Allow constructing Read from a list of underlying Read instances

Author: jatcwang

build.sbt

@@ -15,6 +15,7 @@ lazy val log4catsVersion = "2.7.0"
 lazy val postGisVersion = "2024.1.0"
 lazy val postgresVersion = "42.7.5"
 lazy val refinedVersion = "0.11.3"
+lazy val scalaCollectionCompatVersion = "2.13.0"
 lazy val scalaCheckVersion = "1.15.4"
 lazy val scalatestVersion = "3.2.18"
 lazy val munitVersion = "1.1.0"
@@ -249,12 +250,17 @@ lazy val core = project
     name := "doobie-core",
     description := "Pure functional JDBC layer for Scala.",
     libraryDependencies ++= Seq(
-      "com.chuusai" %% "shapeless" % shapelessVersion
-    ).filterNot(_ => tlIsScala3.value) ++ Seq(
       "org.tpolecat" %% "typename" % "1.1.0",
       "com.h2database" % "h2" % h2Version % "test",
       "org.postgresql" % "postgresql" % postgresVersion % "test"
     ),
+    libraryDependencies ++= (if (tlIsScala3.value)
+                               Seq.empty
+                             else
+                               Seq("com.chuusai" %% "shapeless" % shapelessVersion)),
+    libraryDependencies ++= (if (scalaVersion.value == scala212Version)
+                               Seq("org.scala-lang.modules" %% "scala-collection-compat" % scalaCollectionCompatVersion)
+                             else Seq.empty),
     Compile / unmanagedSourceDirectories += {
       val sourceDir = (Compile / sourceDirectory).value
       CrossVersion.partialVersion(scalaVersion.value) match {
@@ -334,7 +340,7 @@ lazy val postgres = project
       "co.fs2" %% "fs2-io" % fs2Version,
       "org.postgresql" % "postgresql" % postgresVersion,
       postgisDep % "provided",
-      "org.scala-lang.modules" %% "scala-collection-compat" % "2.13.0" % Test
+      "org.scala-lang.modules" %% "scala-collection-compat" % scalaCollectionCompatVersion % Test
     ),
     freeGen2Dir := (Compile / scalaSource).value / "doobie" / "postgres" / "free",
     freeGen2Package := "doobie.postgres.free",
@@ -496,11 +502,6 @@ lazy val bench = project
   .enablePlugins(NoPublishPlugin)
   .enablePlugins(AutomateHeaderPlugin)
   .enablePlugins(JmhPlugin)
-  .settings(
-    libraryDependencies ++= (if (scalaVersion.value == scala212Version)
-                               Seq("org.scala-lang.modules" %% "scala-collection-compat" % "2.13.0")
-                             else Seq.empty)
-  )
   .dependsOn(core, postgres, hikari)
   .settings(doobieSettings)
 

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

@@ -4,13 +4,28 @@
 
 package doobie
 
+import scala.reflect.ClassTag
+
 /** Collection of modules for typeclasses and other helpful bits. */
 package object util {
-
   val unlabeled: String = "unlabeled"
 
   private[util] def void(a: Any*): Unit = {
     val _ = a
   }
 
+  private[doobie] def arraySequence[A: ClassTag](arr: Array[Option[A]]): Option[Array[A]] = {
+    val result = new Array[A](arr.length)
+    var i = 0
+    while (i < arr.length) {
+      arr(i) match {
+        case None => return None
+        case Some(value) =>
+          result(i) = value
+          i += 1
+      }
+    }
+    Some(result)
+  }
+
 }

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.reflect.ClassTag
 
 @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 +146,30 @@ object Read extends LowerPriority1Read {
     override lazy val length: Int = read0.length + read1.length
   }
 
+  /** A Composite made up of a list of underlying Read instances. This class but is intended to provide a simpler
+    * interface for other libraries that uses Doobie. This isn't used by Doobie itself for its derived instances.
+    *
+    * For large number of columns, this class may be more performant than chain of Read.Composite.
+    */
+  class CompositeOfInstances[A: ClassTag](readInstances: Array[Read[A]]) extends Read[Array[A]] {
+    override def unsafeGet(rs: ResultSet, startIdx: Int): Array[A] = {
+      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[Array[A]]] = {
+      new CompositeOfInstances(readInstances.map(_.toOpt)).map(arraySequence)
+    }
+
+    override def length: Int = readInstances.map(_.length).sum
+  }
+
 }
 
 trait LowerPriority1Read extends LowerPriority2Read {

modules/core/src/test/scala/doobie/util/ReadSuite.scala

@@ -201,6 +201,56 @@ class ReadSuite extends munit.CatsEffectSuite with ReadSuitePlatform {
       q2.transact(xa).assertEquals(List(Some((1, 2, 3, 4))))
   }
 
+  test("Read.CompositeOfInstances reads columns correctly") {
+    import cats.syntax.functor.*
+    val rscc: Read[SimpleCaseClass] = Read.derived[SimpleCaseClass]
+
+    implicit val read: Read[ComplexCaseClass] =
+      new Read.CompositeOfInstances(Array(
+        rscc.widen[Any],
+        rscc.toOpt.widen[Any],
+        Read[Option[Int]].widen[Any],
+        Read[String].widen[Any]))
+        .map { arr =>
+          ComplexCaseClass(
+            arr(0).asInstanceOf[SimpleCaseClass],
+            arr(1).asInstanceOf[Option[SimpleCaseClass]],
+            arr(2).asInstanceOf[Option[Int]],
+            arr(3).asInstanceOf[String])
+        }
+
+    for {
+      _ <- IO(assertEquals(read.length, 8))
+      _ <-
+        sql"SELECT 1, '2', '3', 4, '5', '6', 7, '8'"
+          .query[ComplexCaseClass].unique.transact(xa)
+          .assertEquals(
+            ComplexCaseClass(
+              SimpleCaseClass(Some(1), "2", Some("3")),
+              Some(SimpleCaseClass(Some(4), "5", Some("6"))),
+              Some(7),
+              "8"
+            )
+          )
+      _ <-
+        // The 's' field in Option[SimpleCaseClass] is NULL, so whole case class value is None
+        sql"SELECT NULL, '2', '3', 4, NULL, '6', 7, '8'"
+          .query[ComplexCaseClass].unique.transact(xa)
+          .assertEquals(
+            ComplexCaseClass(
+              SimpleCaseClass(None, "2", Some("3")),
+              None,
+              Some(7),
+              "8"
+            )
+          )
+      _ <-
+        sql"SELECT 1, NULL, '3', 4, '5', '6', 7, '8'"
+          .query[Option[ComplexCaseClass]].unique.transact(xa)
+          .assertEquals(None)
+    } yield ()
+  }
+
   test("Read should select correct columns when combined with `ap`") {
     import cats.syntax.all.*
     import doobie.implicits.*