refactor!: sizeable changes across the board

binary.go

@@ -10,7 +10,7 @@ import (
 // interface.
 type Binary struct{}
 
-var _ Renderer = (*Binary)(nil)
+var _ FormatRenderer = (*Binary)(nil)
 
 // Render writes result of calling MarshalBinary() on v. If v does not implment
 // encoding.BinaryMarshaler the ErrCannotRander error will be returned.
@@ -22,12 +22,12 @@ func (bm *Binary) Render(w io.Writer, v any) error {
 
 	b, err := x.MarshalBinary()
 	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
+		return fmt.Errorf("%w: %w", ErrFailed, err)
 	}
 
 	_, err = w.Write(b)
 	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
+		return fmt.Errorf("%w: %w", ErrFailed, err)
 	}
 
 	return nil

binary_test.go

@@ -45,15 +45,15 @@ func TestBinary_Render(t *testing.T) {
 				data: []byte("test string"),
 				err:  errors.New("marshal error!!1"),
 			},
-			wantErr:   "render: marshal error!!1",
+			wantErr:   "render: failed: marshal error!!1",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 		{
 			name:      "error writing to writer",
 			writeErr:  errors.New("write error!!1"),
 			value:     &mockBinaryMarshaler{data: []byte("test string")},
-			wantErr:   "render: write error!!1",
+			wantErr:   "render: failed: write error!!1",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 	}
 	for _, tt := range tests {

format.go

@@ -1,32 +0,0 @@
-package render
-
-import (
-	"fmt"
-	"io"
-)
-
-var ErrUnsupportedFormat = fmt.Errorf("%w: unsupported format", Err)
-
-// FormatRenderer is a renderer that delegates rendering to another renderer
-// based on a format value.
-type FormatRenderer struct {
-	// Renderers is a map of format names to renderers. When Render is called,
-	// the format is used to look up the renderer to use.
-	Renderers map[string]Renderer
-}
-
-// Render renders a value to an io.Writer using the specified format. If the
-// format is not supported, ErrUnsupportedFormat is returned.
-//
-// If the format is supported, but the value cannot be rendered to the format,
-// the error returned by the renderer is returned. In most cases this will be
-// ErrCannotRender, but it could be a different error if the renderer returns
-// one.
-func (r *FormatRenderer) Render(w io.Writer, format string, v any) error {
-	renderer, ok := r.Renderers[format]
-	if ok {
-		return renderer.Render(w, v)
-	}
-
-	return ErrUnsupportedFormat
-}

format_test.go

@@ -1,74 +0,0 @@
-package render_test
-
-import (
-	"bytes"
-	"errors"
-	"testing"
-
-	"github.com/jimeh/go-render"
-	"github.com/stretchr/testify/assert"
-)
-
-func TestFormatRenderer_Render(t *testing.T) {
-	tests := []struct {
-		name      string
-		renderers map[string]render.Renderer
-		format    string
-		value     interface{}
-		want      string
-		wantErr   string
-		wantErrIs []error
-	}{
-		{
-			name: "existing renderer",
-			renderers: map[string]render.Renderer{
-				"mock": &mockRenderer{output: "mock output"},
-			},
-			format: "mock",
-			value:  struct{}{},
-			want:   "mock output",
-		},
-		{
-			name: "existing renderer returns error",
-			renderers: map[string]render.Renderer{
-				"other": &mockRenderer{
-					output: "mock output",
-					err:    errors.New("mock error"),
-				},
-			},
-			format:  "other",
-			value:   struct{}{},
-			wantErr: "mock error",
-		},
-		{
-			name:      "non-existing renderer",
-			renderers: map[string]render.Renderer{},
-			format:    "unknown",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
-		},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			fr := &render.FormatRenderer{
-				Renderers: tt.renderers,
-			}
-			var buf bytes.Buffer
-
-			err := fr.Render(&buf, tt.format, tt.value)
-			got := buf.String()
-
-			if tt.wantErr != "" {
-				assert.EqualError(t, err, tt.wantErr)
-			}
-			for _, e := range tt.wantErrIs {
-				assert.ErrorIs(t, err, e)
-			}
-
-			if tt.wantErr == "" && len(tt.wantErrIs) == 0 {
-				assert.NoError(t, err)
-				assert.Equal(t, tt.want, got)
-			}
-		})
-	}
-}

json.go

@@ -21,7 +21,7 @@ type JSON struct {
 	Indent string
 }
 
-var _ Renderer = (*JSON)(nil)
+var _ FormatRenderer = (*JSON)(nil)
 
 // Render marshals the given value to JSON.
 func (j *JSON) Render(w io.Writer, v any) error {
@@ -38,7 +38,7 @@ func (j *JSON) Render(w io.Writer, v any) error {
 
 	err := enc.Encode(v)
 	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
+		return fmt.Errorf("%w: %w", ErrFailed, err)
 	}
 
 	return nil

json_test.go

@@ -69,14 +69,14 @@ func TestJSON_Render(t *testing.T) {
 		{
 			name:      "error from json.Marshaler",
 			value:     &mockJSONMarshaler{err: errors.New("marshal error!!1")},
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 		{
 			name:      "invalid value",
 			pretty:    false,
 			value:     make(chan int),
-			wantErr:   "render: json: unsupported type: chan int",
+			wantErr:   "render: failed: json: unsupported type: chan int",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 	}
 	for _, tt := range tests {

multi.go

@@ -7,10 +7,10 @@ import (
 
 // MultiRenderer is a renderer that tries multiple renderers until one succeeds.
 type MultiRenderer struct {
-	Renderers []Renderer
+	Renderers []FormatRenderer
 }
 
-var _ Renderer = (*MultiRenderer)(nil)
+var _ FormatRenderer = (*MultiRenderer)(nil)
 
