Add context and dest for mapper functions

README.md

@@ -135,8 +135,8 @@ type TodoMapperHelper interface {
 }
 
 type TodoMapper interface {
-	TodoModelToTodo(pkg00000.Context, *pkg00001.TodoModel) (*pkg00002.Todo, error)
-	TodoToTodoModel(pkg00000.Context, *pkg00002.Todo) (*pkg00001.TodoModel, error)
+	TodoModelToTodo(pkg00000.Context, *pkg00001.TodoModel, *pkg00002.Todo) error
+	TodoToTodoModel(pkg00000.Context, *pkg00002.Todo, *pkg00001.TodoModel) error
 }
 
 // ... (TodoMapper default implementation)
@@ -162,7 +162,8 @@ Mapping codes look like the following:
        t.Fatal(err)
    }
    todoMapper, _ := obj.(TodoMapper)
-   entity, err := todoMapper.ModelToEntity(ctx, model) 
+   var entity Todo
+   err := todoMapper.ModelToEntity(ctx, model, &entity) 
    ```
 
 ### Custom mappers
@@ -180,16 +181,18 @@ Example: `string <-> time.Time` mapper
 type TimeStringMapper struct {
 }
 
-func (m *TimeStringMapper) StringToTime(ctx context.Context, source string) (*time.Time, error) {
-    t, err := time.Parse(time.RFC3339, source)
-    if err != nil {
-        return nil, err
-    }
-    return &t, nil
+func (m *TimeStringMapper) StringToTime(ctx context.Context, source string, dest *time.Time) error {
+	t, err := time.Parse(time.RFC3339, source)
+	if err != nil {
+		return err
+	}
+	*dest = t
+	return nil
 }
 
-func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time) (string, error) {
-    return source.Format(time.RFC3339), nil
+func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time, dest *string) error {
+	*dest = source.Format(time.RFC3339)
+	return nil
 }
 
 type Mappers interface {
@@ -224,12 +227,14 @@ func AddTimeToStringMapper(mappers Mappers) {
 
 `Mappers.AddMapperFuncFactory` takes qualified type names as arguments. A qualified type name is `FULL_PACKAGE_PATH#TYPENAME`(i.e. `time#Time`, `example.com/testmod/domain#Todo`).
 
-Argument types and return types in custom mapping functions must be a
+Source argument types in custom mapping functions must be a
 
 - Raw value: primitive types(i.e. `string`, `int`, `slice` ...)
 - Pointer: others
 
-So `func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time) (string, error)` defines source type as a pointer(`*time.Time`) and return type as a raw value(`string`) .
+Destination arguments are pointers.
+
+So `func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time, dest *string) error` defines source type as a pointer(`*time.Time`).
 
 `Mappers.Add` finds given mapper methods name like 'XxxToYyy' and calls `AddMapperFuncFactory`.
 

context.go

@@ -34,10 +34,10 @@ type MapperFuncField struct {
 
 // Signature returns a function signature.
 func (m *MapperFuncField) Signature(mctx *MappingContext) string {
-	return fmt.Sprintf("func(%s.Context, %s) (%s, error)",
+	return fmt.Sprintf("func(%s.Context, %s, %s) error",
 		mctx.GetImportAlias("context"),
 		GetPreferableTypeSource(m.Source, mctx),
-		GetPreferableTypeSource(m.Dest, mctx))
+		GetPointerTypeSource(m.Dest, mctx))
 }
 
 // NewMappingContext returns new [MappingContext] .

gen.go

@@ -390,6 +390,9 @@ type MappingValue interface {
 	// CanGet returns true if this value is readable.
 	CanGet() bool
 
+	// CanAddr returns true if this value with getter is addressable.
+	CanAddr() bool
+
 	// GetSetterSource returns a source code of the setter.
 	GetSetterSource(valueSource string) string
 
@@ -431,6 +434,10 @@ func (v *localMappingValue) CanGet() bool {
 	return true
 }
 
+func (v *localMappingValue) CanAddr() bool {
+	return true
+}
+
 func (v *localMappingValue) GetSetterSource(valueSource string) string {
 	return fmt.Sprintf("%s = %s", v.name, valueSource)
 }
@@ -514,6 +521,19 @@ func (v *objectPropertyMappingValue) GetGetterSource() string {
 	return ""
 }
 
