fix: improve lens generation

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

v2/cli/lens.go

@@ -64,6 +64,7 @@ type fieldInfo struct {
 	TypeName     string
 	BaseType     string // TypeName without leading * for pointer types
 	IsOptional   bool   // true if field is a pointer or has json omitempty tag
+	IsComparable bool   // true if the type is comparable (can use ==)
 }
 
 // templateData holds data for template rendering
@@ -127,12 +128,19 @@ func Make{{.Name}}RefLenses() {{.Name}}RefLenses {
 			func(s *{{$.Name}}) O.Option[{{.TypeName}}] { return iso{{.Name}}.Get(s.{{.Name}}) },
 			func(s *{{$.Name}}, v O.Option[{{.TypeName}}]) *{{$.Name}} { s.{{.Name}} = iso{{.Name}}.ReverseGet(v); return s },
 		),
+{{- else}}
+{{- if .IsComparable}}
+		{{.Name}}: L.MakeLensStrict(
+			func(s *{{$.Name}}) {{.TypeName}} { return s.{{.Name}} },
+			func(s *{{$.Name}}, v {{.TypeName}}) *{{$.Name}} { s.{{.Name}} = v; return s },
+		),
 {{- else}}
 		{{.Name}}: L.MakeLensRef(
 			func(s *{{$.Name}}) {{.TypeName}} { return s.{{.Name}} },
 			func(s *{{$.Name}}, v {{.TypeName}}) *{{$.Name}} { s.{{.Name}} = v; return s },
 		),
 {{- end}}
+{{- end}}
 {{- end}}
 	}
 }
@@ -257,6 +265,111 @@ func isPointerType(expr ast.Expr) bool {
 	return ok
 }
 
+// isComparableType checks if a type expression represents a comparable type.
+// Comparable types in Go include:
+// - Basic types (bool, numeric types, string)
+// - Pointer types
+// - Channel types
+// - Interface types
+// - Structs where all fields are comparable
+// - Arrays where the element type is comparable
+//
+// Non-comparable types include:
+// - Slices
+// - Maps
+// - Functions
+func isComparableType(expr ast.Expr) bool {
+	switch t := expr.(type) {
+	case *ast.Ident:
+		// Basic types and named types
+		// We assume named types are comparable unless they're known non-comparable types
+		name := t.Name
+		// Known non-comparable built-in types
+		if name == "error" {
+			// error is an interface, which is comparable
+			return true
+		}
+		// Most basic types and named types are comparable
+		// We can't determine if a custom type is comparable without type checking,
+		// so we assume it is (conservative approach)
+		return true
+	case *ast.StarExpr:
+		// Pointer types are always comparable
+		return true
+	case *ast.ArrayType:
+		// Arrays are comparable if their element type is comparable
+		if t.Len == nil {
+			// This is a slice (no length), slices are not comparable
+			return false
+		}
+		// Fixed-size array, check element type
+		return isComparableType(t.Elt)
+	case *ast.MapType:
+		// Maps are not comparable
+		return false
+	case *ast.FuncType:
+		// Functions are not comparable
+		return false
+	case *ast.InterfaceType:
+		// Interface types are comparable
+		return true
+	case *ast.StructType:
+		// Structs are comparable if all fields are comparable
+		// We can't easily determine this without full type information,
+		// so we conservatively return false for struct literals
+		return false
+	case *ast.SelectorExpr:
+		// Qualified identifier (e.g., pkg.Type)
+		// We can't determine comparability without type information
+		// Check for known non-comparable types from standard library
+		if ident, ok := t.X.(*ast.Ident); ok {
+			pkgName := ident.Name
+			typeName := t.Sel.Name
+			// Check for known non-comparable types
+			if pkgName == "context" && typeName == "Context" {
+				// context.Context is an interface, which is comparable
+				return true
+			}
+			// For other qualified types, we assume they're comparable
+			// This is a conservative approach
+		}
+		return true
+	case *ast.IndexExpr, *ast.IndexListExpr:
+		// Generic types - we can't determine comparability without type information
+		// For common generic types, we can make educated guesses
+		var baseExpr ast.Expr
+		if idx, ok := t.(*ast.IndexExpr); ok {
+			baseExpr = idx.X
+		} else if idxList, ok := t.(*ast.IndexListExpr); ok {
+			baseExpr = idxList.X
+		}
+
+		if sel, ok := baseExpr.(*ast.SelectorExpr); ok {
+			if ident, ok := sel.X.(*ast.Ident); ok {
+				pkgName := ident.Name
+				typeName := sel.Sel.Name
+				// Check for known non-comparable generic types
+				if pkgName == "option" && typeName == "Option" {
+					// Option types are not comparable (they contain a slice internally)
+					return false
+				}
+				if pkgName == "either" && typeName == "Either" {
+					// Either types are not comparable
+					return false
+				}
+			}
+		}
+		// For other generic types, conservatively assume not comparable
+		return false
+	case *ast.ChanType:
+		// Channel types are comparable
+		return true
+	default:
+		// Unknown type, conservatively assume not comparable
+		return false
+	}
+}
+
 // parseFile parses a Go file and extracts structs with lens annotations
 func parseFile(filename string) ([]structInfo, string, error) {
 	fset := token.NewFileSet()
@@ -331,6 +444,7 @@ func parseFile(filename string) ([]structInfo, string, error) {
 					typeName := getTypeName(field.Type)
 					isOptional := false
 					baseType := typeName
+					isComparable := false
 