 // Render tries each renderer in order until one succeeds. If none succeed,
 // ErrCannotRender is returned. If a renderer returns an error that is not

multi_test.go

@@ -16,7 +16,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 
 	tests := []struct {
 		name      string
-		renderers []render.Renderer
+		renderers []render.FormatRenderer
 		value     interface{}
 		want      string
 		wantErr   string
@@ -24,7 +24,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 	}{
 		{
 			name: "no renderer can render",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				cannotRenderer,
 				cannotRenderer,
 			},
@@ -33,7 +33,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 		},
 		{
 			name: "one renderer can render",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				cannotRenderer,
 				successRenderer,
 				cannotRenderer,
@@ -43,7 +43,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 		},
 		{
 			name: "multiple renderers can render",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				&mockRenderer{err: render.ErrCannotRender},
 				&mockRenderer{output: "first output"},
 				&mockRenderer{output: "second output"},
@@ -53,7 +53,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 		},
 		{
 			name: "first renderer fails",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				failRenderer,
 				successRenderer,
 			},
@@ -62,7 +62,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 		},
 		{
 			name: "fails after cannot render",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				cannotRenderer,
 				failRenderer,
 				successRenderer,
@@ -72,7 +72,7 @@ func TestMultiRenderer_Render(t *testing.T) {
 		},
 		{
 			name: "fails after success render",
-			renderers: []render.Renderer{
+			renderers: []render.FormatRenderer{
 				successRenderer,
 				failRenderer,
 				cannotRenderer,

render.go

@@ -1,3 +1,14 @@
+// Package render provides a simple and flexible way to render a value to a
+// io.Writer using different formats based on a format string argument.
+//
+// It allows rendering a custom type which can be marshaled to JSON, YAML, XML,
+// while also supporting plain text by implementing fmt.Stringer or io.WriterTo.
+// Binary output is also supported by implementing the encoding.BinaryMarshaler
+// interface.
+//
+// Originally intended to easily implement CLI tools which can output their data
+// as plain text, as well as JSON/YAML with a simple switch of a format string.
+// But it can just as easily render to any io.Writer.
 package render
 
 import (
@@ -6,24 +17,43 @@ import (
 )
 
 var (
-	Err = fmt.Errorf("render")
+	// Err is the base error for the package. All errors returned by this
+	// package are wrapped with this error.
+	Err       = fmt.Errorf("render")
+	ErrFailed = fmt.Errorf("%w: failed", Err)
 
 	// ErrCannotRender is returned when a value cannot be rendered. This may be
 	// due to the value not supporting the format, or the value itself not being
-	// renderable.
+	// renderable. Only Renderer implementations should return this error.
 	ErrCannotRender = fmt.Errorf("%w: cannot render", Err)
 )
 
-// Renderer is the interface that that individual renderers must implement.
+// FormatRenderer interface is for single format renderers, which can only
-type Renderer interface {
+// render a single format.
+type FormatRenderer interface {
+	// Render writes v into w in the format that the FormatRenderer supports.
+	//
+	// If v does not implement a required interface, or otherwise cannot be
+	// rendered to the format in question, then a ErrCannotRender error must be
+	// returned. Any other errors should be returned as is.
 	Render(w io.Writer, v any) error
 }
 
 var (
+	// DefaultBinary is the default binary marshaler renderer. It
+	// renders values using the encoding.BinaryMarshaler interface.
+	DefaultBinary = &Binary{}
+
 	// DefaultJSON is the default JSON renderer. It renders values using the
 	// encoding/json package, with pretty printing enabled.
 	DefaultJSON = &JSON{Pretty: true}
 
+	// DefaultText is the default text renderer, used by the package level
+	// Render function. It renders values using the DefaultStringer and
+	// DefaultWriterTo renderers. This means a value must implement either the
+	// fmt.Stringer or io.WriterTo interfaces to be rendered.
+	DefaultText = &Text{}
+
 	// DefaultXML is the default XML renderer. It renders values using the
 	// encoding/xml package, with pretty printing enabled.
 	DefaultXML = &XML{Pretty: true}
@@ -32,39 +62,10 @@ var (
 	// gopkg.in/yaml.v3 package, with an indentation of 2 spaces.
 	DefaultYAML = &YAML{Indent: 2}
 
-	// DefaultWriterTo is the default writer to renderer. It renders values
+	// DefaultRenderer is used by the package level Render function. It supports
-	// using the io.WriterTo interface.
+	// the text", "json", and "yaml" formats. If you need to support another set
-	DefaultWriterTo = &WriterTo{}
+	// of formats, use the New function to create a custom FormatRenderer.
-
+	DefaultRenderer = MustNew("json", "text", "yaml")
-	// DefaultStringer is the default stringer renderer. It renders values
-	// using the fmt.Stringer interface.
-	DefaultStringer = &Stringer{}
-
-	// DefaultText is the default text renderer, used by the package level
-	// Render function. It renders values using the DefaultStringer and
-	// DefaultWriterTo renderers. This means a value must implement either the
-	// fmt.Stringer or io.WriterTo interfaces to be rendered.
-	DefaultText = &MultiRenderer{
-		Renderers: []Renderer{DefaultStringer, DefaultWriterTo},
-	}
-
-	// DefaultBinary is the default binary marshaler renderer. It
-	// renders values using the encoding.BinaryMarshaler interface.
-	DefaultBinary = &Binary{}
-
-	// DefaultRenderer is the default renderer, used by the package level Render
-	// function.
-	DefaultRenderer = &FormatRenderer{map[string]Renderer{
-		"bin":    DefaultBinary,
-		"binary": DefaultBinary,
-		"json":   DefaultJSON,
-		"plain":  DefaultText,
-		"text":   DefaultText,
-		"txt":    DefaultText,
-		"xml":    DefaultXML,
-		"yaml":   DefaultYAML,
-		"yml":    DefaultYAML,
-	}}
 )
 
