feat!: make transform an interface type

This change introduces a new `Transform` type with a canonical implementation
`TransformFunc` that can be used to transform a string at render time. This
change also updates the `Style` type to use the new `Transform` type instead of
a function type.

We need this to support complex transforms and to be able to compare styles
using the `==` operator.

get.go

@@ -411,7 +411,7 @@ func (s Style) GetFrameSize() (x, y int) {
 
 // GetTransform returns the transform set on the style. If no transform is set
 // nil is returned.
-func (s Style) GetTransform() func(string) string {
+func (s Style) GetTransform() Transform {
 	return s.getAsTransform(transformKey)
 }
 
@@ -520,7 +520,7 @@ func (s Style) getBorderStyle() Border {
 	return s.borderStyle
 }
 
-func (s Style) getAsTransform(propKey) func(string) string {
+func (s Style) getAsTransform(propKey) Transform {
 	if !s.isSet(transformKey) {
 		return nil
 	}

list/list_test.go

@@ -306,7 +306,7 @@ func TestEnumeratorsTransform(t *testing.T) {
 	}{
 		"alphabet lower": {
 			enumeration: list.Alphabet,
-			style:       lipgloss.NewStyle().PaddingRight(1).Transform(strings.ToLower),
+			style:       lipgloss.NewStyle().PaddingRight(1).Transform(lipgloss.TransformFunc(strings.ToLower)),
 			expected: `
 a. Foo
 b. Bar
@@ -315,9 +315,9 @@ c. Baz
 		},
 		"arabic)": {
 			enumeration: list.Arabic,
-			style: lipgloss.NewStyle().PaddingRight(1).Transform(func(s string) string {
+			style: lipgloss.NewStyle().PaddingRight(1).Transform(lipgloss.TransformFunc(func(s string) string {
 				return strings.Replace(s, ".", ")", 1)
-			}),
+			})),
 			expected: `
 1) Foo
 2) Bar
@@ -326,9 +326,9 @@ c. Baz
 		},
 		"roman within ()": {
 			enumeration: list.Roman,
-			style: lipgloss.NewStyle().Transform(func(s string) string {
+			style: lipgloss.NewStyle().Transform(lipgloss.TransformFunc(func(s string) string {
 				return "(" + strings.Replace(strings.ToLower(s), ".", "", 1) + ") "
-			}),
+			})),
 			expected: `
   (i) Foo
  (ii) Bar
@@ -337,9 +337,9 @@ c. Baz
 		},
 		"bullet is dash": {
 			enumeration: list.Bullet,
-			style: lipgloss.NewStyle().Transform(func(s string) string {
+			style: lipgloss.NewStyle().Transform(lipgloss.TransformFunc(func(s string) string {
 				return "- " // this is better done by replacing the enumerator.
-			}),
+			})),
 			expected: `
 - Foo
 - Bar

set.go

@@ -1,6 +1,8 @@
 package lipgloss
 
-import "image/color"
+import (
+	"image/color"
+)
 
 // Set a value on the underlying rules map.
 func (s *Style) set(key propKey, value interface{}) {
@@ -66,7 +68,7 @@ func (s *Style) set(key propKey, value interface{}) {
 		// that negative value can be no less than -1).
 		s.tabWidth = value.(int)
 	case transformKey:
-		s.transform = value.(func(string) string)
+		s.transform = value.(Transform)
 	default:
 		if v, ok := value.(bool); ok { //nolint:nestif
 			if v {
@@ -675,15 +677,31 @@ func (s Style) StrikethroughSpaces(v bool) Style {
 	return s
 }
 
+// Transform is a method for setting a function that will be applied to the
+// string at render time.
+type Transform interface {
+	Transform(string) string
+}
+
+// TransformFunc is a function that can be used to transform a string at render
+// time.
+type TransformFunc func(string) string
+
+// Transform applies a given function to a string at render time, allowing for
+// the string being rendered to be manipuated.
+func (t TransformFunc) Transform(s string) string {
+	return t(s)
+}
+
 // Transform applies a given function to a string at render time, allowing for
 // the string being rendered to be manipuated.
 //
 // Example:
 //
-//	s := NewStyle().Transform(strings.ToUpper)
+//	s := NewStyle().Transform(lipgloss.TransformFunc(strings.ToUpper))
 //	fmt.Println(s.Render("raow!") // "RAOW!"
-func (s Style) Transform(fn func(string) string) Style {
-	s.set(transformKey, fn)
+func (s Style) Transform(t Transform) Style {
+	s.set(transformKey, t)
 	return s
 }
 

style.go

@@ -145,7 +145,7 @@ type Style struct {
 	maxHeight int
 	tabWidth  int
 
-	transform func(string) string
+	transform Transform
 }
 
 // joinString joins a list of strings into a single string separated with a
@@ -271,7 +271,7 @@ func (s Style) Render(strs ...string) string {
 	)
 
 	if transform != nil {
-		str = transform(str)
+		str = transform.Transform(str)
 	}
 
 	if s.props == 0 {

style_test.go

@@ -456,7 +456,7 @@ func TestStringTransform(t *testing.T) {
 			"犬 yzal eht revo depmuj 狐 nworb kciuq ehT",
 		},
 	} {
-		res := NewStyle().Bold(true).Transform(tc.fn).Render(tc.input)
+		res := NewStyle().Bold(true).Transform(TransformFunc(tc.fn)).Render(tc.input)
 		expected := "\x1b[1m" + tc.expected + "\x1b[m"
 		if res != expected {
 			t.Errorf("Test #%d:\nExpected: %q\nGot:      %q", i+1, expected, res)