There is no instance of Composite[List[A]] where A is a case class

[asoltysik]

Doobie version: 0.5.0

import doobie._
import doobie.implicits._
import doobie.postgres.implicits._

case class Foo(a: Int)

Composite[List[Foo]]

Errors:

Could not find or construct Composite[List[Foo]].

  If List[Foo] is a simple type (or option thereof) that maps to a single column, you're
  probably missing a Meta instance. If List[Foo] is a product type (typically a case class,
  tuple, or HList) then probably one of its component types is missing a Composite instance. You can
  usually diagnose this by evaluating Composite[Foo] for each element Foo of the product type in
  question. See the FAQ in the Book of Doobie for more hints.

not enough arguments for method apply: (implicit A: doobie.util.composite.Composite[List[Foo]])doobie.util.composite.Composite[List[Foo]] in object Composite.
Unspecified value parameter A.

[wedens]

Which column type do you want to map to List?

You can probably do that for arrays. There are instances for postgres arrays of some primitive types (int4, bit and some others). In this case you can probably do val x: Meta[List[Foo]] = Meta[Array[Int]].xmap(_.toList.map(Foo(_)), _.toArray.map(_.a)).

[tpolecat]

Given Meta[List[A]] and Generic[B] { type Repr = A :: HNil } we can derive Meta[List[B]] but it's marginal enough that I don't think I want to add the derivation implicitly. I think the answer is xmap, at least for now.