 // Render renders the given value to the given writer using the given format.
@@ -72,23 +73,63 @@ var (
 //
 // By default it supports the following formats:
 //
+//   - "text": Renders values using the fmt.Stringer and io.WriterTo interfaces.
 //   - "json": Renders values using the encoding/json package, with pretty
 //     printing enabled.
 //   - "yaml": Renders values using the gopkg.in/yaml.v3 package, with an
 //     indentation of 2 spaces.
-//   - "yml": Alias for "yaml".
-//   - "xml": Renders values using the encoding/xml package, with pretty
-//     printing enabled.
-//   - "text": Renders values using the fmt.Stringer and io.WriterTo interfaces.
-//     This means a value must implement either the fmt.Stringer or io.WriterTo
-//     interfaces to be rendered.
-//   - "txt": Alias for "text".
-//   - "plain": Alias for "text".
-//   - "binary": Renders values using the encoding.BinaryMarshaler interface.
-//   - "bin": Alias for "binary".
 //
 // If the format is not supported, a ErrUnsupportedFormat error will be
 // returned.
+//
+// If you need to support a custom set of formats, use the New function to
+// create a new FormatRenderer with the formats you need. If you need new custom
+// renderers, manually create a new FormatRenderer.
 func Render(w io.Writer, format string, v any) error {
 	return DefaultRenderer.Render(w, format, v)
 }
+
+// New creates a new *FormatRenderer with support for the given formats.
+//
+// Supported formats are:
+//
+//   - "binary": Renders values using DefaultBinary.
+//   - "json": Renders values using DefaultJSON.
+//   - "text": Renders values using DefaultText.
+//   - "xml": Renders values using DefaultXML.
+//   - "yaml": Renders values using DefaultYAML.
+//
+// If an unsupported format is given, an ErrUnsupportedFormat error will be
+// returned.
+func New(formats ...string) (*Renderer, error) {
+	renderers := map[string]FormatRenderer{}
+
+	for _, format := range formats {
+		switch format {
+		case "binary":
+			renderers[format] = DefaultBinary
+		case "json":
+			renderers[format] = DefaultJSON
+		case "text":
+			renderers[format] = DefaultText
+		case "xml":
+			renderers[format] = DefaultXML
+		case "yaml":
+			renderers[format] = DefaultYAML
+		default:
+			return nil, fmt.Errorf("%w: %s", ErrUnsupportedFormat, format)
+		}
+	}
+
+	return NewFormatRenderer(renderers), nil
+}
+
+// MustNew is like New, but panics if an error occurs.
+func MustNew(formats ...string) *Renderer {
+	r, err := New(formats...)
+	if err != nil {
+		panic(err)
+	}
+
+	return r
+}

render_example_test.go

@@ -39,7 +39,10 @@ func ExampleRender_json() {
 	buf := &bytes.Buffer{}
 	err := render.Render(buf, "json", data)
 	if err != nil {
-		panic(err)
+		fmt.Printf("err: %s\n", err)
+
+		return
+		// panic(err)
 	}
 	fmt.Println(buf.String())
 
@@ -139,9 +142,13 @@ func ExampleRender_xml() {
 		Tags: []string{"golang", "json", "yaml", "toml"},
 	}
 
+	// Create a new renderer that supports XML in addition to default JSON, YAML
+	// and Text.
+	r := render.MustNew("json", "text", "xml", "yaml")
+
 	// Render the object to XML.
 	buf := &bytes.Buffer{}
-	err := render.Render(buf, "xml", data)
+	err := r.Render(buf, "xml", data)
 	if err != nil {
 		panic(err)
 	}

render_test.go

@@ -35,7 +35,7 @@ type mockRenderer struct {
 	err    error
 }
 
-var _ render.Renderer = (*mockRenderer)(nil)
+var _ render.FormatRenderer = (*mockRenderer)(nil)
 
 func (m *mockRenderer) Render(w io.Writer, _ any) error {
 	_, err := w.Write([]byte(m.output))
@@ -59,23 +59,8 @@ func TestDefaultYAML(t *testing.T) {
 	assert.Equal(t, &render.YAML{Indent: 2}, render.DefaultYAML)
 }
 
-func TestDefaultWriterTo(t *testing.T) {
-	assert.Equal(t, &render.WriterTo{}, render.DefaultWriterTo)
-}
-
-func TestDefaultStringer(t *testing.T) {
-	assert.Equal(t, &render.Stringer{}, render.DefaultStringer)
-}
-
 func TestDefaultText(t *testing.T) {
-	want := &render.MultiRenderer{
+	assert.Equal(t, &render.Text{}, render.DefaultText)
-		Renderers: []render.Renderer{
-			&render.Stringer{},
-			&render.WriterTo{},
-		},
-	}
-
-	assert.Equal(t, want, render.DefaultText)
 }
 
 func TestDefaultBinary(t *testing.T) {
@@ -83,408 +68,253 @@ func TestDefaultBinary(t *testing.T) {
 }
 
 func TestDefaultRenderer(t *testing.T) {
-	want := &render.FormatRenderer{
+	want := &render.Renderer{
-		Renderers: map[string]render.Renderer{
+		Formats: map[string]render.FormatRenderer{
-			"bin":    render.DefaultBinary,
+			"json": render.DefaultJSON,
-			"binary": render.DefaultBinary,
+			"text": render.DefaultText,
-			"json":   render.DefaultJSON,
+			"yaml": render.DefaultYAML,
-			"plain":  render.DefaultText,
-			"text":   render.DefaultText,
-			"txt":    render.DefaultText,
-			"xml":    render.DefaultXML,
-			"yaml":   render.DefaultYAML,
-			"yml":    render.DefaultYAML,
 		},
 	}
 
