refactor: unify the error paths in one Path type

Assisted-by: GLM 5.3 Flash

CHANGELOG.md

@@ -94,6 +94,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- `DecodeError` and `EncodeError` carry one `Path` type, a list of segments
+  (`"items"`, `"[0]"`, `"weight"`) with a `String()` rendering the TOML
+  notation, `items[0].weight`. The decode error used to hold a bare
+  `[]string`, the encode error a plain string. Both messages render the same
+  way now, `interpres: items[0].weight: ...`, with one `interpres:` prefix
+  where the composition used to double it.
 - `omitempty` follows the encoding/json semantics: the field is skipped when
   it holds an empty string, a zero number, `false`, a nil pointer or
   interface, or a nil or empty slice, array or map. In 1.x the option covered

decode_test.go

@@ -725,8 +725,8 @@ func TestDecodeErrorCarriesPath(t *testing.T) {
 	if de.Err == nil || !strings.Contains(de.Err.Error(), "overflows uint8") {
 		t.Fatalf("Err = %v", de.Err)
 	}
-	// The rendered message keeps its shape: segments joined with ": ".
+	// The rendered message uses the Path notation.
-	wantMsg := "items: [0]: weight: interpres: integer 300 overflows uint8"
+	wantMsg := "interpres: items[0].weight: integer 300 overflows uint8"
 	if err.Error() != wantMsg {
 		t.Fatalf("message = %q, want %q", err.Error(), wantMsg)
 	}
@@ -1310,7 +1310,7 @@ func TestDecodeFixedArray(t *testing.T) {
 			Ports [3]int `toml:"ports"`
 		}
 		err := Unmarshal([]byte("ports = [8080, 9090]\n"), &cfg)
-		want := "ports: interpres: cannot assign 2 elements to [3]int"
+		want := "interpres: ports: cannot assign 2 elements to [3]int"
 		if err == nil || err.Error() != want {
 			t.Errorf("err = %v, want %q", err, want)
 		}
@@ -1344,7 +1344,7 @@ func TestRequiredTag(t *testing.T) {
 			Server Config `toml:"server"`
 		}
 		err := Unmarshal([]byte("[server]\nradius = 1\n"), &outer)
-		want := `server: interpres: missing required key "host"`
+		want := `interpres: server: missing required key "host"`
 		if err == nil || err.Error() != want {
 			t.Errorf("err = %v, want %q", err, want)
 		}
@@ -1477,3 +1477,13 @@ func TestDecodeMergesIntoNonEmptyMap(t *testing.T) {
 		t.Errorf("lang = %v, want the key added", dst["lang"])
 	}
 }
+
+func TestPathString(t *testing.T) {
+	p := Path{"server", "ports", "[2]", "host"}
+	if got, want := p.String(), "server.ports[2].host"; got != want {
+		t.Errorf("String() = %q, want %q", got, want)
+	}
+	if got := (Path{}).String(); got != "" {
+		t.Errorf("String() of an empty path = %q, want the empty string", got)
+	}
+}

docs/API.md

@@ -738,26 +738,32 @@ if se, ok := errors.AsType[*interpres.SyntaxError](err); ok {
 followed by a caret line marking the column, for messages the reader sees
 under the input.
 
-### `type DecodeError struct{ Path []string; Err error }`
+### `type DecodeError struct{ Path Path; Err error }`
 
 Wraps a decoding failure with the key path at which it happened. `Path` lists
 one segment per level from the document root, the outermost key first: a key
 contributes its name, an array element its bracketed index, so the path of the
-`weight` field in the first item reads `["items", "[0]", "weight"]`. The
+`weight` field in the first item reads `["items", "[0]", "weight"]` and its
-rendered message is unchanged by the type; read the fields instead of parsing
+`String()` renders `items[0].weight`. Read the fields instead of parsing the
-the message:
+message:
 
 ```go
 if de, ok := errors.AsType[*interpres.DecodeError](err); ok {
-	fmt.Println(de.Path, de.Err)
+	fmt.Println(de.Path.String(), de.Err)
 }
 ```
 
-### `type EncodeError struct{ Path string; Err error }`
+### `type EncodeError struct{ Path Path; Err error }`
 
-Wraps an encoding failure with the key path of the value that failed, in the
+Wraps an encoding failure with the key path of the value that failed, the
-document's own notation: `server.ports[2]`. Read it with `errors.AsType` the
+same `Path` type the decode error carries, so `server.ports[2]` reads the
-same way.
+same on both sides. Read it with `errors.AsType` the same way.
+
+### `type Path []string`
+
+The path both error wrappers carry, one segment per level from the document
+root. `String()` renders the TOML notation: keys join with dots, an index
+attaches to the previous segment in brackets, `items[0].weight`.
 