 					// Check if field is optional:
 					// 1. Pointer types are always optional
@@ -344,6 +458,12 @@ func parseFile(filename string) ([]structInfo, string, error) {
 						isOptional = true
 					}
 
+					// Check if the type is comparable (for non-optional fields)
+					// For optional fields, we don't need to check since they use LensO
+					if !isOptional {
+						isComparable = isComparableType(field.Type)
+					}
+
 					// Extract imports from this field's type
 					fieldImports := make(map[string]string)
 					extractImports(field.Type, fieldImports)
@@ -360,6 +480,7 @@ func parseFile(filename string) ([]structInfo, string, error) {
 						TypeName:     typeName,
 						BaseType:     baseType,
 						IsOptional:   isOptional,
+						IsComparable: isComparable,
 					})
 				}
 			}

v2/cli/lens_test.go

@@ -168,6 +168,91 @@ func TestIsPointerType(t *testing.T) {
 	}
 }
 
+func TestIsComparableType(t *testing.T) {
+	tests := []struct {
+		name     string
+		code     string
+		expected bool
+	}{
+		{
+			name:     "basic type - string",
+			code:     "type T struct { F string }",
+			expected: true,
+		},
+		{
+			name:     "basic type - int",
+			code:     "type T struct { F int }",
+			expected: true,
+		},
+		{
+			name:     "basic type - bool",
+			code:     "type T struct { F bool }",
+			expected: true,
+		},
+		{
+			name:     "pointer type",
+			code:     "type T struct { F *string }",
+			expected: true,
+		},
+		{
+			name:     "slice type - not comparable",
+			code:     "type T struct { F []string }",
+			expected: false,
+		},
+		{
+			name:     "map type - not comparable",
+			code:     "type T struct { F map[string]int }",
+			expected: false,
+		},
+		{
+			name:     "array type - comparable if element is",
+			code:     "type T struct { F [5]int }",
+			expected: true,
+		},
+		{
+			name:     "interface type",
+			code:     "type T struct { F interface{} }",
+			expected: true,
+		},
+		{
+			name:     "channel type",
+			code:     "type T struct { F chan int }",
+			expected: true,
+		},
+		{
+			name:     "function type - not comparable",
+			code:     "type T struct { F func() }",
+			expected: false,
+		},
+		{
+			name:     "struct literal - conservatively not comparable",
+			code:     "type T struct { F struct{ X int } }",
+			expected: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fset := token.NewFileSet()
+			file, err := parser.ParseFile(fset, "", "package test\n"+tt.code, 0)
+			require.NoError(t, err)
+
+			var fieldType ast.Expr
+			ast.Inspect(file, func(n ast.Node) bool {
+				if field, ok := n.(*ast.Field); ok && len(field.Names) > 0 {
+					fieldType = field.Type
+					return false
+				}
+				return true
+			})
+
+			require.NotNil(t, fieldType)
+			result := isComparableType(fieldType)
+			assert.Equal(t, tt.expected, result)
+		})
+	}
+}
+
 func TestHasOmitEmpty(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -337,6 +422,171 @@ type Config struct {
 	assert.False(t, config.Fields[4].IsOptional, "Required field without omitempty should not be optional")
 }
 