 	assert.Equal(t, want, render.DefaultRenderer)
 }
 
+type renderFormatTestCase struct {
+	name      string
+	writeErr  error
+	format    string
+	value     any
+	want      string
+	wantErr   string
+	wantErrIs []error
+	wantPanic string
+}
+
+// "binary" format.
+var binaryFormattestCases = []renderFormatTestCase{
+	{
+		name:   "binary format with binary marshaler",
+		format: "binary",
+		value:  &mockBinaryMarshaler{data: []byte("test string")},
+		want:   "test string",
+	},
+	{
+		name:      "binary format without binary marshaler",
+		format:    "binary",
+		value:     struct{}{},
+		wantErr:   "render: unsupported format: binary",
+		wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+	},
+	{
+		name:   "binary format with error marshaling",
+		format: "binary",
+		value: &mockBinaryMarshaler{
+			data: []byte("test string"),
+			err:  errors.New("marshal error!!1"),
+		},
+		wantErr:   "render: failed: marshal error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "binary format with error writing to writer",
+		format:    "binary",
+		writeErr:  errors.New("write error!!1"),
+		value:     &mockBinaryMarshaler{data: []byte("test string")},
+		wantErr:   "render: failed: write error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "binary format with invalid type",
+		format:    "binary",
+		value:     make(chan int),
+		wantErr:   "render: unsupported format: binary",
+		wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+	},
+}
+
+// "json" format.
+var jsonFormatTestCases = []renderFormatTestCase{
+	{
+		name:   "json format",
+		format: "json",
+		value:  map[string]int{"age": 30},
+		want:   "{\n  \"age\": 30\n}\n",
+	},
+	{
+		name:   "json format with json marshaler",
+		format: "json",
+		value:  &mockJSONMarshaler{data: []byte(`{"age":30}`)},
+		want:   "{\n  \"age\": 30\n}\n",
+	},
+	{
+		name:      "json format with error from json marshaler",
+		format:    "json",
+		value:     &mockJSONMarshaler{err: errors.New("marshal error!!1")},
+		wantErrIs: []error{render.Err},
+	},
+	{
+		name:      "json format with error writing to writer",
+		format:    "json",
+		writeErr:  errors.New("write error!!1"),
+		value:     map[string]int{"age": 30},
+		wantErr:   "render: failed: write error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "json format with invalid type",
+		format:    "json",
+		value:     make(chan int),
+		wantErr:   "render: failed: json: unsupported type: chan int",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+}
+
+// "text" format.
+var textFormatTestCases = []renderFormatTestCase{
+	{
+		name:   "text format with fmt.Stringer",
+		format: "text",
+		value:  &mockStringer{value: "test string"},
+		want:   "test string",
+	},
+	{
+		name:   "text format with io.WriterTo",
+		format: "text",
+		value:  &mockWriterTo{value: "test string"},
+		want:   "test string",
+	},
+	{
+		name:      "text format without fmt.Stringer or io.WriterTo",
+		format:    "text",
+		value:     struct{}{},
+		wantErr:   "render: unsupported format: text",
+		wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+	},
+	{
+		name:      "text format with error writing to writer",
+		format:    "text",
+		writeErr:  errors.New("write error!!1"),
+		value:     &mockStringer{value: "test string"},
+		wantErr:   "render: failed: write error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:   "text format with error from io.WriterTo",
+		format: "text",
+		value: &mockWriterTo{
+			value: "test string",
+			err:   errors.New("WriteTo error!!1"),
+		},
+		wantErr:   "render: failed: WriteTo error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "text format with invalid type",
+		format:    "text",
+		value:     make(chan int),
+		wantErr:   "render: unsupported format: text",
+		wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+	},
+}
+
+// "xml" format.
+var xmlFormatTestCases = []renderFormatTestCase{
+	{
+		name:   "xml format",
+		format: "xml",
+		value: struct {
+			XMLName xml.Name `xml:"user"`
+			Age     int      `xml:"age"`
+		}{Age: 30},
+		want: "<user>\n  <age>30</age>\n</user>",
+	},
+	{
+		name:   "xml format with xml.Marshaler",
+		format: "xml",
+		value:  &mockXMLMarshaler{elm: "test string"},
+		want:   "<mockXMLMarshaler>test string</mockXMLMarshaler>",
+	},
+	{
+		name:      "xml format with error from xml.Marshaler",
+		format:    "xml",
+		value:     &mockXMLMarshaler{err: errors.New("marshal error!!1")},
+		wantErr:   "render: failed: marshal error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:     "xml format with error writing to writer",
+		format:   "xml",
+		writeErr: errors.New("write error!!1"),
+		value: struct {
+			XMLName xml.Name `xml:"user"`
+			Age     int      `xml:"age"`
+		}{Age: 30},
+		wantErr:   "render: failed: write error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "xml format with invalid value",
+		format:    "xml",
+		value:     make(chan int),
+		wantErr:   "render: failed: xml: unsupported type: chan int",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+}
+
+// "yaml" format.
+var yamlFormatTestCases = []renderFormatTestCase{
+	{
+		name:   "yaml format",
+		format: "yaml",
+		value:  map[string]int{"age": 30},
+		want:   "age: 30\n",
+	},
+	{
+		name:   "yaml format with nested structure",
+		format: "yaml",
+		value: map[string]any{
+			"user": map[string]any{
+				"age":  30,
+				"name": "John Doe",
+			},
+		},
+		want: "user:\n  age: 30\n  name: John Doe\n",
+	},
+	{
+		name:   "yaml format with yaml.Marshaler",
+		format: "yaml",
+		value:  &mockYAMLMarshaler{val: map[string]int{"age": 30}},
+		want:   "age: 30\n",
+	},
+	{
+		name:      "yaml format with error from yaml.Marshaler",
+		format:    "yaml",
+		value:     &mockYAMLMarshaler{err: errors.New("mock error")},
+		wantErr:   "render: failed: mock error",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "yaml format with error writing to writer",
+		format:    "yaml",
+		writeErr:  errors.New("write error!!1"),
+		value:     map[string]int{"age": 30},
+		wantErr:   "render: failed: yaml: write error: write error!!1",
+		wantErrIs: []error{render.Err, render.ErrFailed},
+	},
+	{
+		name:      "yaml format with invalid type",
+		format:    "yaml",
+		value:     make(chan int),
+		wantPanic: "cannot marshal type: chan int",
+	},
+}
+
 func TestRender(t *testing.T) {
-	tests := []struct {
+	tests := []renderFormatTestCase{}
-		name      string
+	tests = append(tests, jsonFormatTestCases...)
-		writeErr  error
+	tests = append(tests, textFormatTestCases...)
-		format    string
+	tests = append(tests, yamlFormatTestCases...)
-		value     any
+
-		want      string
-		wantErr   string
-		wantErrIs []error
-		wantPanic string
-	}{
-		// "bin" format.
-		{
-			name:   "bin format with binary marshaler",
-			format: "bin",
-			value:  &mockBinaryMarshaler{data: []byte("test string")},
-			want:   "test string",
-		},
-		{
-			name:      "bin format without binary marshaler",
-			format:    "bin",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:   "bin format with error marshaling",
-			format: "bin",
-			value: &mockBinaryMarshaler{
-				data: []byte("test string"),
-				err:  errors.New("marshal error!!1"),
-			},
-			wantErr:   "render: marshal error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "bin format with error writing to writer",
-			format:    "bin",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockBinaryMarshaler{data: []byte("test string")},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "bin format with invalid type",
-			format:    "bin",
-			value:     make(chan int),
-			wantErr:   "render: cannot render",
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		// "binary" format.
-		{
-			name:   "binary format with binary marshaler",
-			format: "binary",
-			value:  &mockBinaryMarshaler{data: []byte("test string")},
-			want:   "test string",
-		},
-		{
-			name:      "binary format without binary marshaler",
-			format:    "binary",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:   "binary format with error marshaling",
-			format: "binary",
-			value: &mockBinaryMarshaler{
-				data: []byte("test string"),
-				err:  errors.New("marshal error!!1"),
-			},