+func (v *objectPropertyMappingValue) CanAddr() bool {
+	if len(v.exportedFieldName) != 0 {
+		return true
+	}
+	if v.getter != nil {
+		typ := v.getter.Type().(*types.Signature).Results().At(0).Type()
+		if _, ok := typ.(*types.Pointer); ok {
+			return true
+		}
+	}
+	return false
+}
+
 func (v *objectPropertyMappingValue) CanGet() bool {
 	return len(v.exportedFieldName) > 0 || v.getter != nil
 }
@@ -753,7 +773,7 @@ func (g *generator) Generate() error {
 			a := elem.A
 			b := elem.B
 			aArgSource := GetPreferableTypeSource(a.Type(), mctx)
-			bArgSource := GetPreferableTypeSource(b.Type(), mctx)
+			bArgSource := GetPointerTypeSource(b.Type(), mctx)
 			p("type %sHelper interface {", mapping.Name)
 			p("  %s(%s.Context, %s, %s) error", mapping.MethodName(OperandA),
 				mctx.GetImportAlias("context"), aArgSource, bArgSource)
@@ -764,10 +784,10 @@ func (g *generator) Generate() error {
 			p("}")
 			p("")
 			p("type %s interface {", mapping.Name)
-			p("%s(%s.Context, %s) (%s, error) ", mapping.MethodName(OperandA),
+			p("%s(%s.Context, %s, %s) error", mapping.MethodName(OperandA),
 				mctx.GetImportAlias("context"), aArgSource, bArgSource)
 			if mapping.Bidirectional {
-				p("%s(%s.Context, %s) (%s, error) ", mapping.MethodName(OperandB),
+				p("%s(%s.Context, %s, %s) error", mapping.MethodName(OperandB),
 					mctx.GetImportAlias("context"), bArgSource, aArgSource)
 			}
 			p("}")
@@ -921,19 +941,18 @@ func genMapFunc(printer Printer, mapping *Mapping,
 	source types.Object, dest types.Object, typ OperandType, mctx *MappingContext) error {
 	p := printer.P
 
-	p("func (m *%s) %s(ctx %s.Context, source *%s) (*%s, error) {",
+	p("func (m *%s) %s(ctx %s.Context, source *%s, dest *%s) error {",
 		mapping.PrivateName(), mapping.MethodName(typ), mctx.GetImportAlias("context"),
 		GetSource(source.Type(), mctx), GetSource(dest.Type(), mctx))
-	p("  dest := &%s{}", GetSource(dest.Type(), mctx))
 	if err := genMapFuncBody(printer, source, "source", dest, "dest", &mapping.ObjectMapping, typ, mctx); err != nil {
 		return err
 	}
 	p("  if m.helper != nil {")
 	p("     if err := m.helper.%s(ctx, source, dest); err != nil {", mapping.MethodName(typ))
-	p("       return nil, err")
+	p("       return err")
 	p("     }")
 	p("  }")
-	p("  return dest, nil")
+	p("  return nil")
 	p("}")
 
 	return nil
@@ -1184,39 +1203,53 @@ func genAssignStmt(printer Printer,
 	sourceIsPointerPreferable := IsPointerPreferableType(sourceType)
 	destTypeName := GetQualifiedTypeName(destType)
 	_, destIsPointer := destType.(*types.Pointer)
-	destIsPointerPreferable := IsPointerPreferableType(destType)
 