+func TestParseFileWithComparableTypes(t *testing.T) {
+	// Create a temporary test file
+	tmpDir := t.TempDir()
+	testFile := filepath.Join(tmpDir, "test.go")
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type TypeTest struct {
+	Name      string
+	Age       int
+	Pointer   *string
+	Slice     []string
+	Map       map[string]int
+	Channel   chan int
+}
+`
+
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Parse the file
+	structs, pkg, err := parseFile(testFile)
+	require.NoError(t, err)
+
+	// Verify results
+	assert.Equal(t, "testpkg", pkg)
+	assert.Len(t, structs, 1)
+
+	// Check TypeTest struct
+	typeTest := structs[0]
+	assert.Equal(t, "TypeTest", typeTest.Name)
+	assert.Len(t, typeTest.Fields, 6)
+
+	// Name - string is comparable
+	assert.Equal(t, "Name", typeTest.Fields[0].Name)
+	assert.Equal(t, "string", typeTest.Fields[0].TypeName)
+	assert.False(t, typeTest.Fields[0].IsOptional)
+	assert.True(t, typeTest.Fields[0].IsComparable, "string should be comparable")
+
+	// Age - int is comparable
+	assert.Equal(t, "Age", typeTest.Fields[1].Name)
+	assert.Equal(t, "int", typeTest.Fields[1].TypeName)
+	assert.False(t, typeTest.Fields[1].IsOptional)
+	assert.True(t, typeTest.Fields[1].IsComparable, "int should be comparable")
+
+	// Pointer - pointer is optional, IsComparable not checked for optional fields
+	assert.Equal(t, "Pointer", typeTest.Fields[2].Name)
+	assert.Equal(t, "*string", typeTest.Fields[2].TypeName)
+	assert.True(t, typeTest.Fields[2].IsOptional)
+	assert.False(t, typeTest.Fields[2].IsComparable, "IsComparable not set for optional fields")
+
+	// Slice - not comparable
+	assert.Equal(t, "Slice", typeTest.Fields[3].Name)
+	assert.Equal(t, "[]string", typeTest.Fields[3].TypeName)
+	assert.False(t, typeTest.Fields[3].IsOptional)
+	assert.False(t, typeTest.Fields[3].IsComparable, "slice should not be comparable")
+
+	// Map - not comparable
+	assert.Equal(t, "Map", typeTest.Fields[4].Name)
+	assert.Equal(t, "map[string]int", typeTest.Fields[4].TypeName)
+	assert.False(t, typeTest.Fields[4].IsOptional)
+	assert.False(t, typeTest.Fields[4].IsComparable, "map should not be comparable")
+
+	// Channel - comparable (note: getTypeName returns "any" for channel types, but isComparableType correctly identifies them)
+	assert.Equal(t, "Channel", typeTest.Fields[5].Name)
+	assert.Equal(t, "any", typeTest.Fields[5].TypeName) // getTypeName doesn't handle chan types specifically
+	assert.False(t, typeTest.Fields[5].IsOptional)
+	assert.True(t, typeTest.Fields[5].IsComparable, "channel should be comparable")
+}
+
+func TestLensRefTemplatesWithComparable(t *testing.T) {
+	s := structInfo{
+		Name: "TestStruct",
+		Fields: []fieldInfo{
+			{Name: "Name", TypeName: "string", IsOptional: false, IsComparable: true},
+			{Name: "Age", TypeName: "int", IsOptional: false, IsComparable: true},
+			{Name: "Data", TypeName: "[]byte", IsOptional: false, IsComparable: false},
+			{Name: "Pointer", TypeName: "*string", IsOptional: true, IsComparable: false},
+		},
+	}
+
+	// Test constructor template for RefLenses
+	var constructorBuf bytes.Buffer
+	err := constructorTmpl.Execute(&constructorBuf, s)
+	require.NoError(t, err)
+
+	constructorStr := constructorBuf.String()
+
+	// Check that MakeLensStrict is used for comparable types in RefLenses
+	assert.Contains(t, constructorStr, "func MakeTestStructRefLenses() TestStructRefLenses")
+
+	// Name field - comparable, should use MakeLensStrict
+	assert.Contains(t, constructorStr, "Name: L.MakeLensStrict(",
+		"comparable field Name should use MakeLensStrict in RefLenses")
+
+	// Age field - comparable, should use MakeLensStrict
+	assert.Contains(t, constructorStr, "Age: L.MakeLensStrict(",
+		"comparable field Age should use MakeLensStrict in RefLenses")
+
+	// Data field - not comparable, should use MakeLensRef
+	assert.Contains(t, constructorStr, "Data: L.MakeLensRef(",
+		"non-comparable field Data should use MakeLensRef in RefLenses")
+
+	// Pointer field - optional, should use MakeLensRef
+	assert.Contains(t, constructorStr, "Pointer: L.MakeLensRef(",
+		"optional field Pointer should use MakeLensRef in RefLenses")
+}
+
+func TestGenerateLensHelpersWithComparable(t *testing.T) {
+	// Create a temporary directory with test files
+	tmpDir := t.TempDir()
+
+	testCode := `package testpkg
+
+// fp-go:Lens
+type TestStruct struct {
+	Name  string
+	Count int
+	Data  []byte
+}
+`
+
+	testFile := filepath.Join(tmpDir, "test.go")
+	err := os.WriteFile(testFile, []byte(testCode), 0644)
+	require.NoError(t, err)
+
+	// Generate lens code
+	outputFile := "gen.go"
+	err = generateLensHelpers(tmpDir, outputFile, false)
+	require.NoError(t, err)
+
+	// Verify the generated file exists
+	genPath := filepath.Join(tmpDir, outputFile)
+	_, err = os.Stat(genPath)
+	require.NoError(t, err)
+
+	// Read and verify the generated content
+	content, err := os.ReadFile(genPath)
+	require.NoError(t, err)
+
+	contentStr := string(content)
+
+	// Check for expected content in RefLenses
+	assert.Contains(t, contentStr, "MakeTestStructRefLenses")
+
+	// Name and Count are comparable, should use MakeLensStrict
+	assert.Contains(t, contentStr, "L.MakeLensStrict",
+		"comparable fields should use MakeLensStrict in RefLenses")
+
+	// Data is not comparable (slice), should use MakeLensRef
+	assert.Contains(t, contentStr, "L.MakeLensRef",
+		"non-comparable fields should use MakeLensRef in RefLenses")
+
+	// Verify the pattern appears for Name field (comparable)
+	namePattern := "Name: L.MakeLensStrict("
+	assert.Contains(t, contentStr, namePattern,
+		"Name field should use MakeLensStrict")
+
+	// Verify the pattern appears for Data field (not comparable)
+	dataPattern := "Data: L.MakeLensRef("
+	assert.Contains(t, contentStr, dataPattern,
+		"Data field should use MakeLensRef")
+}
+
 func TestGenerateLensHelpers(t *testing.T) {
 	// Create a temporary directory with test files
 	tmpDir := t.TempDir()