-			wantErr:   "render: marshal error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "binary format with error writing to writer",
-			format:    "binary",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockBinaryMarshaler{data: []byte("test string")},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "binary format with invalid type",
-			format:    "binary",
-			value:     make(chan int),
-			wantErr:   "render: cannot render",
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		// "json" format.
-		{
-			name:   "json format",
-			format: "json",
-			value:  map[string]int{"age": 30},
-			want:   "{\n  \"age\": 30\n}\n",
-		},
-		{
-			name:   "json format with json marshaler",
-			format: "json",
-			value:  &mockJSONMarshaler{data: []byte(`{"age":30}`)},
-			want:   "{\n  \"age\": 30\n}\n",
-		},
-		{
-			name:      "json format with error from json marshaler",
-			format:    "json",
-			value:     &mockJSONMarshaler{err: errors.New("marshal error!!1")},
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "json format with error writing to writer",
-			format:    "json",
-			writeErr:  errors.New("write error!!1"),
-			value:     map[string]int{"age": 30},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "json format with invalid type",
-			format:    "json",
-			value:     make(chan int),
-			wantErr:   "render: json: unsupported type: chan int",
-			wantErrIs: []error{render.Err},
-		},
-		// "plain" format.
-		{
-			name:   "plain format with fmt.Stringer",
-			format: "plain",
-			value:  &mockStringer{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:   "plain format with io.WriterTo",
-			format: "plain",
-			value:  &mockWriterTo{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:      "plain format without fmt.Stringer or io.WriterTo",
-			format:    "plain",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:      "plain format with error writing to writer",
-			format:    "plain",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockStringer{value: "test string"},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:   "plain format with error from io.WriterTo",
-			format: "plain",
-			value: &mockWriterTo{
-				value: "test string",
-				err:   errors.New("WriteTo error!!1"),
-			},
-			wantErr:   "render: WriteTo error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "plain format with invalid type",
-			format:    "plain",
-			value:     make(chan int),
-			wantErr:   "render: cannot render",
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		// "text" format.
-		{
-			name:   "text format with fmt.Stringer",
-			format: "text",
-			value:  &mockStringer{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:   "text format with io.WriterTo",
-			format: "text",
-			value:  &mockWriterTo{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:      "text format without fmt.Stringer or io.WriterTo",
-			format:    "text",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:      "text format with error writing to writer",
-			format:    "text",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockStringer{value: "test string"},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:   "text format with error from io.WriterTo",
-			format: "text",
-			value: &mockWriterTo{
-				value: "test string",
-				err:   errors.New("WriteTo error!!1"),
-			},
-			wantErr:   "render: WriteTo error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "text format with invalid type",
-			format:    "text",
-			value:     make(chan int),
-			wantErr:   "render: cannot render",
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		// "txt" format.
-		{
-			name:   "txt format with fmt.Stringer",
-			format: "txt",
-			value:  &mockStringer{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:   "txt format with io.WriterTo",
-			format: "txt",
-			value:  &mockWriterTo{value: "test string"},
-			want:   "test string",
-		},
-		{
-			name:      "txt format without fmt.Stringer or io.WriterTo",
-			format:    "txt",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:      "txt format with error writing to writer",
-			format:    "txt",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockStringer{value: "test string"},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:   "txt format with error from io.WriterTo",
-			format: "txt",
-			value: &mockWriterTo{
-				value: "test string",
-				err:   errors.New("WriteTo error!!1"),
-			},
-			wantErr:   "render: WriteTo error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "txt format with invalid type",
-			format:    "txt",
-			value:     make(chan int),
-			wantErr:   "render: cannot render",
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		// "xml" format.
-		{
-			name:   "xml format",
-			format: "xml",
-			value: struct {
-				XMLName xml.Name `xml:"user"`
-				Age     int      `xml:"age"`
-			}{Age: 30},
-			want: "<user>\n  <age>30</age>\n</user>",
-		},
-		{
-			name:   "xml format with xml.Marshaler",
-			format: "xml",
-			value:  &mockXMLMarshaler{elm: "test string"},
-			want:   "<mockXMLMarshaler>test string</mockXMLMarshaler>",
-		},
-		{
-			name:      "xml format with error from xml.Marshaler",
-			format:    "xml",
-			value:     &mockXMLMarshaler{err: errors.New("marshal error!!1")},
-			wantErr:   "render: marshal error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:     "xml format with error writing to writer",
-			format:   "xml",
-			writeErr: errors.New("write error!!1"),
-			value: struct {
-				XMLName xml.Name `xml:"user"`
-				Age     int      `xml:"age"`
-			}{Age: 30},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "xml format with invalid value",
-			format:    "xml",
-			value:     make(chan int),
-			wantErr:   "render: xml: unsupported type: chan int",
-			wantErrIs: []error{render.Err},
-		},
-		// "yaml" format.
-		{
-			name:   "yaml format",
-			format: "yaml",
-			value:  map[string]int{"age": 30},
-			want:   "age: 30\n",
-		},
-		{
-			name:   "yaml format with nested structure",
-			format: "yaml",
-			value: map[string]any{
-				"user": map[string]any{
-					"age":  30,
-					"name": "John Doe",
-				},
-			},
-			want: "user:\n  age: 30\n  name: John Doe\n",
-		},
-		{
-			name:   "yaml format with yaml.Marshaler",
-			format: "yaml",
-			value:  &mockYAMLMarshaler{val: map[string]int{"age": 30}},
-			want:   "age: 30\n",
-		},
-		{
-			name:      "yaml format with error from yaml.Marshaler",
-			format:    "yaml",
-			value:     &mockYAMLMarshaler{err: errors.New("mock error")},
-			wantErr:   "render: mock error",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "yaml format with error writing to writer",
-			format:    "yaml",
-			writeErr:  errors.New("write error!!1"),
-			value:     map[string]int{"age": 30},
-			wantErr:   "render: yaml: write error: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "yaml format with invalid type",
-			format:    "yaml",
-			value:     make(chan int),
-			wantPanic: "cannot marshal type: chan int",
-		},
-		// "yml" format.
-		{
-			name:   "yml format",
-			format: "yml",
-			value:  map[string]int{"age": 30},
-			want:   "age: 30\n",
-		},
-		{
-			name:   "yml format with nested structure",
-			format: "yml",
-			value: map[string]any{
-				"user": map[string]any{
-					"age":  30,
-					"name": "John Doe",
-				},
-			},
-			want: "user:\n  age: 30\n  name: John Doe\n",
-		},
-		{
-			name:   "yml format with yaml.Marshaler",
-			format: "yml",
-			value:  &mockYAMLMarshaler{val: map[string]int{"age": 30}},
-			want:   "age: 30\n",
-		},
-		{
-			name:      "yml format with error from yaml.Marshaler",
-			format:    "yml",
-			value:     &mockYAMLMarshaler{err: errors.New("mock error")},
-			wantErr:   "render: mock error",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "yml format with error writing to writer",
-			format:    "yml",
-			writeErr:  errors.New("write error!!1"),
-			value:     map[string]int{"age": 30},
-			wantErr:   "render: yaml: write error: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-		{
-			name:      "yml format with invalid type",
-			format:    "yml",
-			value:     make(chan int),
-			wantPanic: "cannot marshal type: chan int",
-		},
-	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			w := &mockWriter{WriteErr: tt.writeErr}

