Improve error messages for the Focus macro.

build.sbt

@@ -51,7 +51,12 @@ lazy val buildSettings = Seq(
   Compile / console / scalacOptions -= "-Ywarn-unused:imports",
   scalacOptions ++= {
     if (tlIsScala3.value)
-      Seq("-source:3.0-migration", "-Ykind-projector", "-language:implicitConversions,higherKinds,postfixOps")
+      Seq(
+        "-source:3.0-migration",
+        "-Ykind-projector",
+        "-language:implicitConversions,higherKinds,postfixOps",
+        "-Wunused:all"
+      )
     else
       Seq(
         "-Ymacro-annotations",
@@ -158,7 +163,9 @@ lazy val core = crossProject(JVMPlatform, JSPlatform, NativePlatform)
           ProblemFilters.exclude[MissingClassProblem]("monocle.syntax.AsPrism"),
           ProblemFilters.exclude[MissingClassProblem]("monocle.syntax.AsPrism$"),
           ProblemFilters.exclude[MissingClassProblem]("monocle.syntax.AsPrismImpl"),
-          ProblemFilters.exclude[MissingClassProblem]("monocle.syntax.AsPrismImpl$")
+          ProblemFilters.exclude[MissingClassProblem]("monocle.syntax.AsPrismImpl$"),
+          // ignore mima for classes only used by `Focus` macro
+          ProblemFilters.exclude[DirectMissingMethodProblem]("monocle.internal.focus.*")
         )
       else Nil
     }

core/shared/src/main/scala-3/monocle/internal/focus/ErrorHandling.scala

@@ -3,19 +3,52 @@ package monocle.internal.focus
 private[focus] trait ErrorHandling {
   this: FocusBase =>
 
-  def errorMessage(error: FocusError): String = error match {
-    case FocusError.NotACaseClass(fromClass, fieldName) =>
-      s"Cannot generate Lens for field '$fieldName', because '$fromClass' is not a case class"
+  def errorReport(error: FocusError): (String, Option[Position]) = error match {
+    case FocusError.NotACaseClass(fromClass, fieldName, pos) =>
+      (
+        s"Cannot generate Lens for field '$fieldName', because '$fromClass' is not a case class",
+        Some(pos)
+      )
     case FocusError.NotAConcreteClass(fromClass) =>
-      s"Expecting a concrete case class in the 'From' position; cannot reify type $fromClass"
+      (
+        s"Expecting a concrete case class in the 'From' position; cannot reify type $fromClass",
+        None
+      )
     case FocusError.NotASimpleLambdaFunction =>
-      s"Expecting a lambda function that directly accesses a field. Example: `Focus[Address](_.streetNumber)`"
-    case FocusError.CouldntUnderstandKeywordContext => s"Internal error in monocle.Focus; cannot access special syntax."
+      (
+        s"Expecting a lambda function that directly accesses a field. Example: `Focus[Address](_.streetNumber)`",
+        None
+      )
+    case FocusError.CouldntUnderstandKeywordContext =>
+      (
+        s"Internal error in monocle.Focus; cannot access special syntax.",
+        None
+      )
     case FocusError.DidNotDirectlyAccessArgument(argName) =>
-      s"Expecting a lambda function that directly accesses the argument; other variable `$argName` found. Example: `Focus[Address](_.streetNumber)`"
-    case FocusError.ComposeMismatch(type1, type2)             => s"Could not compose $type1.andThen($type2)"
-    case FocusError.UnexpectedCodeStructure(code)             => s"Unexpected code structure: $code"
-    case FocusError.CouldntFindFieldType(fromType, fieldName) => s"Couldn't find type for $fromType.$fieldName"
-    case FocusError.InvalidDowncast(fromType, toType)         => s"Type '$fromType' could not be cast to '$toType'"
+      (
+        s"Expecting a lambda function that directly accesses the argument; other variable `$argName` found. Example: `Focus[Address](_.streetNumber)`",
+        None
+      )
+    case FocusError.ComposeMismatch(type1, type2) =>
+      (
+        s"Could not compose $type1.andThen($type2)",
+        None
+      )
+    case FocusError.UnexpectedCodeStructure(code) =>
+      (
+        s"Unexpected code structure: $code",
+        None
+      )
+    case FocusError.CouldntFindFieldType(fromType, fieldName, pos) =>
+      (
+        s"Couldn't find type for $fromType.$fieldName",
+        Some(pos)
+      )
+    case FocusError.InvalidDowncast(fromType, toType) =>
+      (
+        s"Type '$fromType' could not be cast to '$toType'",
+        None
+      )
   }
+
 }

core/shared/src/main/scala-3/monocle/internal/focus/FocusBase.scala

@@ -9,6 +9,7 @@ private[focus] trait FocusBase {
 
   type Term     = macroContext.reflect.Term
   type TypeRepr = macroContext.reflect.TypeRepr
+  type Position = macroContext.reflect.Position
 
   case class LambdaConfig(argName: String, lambdaBody: Term)
 