 	// Try to execute custom mapper
-	argName := ""
-	switch {
-	case sourceIsPointerPreferable && sourceIsPointer:
-		argName = sourceSig
-	case sourceIsPointerPreferable && !sourceIsPointer:
-		argName = "&(" + sourceSig + ")"
-	case !sourceIsPointerPreferable && sourceIsPointer:
-		argName = "*(" + sourceSig + ")"
-	case !sourceIsPointerPreferable && !sourceIsPointer:
-		argName = sourceSig
-	}
-
 	mf := mctx.GetMapperFuncFieldName(sourceType, destType)
 	if mf != nil {
 		p("if m.%s != nil {", mf.FieldName)
-		p("  if v, err := m.%s(ctx, %s); err != nil {", mf.FieldName, argName)
-		p("    return nil, err")
-		p("  } else {")
+		var argName string
 		switch {
-		case destIsPointer && destIsPointerPreferable:
-			p(destValue.GetSetterSource("v"))
-		case destIsPointer && !destIsPointerPreferable:
-			p(destValue.GetSetterSource("&v"))
-		case !destIsPointer && destIsPointerPreferable:
-			p(destValue.GetSetterSource("*v"))
-		case !destIsPointer && !destIsPointerPreferable:
+		case sourceIsPointerPreferable && sourceIsPointer:
+			argName = sourceSig
+		case sourceIsPointerPreferable && !sourceIsPointer:
+			if sourceValue.CanAddr() {
+				argName = "&(" + sourceSig + ")"
+			} else {
+				p("s := %s", sourceSig)
+				argName = "&s"
+			}
+		case !sourceIsPointerPreferable && sourceIsPointer:
+			argName = "*(" + sourceSig + ")"
+		case !sourceIsPointerPreferable && !sourceIsPointer:
+			argName = sourceSig
+		}
+
+		var destName string
+		if destValue.CanAddr() {
+			if destIsPointer {
+				destName = destValue.GetGetterSource()
+			} else {
+				destName = "&(" + destValue.GetGetterSource() + ")"
+			}
+		} else {
+			p("var v %s", GetSource(destType, mctx))
+			if destIsPointer {
+				destName = "v"
+			} else {
+				destName = "&v"
+			}
+		}
+		p("  if err := m.%s(ctx, %s, %s); err != nil {", mf.FieldName, argName, destName)
+		p("    return err")
+		if destValue.CanAddr() {
+			p("}")
+		} else {
+			p("  } else {")
 			p(destValue.GetSetterSource("v"))
+			p("  }")
 		}
-		p("  }")
-		p("}")
+		p("} else { ")
 	}
 
 	if sourceTypeName == destTypeName {
@@ -1226,19 +1259,34 @@ func genAssignStmt(printer Printer,
 		case sourceIsPointer && !destIsPointer:
 			p(destValue.GetSetterSource("*(" + sourceSig + ")"))
 		case !sourceIsPointer && destIsPointer:
-			p(destValue.GetSetterSource("&(" + sourceSig + ")"))
+			if sourceValue.CanAddr() {
+				p(destValue.GetSetterSource("&(" + sourceSig + ")"))
+			} else {
+				a := mctx.NextVarCount()
+				p("s%d := ", a, sourceSig)
+				p(destValue.GetSetterSource(fmt.Sprintf("&s%d", a)))
+			}
 		case !sourceIsPointer && !destIsPointer:
 			p(destValue.GetSetterSource(sourceSig))
 
 		}
+		if mf != nil {
+			p("}")
+		}
 		return
 	}
 
 	if CanCast(sourceType, destType) {
 		genAssignStmt(printer,
 			NewLocalMappingValue(fmt.Sprintf("%s(%s)", GetSource(destType, mctx), sourceSig), destType),
 			destValue, mctx)
+		if mf != nil {
+			p("}")
+		}
 		return
 	}
+	if mf != nil {
+		p("}")
+	}
 
 }

mappers.go

@@ -107,14 +107,14 @@ func (d *mappers) Add(name string, obj any) {
 		method := typ.Method(i)
 		if mapperFuncNamePattern.MatchString(method.Name) {
 			ft := reflect.TypeOf(method.Func.Interface())
-			if ft.NumIn() != 3 || ft.NumOut() != 2 {
+			if ft.NumIn() != 4 || ft.NumOut() != 1 {
 				continue
 			}
 			in := ft.In(2)
 			if in.Kind() == reflect.Ptr {
 				in = in.Elem()
 			}
-			out := ft.Out(0)
+			out := ft.In(3)
 			if out.Kind() == reflect.Ptr {
 				out = out.Elem()
 			}

testdata/testmod/mapper/my_mappers.go

@@ -8,16 +8,18 @@ import (
 type TimeStringMapper struct {
 }
 
-func (m *TimeStringMapper) StringToTime(ctx context.Context, source string) (*time.Time, error) {
+func (m *TimeStringMapper) StringToTime(ctx context.Context, source string, dest *time.Time) error {
 	t, err := time.Parse(time.RFC3339, source)
 	if err != nil {
-		return nil, err
+		return err
 	}
-	return &t, nil
+	*dest = t
+	return nil
 }
 
-func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time) (string, error) {
-	return source.Format(time.RFC3339), nil
+func (m *TimeStringMapper) TimeToString(ctx context.Context, source *time.Time, dest *string) error {
+	*dest = source.Format(time.RFC3339)
+	return nil
 }
 
 func AddTimeToStringMapper(mappers interface {