renderer.go

@@ -0,0 +1,53 @@
+package render
+
+import (
+	"errors"
+	"fmt"
+	"io"
+)
+
+// ErrUnsupportedFormat is returned when a format is not supported by a
+// renderer. Any method that accepts a format string may return this error.
+var ErrUnsupportedFormat = fmt.Errorf("%w: unsupported format", Err)
+
+// Renderer is a renderer that delegates rendering to another renderer
+// based on a format value.
+type Renderer struct {
+	// Formats is a map of format names to renderers. When Render is called,
+	// the format is used to look up the renderer to use.
+	Formats map[string]FormatRenderer
+}
+
+// NewFormatRenderer returns a new FormatRenderer that delegates rendering to
+// the specified renderers.
+func NewFormatRenderer(formats map[string]FormatRenderer) *Renderer {
+	return &Renderer{Formats: formats}
+}
+
+// Render renders a value to an io.Writer using the specified format. If the
+// format is not supported, ErrUnsupportedFormat is returned.
+//
+// If the format is supported, but the value cannot be rendered to the format,
+// the error returned by the renderer is returned. In most cases this will be
+// ErrCannotRender, but it could be a different error if the renderer returns
+// one.
+func (r *Renderer) Render(w io.Writer, format string, v any) error {
+	renderer, ok := r.Formats[format]
+	if !ok {
+		return fmt.Errorf("%w: %s", ErrUnsupportedFormat, format)
+	}
+
+	err := renderer.Render(w, v)
+	if err != nil {
+		if errors.Is(err, ErrCannotRender) {
+			return fmt.Errorf("%w: %s", ErrUnsupportedFormat, format)
+		}
+		if !errors.Is(err, ErrFailed) {
+			return fmt.Errorf("%w: %w", ErrFailed, err)
+		}
+
+		return err
+	}
+
+	return nil
+}

