Allow constructing Read/Write from a list of underlying instances

Author: jatcwang

modules/core/src/main/scala/doobie/util/read.scala

@@ -12,6 +12,7 @@ import doobie.free.resultset as IFRS
 
 import java.sql.ResultSet
 import scala.annotation.implicitNotFound
+import scala.collection.immutable.ArraySeq
 
 @implicitNotFound("""
 Cannot find or construct a Read instance for type:
@@ -119,7 +120,7 @@ object Read extends LowerPriority1Read {
     override lazy val length: Int = underlyingRead.length
   }
 
-  /** A Read instance consists of multiple underlying Read instances */
+  /** A Read instance consists of two underlying Read instances */
   class Composite[A, S0, S1](read0: Read[S0], read1: Read[S1], f: (S0, S1) => A) extends Read[A] {
     override def unsafeGet(rs: ResultSet, startIdx: Int): A = {
       val r0 = read0.unsafeGet(rs, startIdx)
@@ -145,6 +146,26 @@ object Read extends LowerPriority1Read {
     override lazy val length: Int = read0.length + read1.length
   }
 
+  class CompositeOfList(readInstances: ArraySeq[Read[Any]]) extends Read[ArraySeq[Any]] {
+    override def unsafeGet(rs: ResultSet, startIdx: Int): ArraySeq[Any] = {
+      var columnIdx = startIdx
+      readInstances.map { r =>
+        val res = r.unsafeGet(rs, columnIdx)
+        columnIdx += r.length // This Read instance "consumed" x number of columns
+        res
+      }
+    }
+
+    override def gets: List[(Get[_], NullabilityKnown)] = readInstances.flatMap(_.gets).toList
+
+    override def toOpt: Read[Option[ArraySeq[Any]]] = {
+      import cats.syntax.all.*
+      readInstances.traverse(_.toOpt).map(arrayOfOptions => arrayOfOptions.sequence)
+    }
+
+    override def length: Int = readInstances.map(_.length).sum
+  }
+
 }
 
 trait LowerPriority1Read extends LowerPriority2Read {