t]].compare(x.value, y.value)     } }

now can use ordering using import container._ needed.

(ps: real trick of course make ordering implicit free defining every single ordering each possible type.)


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