renderer_test.go

@@ -0,0 +1,138 @@
+package render_test
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"testing"
+
+	"github.com/jimeh/go-render"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRenderer_Render(t *testing.T) {
+	tests := []struct {
+		name      string
+		renderers map[string]render.FormatRenderer
+		format    string
+		value     interface{}
+		want      string
+		wantErr   string
+		wantErrIs []error
+	}{
+		{
+			name: "existing renderer",
+			renderers: map[string]render.FormatRenderer{
+				"mock": &mockRenderer{output: "mock output"},
+			},
+			format: "mock",
+			value:  struct{}{},
+			want:   "mock output",
+		},
+		{
+			name: "existing renderer returns error",
+			renderers: map[string]render.FormatRenderer{
+				"other": &mockRenderer{
+					output: "mock output",
+					err:    errors.New("mock error"),
+				},
+			},
+			format:  "other",
+			value:   struct{}{},
+			wantErr: "render: failed: mock error",
+		},
+		{
+			name: "existing renderer returns ErrCannotRender",
+			renderers: map[string]render.FormatRenderer{
+				"other": &mockRenderer{
+					output: "mock output",
+					err:    fmt.Errorf("%w: mock", render.ErrCannotRender),
+				},
+			},
+			format:    "other",
+			value:     struct{}{},
+			wantErr:   "render: unsupported format: other",
+			wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+		},
+		{
+			name:      "non-existing renderer",
+			renderers: map[string]render.FormatRenderer{},
+			format:    "unknown",
+			value:     struct{}{},
+			wantErr:   "render: unsupported format: unknown",
+			wantErrIs: []error{render.Err, render.ErrUnsupportedFormat},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			fr := &render.Renderer{
+				Formats: tt.renderers,
+			}
+			var buf bytes.Buffer
+
+			err := fr.Render(&buf, tt.format, tt.value)
+			got := buf.String()
+
+			if tt.wantErr != "" {
+				assert.EqualError(t, err, tt.wantErr)
+			}
+			for _, e := range tt.wantErrIs {
+				assert.ErrorIs(t, err, e)
+			}
+
+			if tt.wantErr == "" && len(tt.wantErrIs) == 0 {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, got)
+			}
+		})
+	}
+}
+
+func TestRenderer_RenderAllFormats(t *testing.T) {
+	tests := []renderFormatTestCase{}
+	tests = append(tests, binaryFormattestCases...)
+	tests = append(tests, jsonFormatTestCases...)
+	tests = append(tests, textFormatTestCases...)
+	tests = append(tests, xmlFormatTestCases...)
+	tests = append(tests, yamlFormatTestCases...)
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := &mockWriter{WriteErr: tt.writeErr}
+
+			var err error
+			var panicRes any
+			renderer, err := render.New("binary", "json", "text", "xml", "yaml")
+			require.NoError(t, err)
+
+			func() {
+				defer func() {
+					if r := recover(); r != nil {
+						panicRes = r
+					}
+				}()
+				err = renderer.Render(w, tt.format, tt.value)
+			}()
+
+			got := w.String()
+
+			if tt.wantPanic != "" {
+				assert.Equal(t, tt.wantPanic, panicRes)
+			}
+
+			if tt.wantErr != "" {
+				assert.EqualError(t, err, tt.wantErr)
+			}
+			for _, e := range tt.wantErrIs {
+				assert.ErrorIs(t, err, e)
+			}
+
+			if tt.wantPanic == "" &&
+				tt.wantErr == "" && len(tt.wantErrIs) == 0 {
+				assert.NoError(t, err)
+				assert.Equal(t, tt.want, got)
+			}
+		})
+	}
+}

stringer.go

@@ -1,28 +0,0 @@
-package render
-
-import (
-	"fmt"
-	"io"
-)
-
-// Stringer is a renderer that renders a value to an io.Writer using the
-// String method.
-type Stringer struct{}
-
-var _ Renderer = (*Stringer)(nil)
-
-// Render renders a value to an io.Writer using the String method. If the value
-// does not implement fmt.Stringer, ErrCannotRender is returned.
-func (s *Stringer) Render(w io.Writer, v any) error {
-	x, ok := v.(fmt.Stringer)
-	if !ok {
-		return ErrCannotRender
-	}
-
-	_, err := w.Write([]byte(x.String()))
-	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
-	}
-
-	return nil
-}

stringer_test.go

@@ -1,70 +0,0 @@
-package render_test
-
-import (
-	"errors"
-	"fmt"
-	"testing"
-
-	"github.com/jimeh/go-render"
-	"github.com/stretchr/testify/assert"
-)
-
-type mockStringer struct {
-	value string
-}
-
-var _ fmt.Stringer = (*mockStringer)(nil)
-
-func (ms *mockStringer) String() string {
-	return ms.value
-}
-
-func TestStringer_Render(t *testing.T) {
-	tests := []struct {
-		name      string
-		writeErr  error
-		value     any
-		want      string
-		wantErr   string
-		wantErrIs []error
-	}{
-		{
-			name:  "implements fmt.Stringer",
-			value: &mockStringer{value: "test string"},
-			want:  "test string",
-		},
-		{
-			name:      "does not implement fmt.Stringer",
-			value:     struct{}{},
-			wantErrIs: []error{render.Err, render.ErrCannotRender},
-		},
-		{
-			name:      "error writing to writer",
-			writeErr:  errors.New("write error!!1"),
-			value:     &mockStringer{value: "test string"},
-			wantErr:   "render: write error!!1",
-			wantErrIs: []error{render.Err},
-		},
-	}
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			s := &render.Stringer{}
-			w := &mockWriter{WriteErr: tt.writeErr}
-
-			err := s.Render(w, tt.value)
-			got := w.String()
-
-			if tt.wantErr != "" {
-				assert.EqualError(t, err, tt.wantErr)
-			}
-			for _, e := range tt.wantErrIs {
-				assert.ErrorIs(t, err, e)
-			}
-
-			if tt.wantErr == "" && len(tt.wantErrIs) == 0 {
-				assert.NoError(t, err)
-				assert.Equal(t, tt.want, got)
-			}
-		})
-	}
-}

text.go

@@ -0,0 +1,28 @@
+package render
+
+import (
+	"fmt"
+	"io"
+)
+
+type Text struct{}
+
+var _ FormatRenderer = (*Text)(nil)
+
+func (t *Text) Render(w io.Writer, v any) error {
+	var err error
+	switch x := v.(type) {
+	case fmt.Stringer:
+		_, err = w.Write([]byte(x.String()))
+	case io.WriterTo:
+		_, err = x.WriteTo(w)
+	default:
+		return ErrCannotRender
+	}
+
+	if err != nil {
+		return fmt.Errorf("%w: %w", ErrFailed, err)
+	}
+
+	return nil
+}

text_test.go

@@ -1,8 +1,8 @@
 package render_test
 
 import (
-	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"testing"
 