v2/samples/lens/example.go

@@ -17,6 +17,8 @@ package lens
 
 import "github.com/IBM/fp-go/v2/optics/lens/option"
 
+//go:generate go run ../../main.go lens --dir . --filename gen_lens.go
+
 // fp-go:Lens
 type Person struct {
 	Name  string

v2/samples/lens/example_test.go

@@ -153,3 +153,46 @@ func TestLensComposition(t *testing.T) {
 	assert.Equal(t, 55, olderCEO.CEO.Age)
 	assert.Equal(t, 50, company.CEO.Age) // Original unchanged
 }
+
+func TestPersonRefLensesIdempotent(t *testing.T) {
+	// Create a person pointer
+	person := &Person{
+		Name:  "Alice",
+		Age:   30,
+		Email: "alice@example.com",
+	}
+
+	// Create ref lenses
+	refLenses := MakePersonRefLenses()
+
+	// Test that setting the same value returns the identical pointer (idempotent)
+	// This works because Name, Age, and Email use MakeLensStrict which has equality optimization
+
+	// Test Name field - setting same value should return same pointer
+	sameName := refLenses.Name.Set("Alice")(person)
+	assert.Same(t, person, sameName, "Setting Name to same value should return identical pointer")
+
+	// Test Age field - setting same value should return same pointer
+	sameAge := refLenses.Age.Set(30)(person)
+	assert.Same(t, person, sameAge, "Setting Age to same value should return identical pointer")
+
+	// Test Email field - setting same value should return same pointer
+	sameEmail := refLenses.Email.Set("alice@example.com")(person)
+	assert.Same(t, person, sameEmail, "Setting Email to same value should return identical pointer")
+
+	// Test that setting a different value creates a new pointer
+	differentName := refLenses.Name.Set("Bob")(person)
+	assert.NotSame(t, person, differentName, "Setting Name to different value should return new pointer")
+	assert.Equal(t, "Bob", differentName.Name)
+	assert.Equal(t, "Alice", person.Name, "Original should be unchanged")
+
+	differentAge := refLenses.Age.Set(31)(person)
+	assert.NotSame(t, person, differentAge, "Setting Age to different value should return new pointer")
+	assert.Equal(t, 31, differentAge.Age)
+	assert.Equal(t, 30, person.Age, "Original should be unchanged")
+
+	differentEmail := refLenses.Email.Set("bob@example.com")(person)
+	assert.NotSame(t, person, differentEmail, "Setting Email to different value should return new pointer")
+	assert.Equal(t, "bob@example.com", differentEmail.Email)
+	assert.Equal(t, "alice@example.com", person.Email, "Original should be unchanged")
+}

