From b04c7732871fe558d8f713c3bf6dbb53779ecb7d Mon Sep 17 00:00:00 2001
From: felix <felix.hedenstrom@rocker.com>
Date: Thu, 22 Jan 2026 10:13:47 +0100
Subject: [PATCH 1/2] Added an emap method

---
 modules/core/src/main/scala/doobie/util/read.scala | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/modules/core/src/main/scala/doobie/util/read.scala b/modules/core/src/main/scala/doobie/util/read.scala
index 42c43f80d..5d7463002 100644
--- a/modules/core/src/main/scala/doobie/util/read.scala
+++ b/modules/core/src/main/scala/doobie/util/read.scala
@@ -4,11 +4,12 @@
 
 package doobie.util
 
-import cats.Applicative
+import cats.{Applicative, Show}
 import doobie.ResultSetIO
 import doobie.enumerated.Nullability
 import doobie.enumerated.Nullability.{NoNulls, NullabilityKnown}
 import doobie.free.resultset as IFRS
+import doobie.util.invariant.InvalidValue
 
 import java.sql.ResultSet
 import scala.annotation.implicitNotFound
@@ -58,6 +59,16 @@ trait Read[A] {
   final def ap[B](ff: Read[A => B]): Read[B] = {
     new Read.Composite[B, A => B, A](ff, this, (f, a) => f(a))
   }
+
+  /** Equivalent to `map`, but allows the conversion to fail with an error message.
+   */
+  final def emap[B](f: A => Either[String, B])(implicit sA: Show[A]): Read[B] =
+    map { a =>
+      f(a) match {
+        case Left(reason) => throw InvalidValue[A, B](a, reason)
+        case Right(b)     => b
+      }
+    }
 }
 
 object Read extends ReadPlatform {

From d4cddc3989507752787538ee501f80a275889692 Mon Sep 17 00:00:00 2001
From: felix <felix.hedenstrom@rocker.com>
Date: Mon, 26 Jan 2026 09:26:31 +0100
Subject: [PATCH 2/2] Added emap tests

---
 .../test/scala/doobie/util/ReadSuite.scala    | 27 +++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/modules/core/src/test/scala/doobie/util/ReadSuite.scala b/modules/core/src/test/scala/doobie/util/ReadSuite.scala
index e98554352..3387e83f8 100644
--- a/modules/core/src/test/scala/doobie/util/ReadSuite.scala
+++ b/modules/core/src/test/scala/doobie/util/ReadSuite.scala
@@ -4,6 +4,7 @@
 
 package doobie.util
 
+import cats.Show
 import cats.effect.IO
 import doobie.util.TestTypes.*
 import doobie.util.transactor.Transactor
@@ -155,6 +156,32 @@ class ReadSuite extends munit.CatsEffectSuite with ReadSuitePlatform {
     insertTuple3AndCheckRead((1, "s1", "s2"), WrappedSimpleCaseClass(SimpleCaseClass(Some(1), "custom", Some("s2"))))
   }
 
+  test(".emap should correctly transform the value") {
+    import doobie.implicits.*
+    implicit val s: Show[SimpleCaseClass] = _.toString
+    implicit val r: Read[WrappedSimpleCaseClass] = Read[SimpleCaseClass].emap(s =>
+      Right(WrappedSimpleCaseClass(
+        s.copy(s = "custom")
+      )))
+
+    insertTuple3AndCheckRead((1, "s1", "s2"), WrappedSimpleCaseClass(SimpleCaseClass(Some(1), "custom", Some("s2"))))
+  }
+
+  test(".emap should fail a transform") {
+    import doobie.implicits.*
+    implicit val s: Show[SimpleCaseClass] = _.toString
+    implicit val r: Read[WrappedSimpleCaseClass] = Read[SimpleCaseClass].emap(_ =>
+      Left("Invalid transformation")
+    )
+
+    sql"SELECT 1,'a','b'".query[WrappedSimpleCaseClass].unique.transact(xa).attempt.assertEquals(
+      Left(doobie.util.invariant.InvalidValue[SimpleCaseClass, WrappedSimpleCaseClass](
+        SimpleCaseClass(Some(1), "a", Some("b")),
+        "Invalid transformation")
+      )
+    )
+  }
+
   /*
   case class with nested Option case class field
    */