@@ -10,6 +10,16 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+type mockStringer struct {
+	value string
+}
+
+var _ fmt.Stringer = (*mockStringer)(nil)
+
+func (ms *mockStringer) String() string {
+	return ms.value
+}
+
 type mockWriterTo struct {
 	value string
 	err   error
@@ -27,7 +37,7 @@ func (m *mockWriterTo) WriteTo(w io.Writer) (int64, error) {
 	return int64(n), err
 }
 
-func TestWriterTo_Render(t *testing.T) {
+func TestText_Render(t *testing.T) {
 	tests := []struct {
 		name      string
 		writeErr  error
@@ -36,33 +46,45 @@ func TestWriterTo_Render(t *testing.T) {
 		wantErr   string
 		wantErrIs []error
 	}{
+		{
+			name:  "implements fmt.Stringer",
+			value: &mockStringer{value: "test string"},
+			want:  "test string",
+		},
+		{
+			name:      "error writing to writer with fmt.Stringer",
+			writeErr:  errors.New("write error!!1"),
+			value:     &mockStringer{value: "test string"},
+			wantErr:   "render: failed: write error!!1",
+			wantErrIs: []error{render.Err, render.ErrFailed},
+		},
 		{
 			name:  "implements io.WriterTo",
 			value: &mockWriterTo{value: "test string"},
 			want:  "test string",
 		},
 		{
-			name:      "does not implement io.WriterTo",
+			name:      "does not implement fmt.Stringer or io.WriterTo",
 			value:     struct{}{},
 			wantErr:   "render: cannot render",
 			wantErrIs: []error{render.Err, render.ErrCannotRender},
 		},
 		{
-			name: "error writing to writer",
+			name: "error writing to writer with io.WriterTo",
 			value: &mockWriterTo{
 				value: "test string",
 				err:   errors.New("WriteTo error!!1"),
 			},
-			wantErr:   "render: WriteTo error!!1",
+			wantErr:   "render: failed: WriteTo error!!1",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			wt := &render.WriterTo{}
+			s := &render.Text{}
-			w := &bytes.Buffer{}
+			w := &mockWriter{WriteErr: tt.writeErr}
 
-			err := wt.Render(w, tt.value)
+			err := s.Render(w, tt.value)
 			got := w.String()
 
 			if tt.wantErr != "" {

writer_to.go

@@ -1,28 +0,0 @@
-package render
-
-import (
-	"fmt"
-	"io"
-)
-
-// WriterTo is a renderer that renders a value to an io.Writer using the
-// WriteTo method.
-type WriterTo struct{}
-
-var _ Renderer = (*WriterTo)(nil)
-
-// Render renders a value to an io.Writer using the WriteTo method. If the value
-// does not implement io.WriterTo, ErrCannotRender is returned.
-func (wt *WriterTo) Render(w io.Writer, v any) error {
-	x, ok := v.(io.WriterTo)
-	if !ok {
-		return ErrCannotRender
-	}
-
-	_, err := x.WriteTo(w)
-	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
-	}
-
-	return nil
-}

xml.go

@@ -21,7 +21,7 @@ type XML struct {
 	Indent string
 }
 
-var _ Renderer = (*XML)(nil)
+var _ FormatRenderer = (*XML)(nil)
 
 // Render marshals the given value to XML.
 func (x *XML) Render(w io.Writer, v any) error {
@@ -38,7 +38,7 @@ func (x *XML) Render(w io.Writer, v any) error {
 
 	err := enc.Encode(v)
 	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
+		return fmt.Errorf("%w: %w", ErrFailed, err)
 	}
 
 	return nil

xml_test.go

@@ -89,15 +89,15 @@ func TestXML_Render(t *testing.T) {
 		{
 			name:      "error from xml.Marshaler",
 			value:     &mockXMLMarshaler{err: errors.New("mock error")},
-			wantErr:   "render: mock error",
+			wantErr:   "render: failed: mock error",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 		{
 			name:      "invalid value",
 			pretty:    false,
 			value:     make(chan int),
-			wantErr:   "render: xml: unsupported type: chan int",
+			wantErr:   "render: failed: xml: unsupported type: chan int",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 	}
 

yaml.go

@@ -14,7 +14,7 @@ type YAML struct {
 	Indent int
 }
 
-var _ Renderer = (*YAML)(nil)
+var _ FormatRenderer = (*YAML)(nil)
 
 // Render marshals the given value to YAML.
 func (j *YAML) Render(w io.Writer, v any) error {
@@ -29,7 +29,7 @@ func (j *YAML) Render(w io.Writer, v any) error {
 
 	err := enc.Encode(v)
 	if err != nil {
-		return fmt.Errorf("%w: %w", Err, err)
+		return fmt.Errorf("%w: %w", ErrFailed, err)
 	}
 
 	return nil

yaml_test.go

@@ -66,8 +66,8 @@ func TestYAML_Render(t *testing.T) {
 		{
 			name:      "error from yaml.Marshaler",
 			value:     &mockYAMLMarshaler{err: errors.New("mock error")},
-			wantErr:   "render: mock error",
+			wantErr:   "render: failed: mock error",
-			wantErrIs: []error{render.Err},
+			wantErrIs: []error{render.Err, render.ErrFailed},
 		},
 		{
 			name:      "invalid value",