v2/samples/lens/gen_lens.go

@@ -2,7 +2,7 @@ package lens
 
 // Code generated by go generate; DO NOT EDIT.
 // This file was generated by robots at
-// 2025-11-07 16:52:17.4935733 +0100 CET m=+0.003883901
+// 2025-11-12 17:16:40.1431921 +0100 CET m=+0.003694701
 
 import (
 	L "github.com/IBM/fp-go/v2/optics/lens"
@@ -55,15 +55,15 @@ func MakePersonLenses() PersonLenses {
 func MakePersonRefLenses() PersonRefLenses {
 	isoPhone := I.FromZero[*string]()
 	return PersonRefLenses{
-		Name: L.MakeLensRef(
+		Name: L.MakeLensStrict(
 			func(s *Person) string { return s.Name },
 			func(s *Person, v string) *Person { s.Name = v; return s },
 		),
-		Age: L.MakeLensRef(
+		Age: L.MakeLensStrict(
 			func(s *Person) int { return s.Age },
 			func(s *Person, v int) *Person { s.Age = v; return s },
 		),
-		Email: L.MakeLensRef(
+		Email: L.MakeLensStrict(
 			func(s *Person) string { return s.Email },
 			func(s *Person, v string) *Person { s.Email = v; return s },
 		),
@@ -123,19 +123,19 @@ func MakeAddressLenses() AddressLenses {
 func MakeAddressRefLenses() AddressRefLenses {
 	isoState := I.FromZero[*string]()
 	return AddressRefLenses{
-		Street: L.MakeLensRef(
+		Street: L.MakeLensStrict(
 			func(s *Address) string { return s.Street },
 			func(s *Address, v string) *Address { s.Street = v; return s },
 		),
-		City: L.MakeLensRef(
+		City: L.MakeLensStrict(
 			func(s *Address) string { return s.City },
 			func(s *Address, v string) *Address { s.City = v; return s },
 		),
-		ZipCode: L.MakeLensRef(
+		ZipCode: L.MakeLensStrict(
 			func(s *Address) string { return s.ZipCode },
 			func(s *Address, v string) *Address { s.ZipCode = v; return s },
 		),
-		Country: L.MakeLensRef(
+		Country: L.MakeLensStrict(
 			func(s *Address) string { return s.Country },
 			func(s *Address, v string) *Address { s.Country = v; return s },
 		),
@@ -189,15 +189,15 @@ func MakeCompanyLenses() CompanyLenses {
 func MakeCompanyRefLenses() CompanyRefLenses {
 	isoWebsite := I.FromZero[*string]()
 	return CompanyRefLenses{
-		Name: L.MakeLensRef(
+		Name: L.MakeLensStrict(
 			func(s *Company) string { return s.Name },
 			func(s *Company, v string) *Company { s.Name = v; return s },
 		),
-		Address: L.MakeLensRef(
+		Address: L.MakeLensStrict(
 			func(s *Company) Address { return s.Address },
 			func(s *Company, v Address) *Company { s.Address = v; return s },
 		),
-		CEO: L.MakeLensRef(
+		CEO: L.MakeLensStrict(
 			func(s *Company) Person { return s.CEO },
 			func(s *Company, v Person) *Company { s.CEO = v; return s },
 		),