@@ -43,13 +44,13 @@ private[focus] trait FocusBase {
   }
 
   enum FocusError {
-    case NotACaseClass(className: String, fieldName: String)
+    case NotACaseClass(className: String, fieldName: String, pos: Position)
     case NotAConcreteClass(className: String)
     case DidNotDirectlyAccessArgument(argName: String)
     case NotASimpleLambdaFunction
     case CouldntUnderstandKeywordContext
     case UnexpectedCodeStructure(code: String)
-    case CouldntFindFieldType(fromType: String, fieldName: String)
+    case CouldntFindFieldType(fromType: String, fieldName: String, pos: Position)
     case ComposeMismatch(type1: String, type2: String)
     case InvalidDowncast(fromType: String, toType: String)
 

core/shared/src/main/scala-3/monocle/internal/focus/FocusImpl.scala

@@ -25,7 +25,9 @@ private[focus] class FocusImpl(val macroContext: Quotes)
 
     generatedCode match {
       case Right(code) => code.asExpr
-      case Left(error) => report.error(errorMessage(error)); '{ ??? }
+      case Left(error) =>
+        val (msg, pos) = errorReport(error)
+        report.errorAndAbort(msg, pos.getOrElse(lambda.asTerm.pos))
     }
   }
 }

core/shared/src/main/scala-3/monocle/internal/focus/features/SelectParserBase.scala

@@ -26,16 +26,24 @@ private[focus] trait SelectParserBase extends ParserBase {
     case None      => FocusError.NotAConcreteClass(tpe.show).asResult
   }
 
-  def getFieldType(fromType: TypeRepr, fieldName: String): FocusResult[TypeRepr] = {
-    // We need to do this to support tuples, because even though they conform as case classes in other respects,
-    // for some reason their field names (_1, _2, etc) have a space at the end, ie `_1 `.
-    def getTrimmedFieldSymbol(fromTypeSymbol: Symbol): Symbol =
-      fromTypeSymbol.memberFields.find(_.name.trim == fieldName).getOrElse(Symbol.noSymbol)
+  private val tupleFieldPattern = "^_[0-9]+$".r
+
+  def getFieldType(fromType: TypeRepr, fieldName: String, pos: Position): FocusResult[TypeRepr] = {
+    def getFieldSymbol(fromTypeSymbol: Symbol): Symbol = {
+      // We need to do this to support tuples, because even though they conform as case classes in other respects,
+      // for some reason their field names (_1, _2, etc) have a space at the end, ie `_1 `.
+      val f: String => String =
+        if (fromType <:< TypeRepr.of[Tuple] && tupleFieldPattern.matches(fieldName))
+          _.trim
+        else
+          identity
+      fromTypeSymbol.fieldMembers.find(s => f(s.name) == fieldName).getOrElse(Symbol.noSymbol)
+    }
 
     getClassSymbol(fromType).flatMap { fromTypeSymbol =>
-      getTrimmedFieldSymbol(fromTypeSymbol) match {
+      getFieldSymbol(fromTypeSymbol) match {
         case FieldType(possiblyTypeArg) => Right(swapWithSuppliedType(fromType, possiblyTypeArg))
-        case _                          => FocusError.CouldntFindFieldType(fromType.show, fieldName).asResult
+        case _                          => FocusError.CouldntFindFieldType(fromType.show, fieldName, pos).asResult
       }
     }
   }

core/shared/src/main/scala-3/monocle/internal/focus/features/selectfield/SelectFieldParser.scala

@@ -14,19 +14,18 @@ private[focus] trait SelectFieldParser {
 
       case Select(CaseClass(remainingCode), fieldName) =>
         val fromType                = getType(remainingCode)
-        val action                  = getFieldAction(fromType, fieldName)
+        val action                  = getFieldAction(fromType, fieldName, term.pos)
         val remainingCodeWithAction = action.map(a => (RemainingCode(remainingCode), a))
         Some(remainingCodeWithAction)
 
       case Select(remainingCode, fieldName) =>
-        Some(FocusError.NotACaseClass(remainingCode.tpe.show, fieldName).asResult)
-
+        Some(FocusError.NotACaseClass(remainingCode.tpe.widen.show, fieldName, term.pos).asResult)
       case _ => None
     }
   }
 
-  private def getFieldAction(fromType: TypeRepr, fieldName: String): FocusResult[FocusAction] =
-    getFieldType(fromType, fieldName).flatMap { toType =>
+  private def getFieldAction(fromType: TypeRepr, fieldName: String, pos: Position): FocusResult[FocusAction] =
+    getFieldType(fromType, fieldName, pos).flatMap { toType =>
       Right(FocusAction.SelectField(fieldName, fromType, getSuppliedTypeArgs(fromType), toType))
     }
 }

core/shared/src/main/scala-3/monocle/internal/focus/features/selectonlyfield/SelectOnlyFieldParser.scala

@@ -14,17 +14,17 @@ private[focus] trait SelectOnlyFieldParser {
 
       case Select(CaseClass(remainingCode), fieldName) if hasOnlyOneField(remainingCode) =>
         val fromType                = getType(remainingCode)
-        val action                  = getFieldAction(fromType, fieldName)
+        val action                  = getFieldAction(fromType, fieldName, term.pos)
         val remainingCodeWithAction = action.map(a => (RemainingCode(remainingCode), a))
         Some(remainingCodeWithAction)
 
       case _ => None
     }
   }
 
-  private def getFieldAction(fromType: TypeRepr, fieldName: String): FocusResult[FocusAction] =
+  private def getFieldAction(fromType: TypeRepr, fieldName: String, pos: Position): FocusResult[FocusAction] =
     for {
-      toType    <- getFieldType(fromType, fieldName)
+      toType    <- getFieldType(fromType, fieldName, pos)
       companion <- getCompanionObject(fromType)
       supplied = getSuppliedTypeArgs(fromType)
     } yield FocusAction.SelectOnlyField(fieldName, fromType, supplied, companion, toType)