 ### `type Decoder`
 

encode.go

@@ -249,7 +249,7 @@ func buildOrderedDoc(om *OrderedMap, doc *tomlDoc, path encPath) error {
 		case OrderedMap:
 			sub := &tomlDoc{ctx: doc.ctx, opts: doc.opts, depth: doc.depth + 1}
 			if atDepthLimit(sub.depth) {
-				return &EncodeError{Path: path.key(key).String(), Err: errDepthLimit()}
+				return &EncodeError{Path: path.key(key).segments(), Err: errDepthLimit()}
 			}
 			if err := buildOrderedDoc(&x, sub, path.key(key)); err != nil {
 				return err
@@ -262,7 +262,7 @@ func buildOrderedDoc(om *OrderedMap, doc *tomlDoc, path encPath) error {
 			}
 			sub := &tomlDoc{ctx: doc.ctx, opts: doc.opts, depth: doc.depth + 1}
 			if atDepthLimit(sub.depth) {
-				return &EncodeError{Path: path.key(key).String(), Err: errDepthLimit()}
+				return &EncodeError{Path: path.key(key).segments(), Err: errDepthLimit()}
 			}
 			if err := buildOrderedDoc(x, sub, path.key(key)); err != nil {
 				return err
@@ -384,10 +384,14 @@ func (p *encPath) elem(i int) encPath {
 	return encPath{parent: p, index: i}
 }
 
-// String renders the path root first: keys join with dots and an array
+// String renders the path root first, in the notation Path.String uses.
-// element carries its bracketed index, so the third port under server reads
-// "server.ports[2]".
 func (p encPath) String() string {
+	return Path(p.segments()).String()
+}
+
+// segments returns the path's segments, root first: a key contributes its
+// name, an array element its bracketed index.
+func (p encPath) segments() []string {
 	var parts []string
 	for s := &p; s != nil; s = s.parent {
 		switch {
@@ -399,18 +403,8 @@ func (p encPath) String() string {
 			parts = append(parts, s.name)
 		}
 	}
-	var b strings.Builder
+	slices.Reverse(parts)
-	for _, part := range slices.Backward(parts) {
+	return parts
-		if strings.HasPrefix(part, "[") {
-			b.WriteString(part)
-			continue
-		}
-		if b.Len() > 0 {
-			b.WriteByte('.')
-		}
-		b.WriteString(part)
-	}
-	return b.String()
 }
 
 // --- reflection walk: struct ---------------------------------------------
@@ -637,10 +631,10 @@ func addField(doc *tomlDoc, name string, v reflect.Value, path encPath, forceInl
 	if m, ok := marshalerOf(v); ok {
 		mv, err := m.MarshalTOML()
 		if err != nil {
-			return &EncodeError{Path: path.key(name).String(), Err: err}
+			return &EncodeError{Path: path.key(name).segments(), Err: err}
 		}
 		if mv == nil {
-			return &EncodeError{Path: path.key(name).String(), Err: errNilMarshalTOML}
+			return &EncodeError{Path: path.key(name).segments(), Err: errNilMarshalTOML}
 		}
 		v = reflect.ValueOf(mv)
 	}
@@ -648,7 +642,7 @@ func addField(doc *tomlDoc, name string, v reflect.Value, path encPath, forceInl
 	// a scalar kind or a struct.
 	s, isText, err := textValue(v)
 	if err != nil {
-		return &EncodeError{Path: path.key(name).String(), Err: err}
+		return &EncodeError{Path: path.key(name).segments(), Err: err}
 	}
 	if isText {
 		doc.addScalar(name, s)
@@ -668,7 +662,7 @@ func addField(doc *tomlDoc, name string, v reflect.Value, path encPath, forceInl
 		om := v.Interface().(OrderedMap)
 		sub := &tomlDoc{ctx: doc.ctx, opts: doc.opts, depth: doc.depth + 1}
 		if atDepthLimit(sub.depth) {
-			return &EncodeError{Path: path.key(name).String(), Err: errDepthLimit()}
+			return &EncodeError{Path: path.key(name).segments(), Err: errDepthLimit()}
 		}
 		if err := buildOrderedDoc(&om, sub, path.key(name)); err != nil {
 			return err
@@ -700,7 +694,7 @@ func addField(doc *tomlDoc, name string, v reflect.Value, path encPath, forceInl
 func addSubTable(doc *tomlDoc, name string, v reflect.Value, path encPath, forceInline bool) error {
 	sub := &tomlDoc{ctx: doc.ctx, opts: doc.opts, depth: doc.depth + 1}
 	if atDepthLimit(sub.depth) {
-		return &EncodeError{Path: path.key(name).String(), Err: errDepthLimit()}
+		return &EncodeError{Path: path.key(name).segments(), Err: errDepthLimit()}
 	}
 	switch v.Kind() {
 	case reflect.Struct:
@@ -775,7 +769,7 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, path encPath, for
 	// A `,inline` tag on an array of tables asks for a form that would change
 	// the value's Go type on re-parse, so the error is the honest answer.
 	if forceInline && allTables {
-		return &EncodeError{Path: path.key(name).String(), Err: errors.New("an array of tables has no inline form")}
+		return &EncodeError{Path: path.key(name).segments(), Err: errors.New("an array of tables has no inline form")}
 	}
 	if allTables {
 		subs := make([]*tomlDoc, n)
@@ -787,7 +781,7 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, path encPath, for
 			}
 			sub := &tomlDoc{ctx: doc.ctx, opts: doc.opts, depth: doc.depth + 1}
 			if atDepthLimit(sub.depth) {
-				return &EncodeError{Path: apath.elem(i).String(), Err: errDepthLimit()}
+				return &EncodeError{Path: apath.elem(i).segments(), Err: errDepthLimit()}
 			}
 			switch {
 			case ev.Type() == orderedMapType:
@@ -797,7 +791,7 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, path encPath, for
 				}
 			case ev.Kind() == reflect.Struct:
 				if isScalarStruct(ev.Type()) {
-					return &EncodeError{Path: apath.elem(i).String(), Err: errors.New("heterogeneous array contains scalar")}
+					return &EncodeError{Path: apath.elem(i).segments(), Err: errors.New("heterogeneous array contains scalar")}
 				}
 				if err := buildStructDoc(ev, sub, apath.elem(i)); err != nil {
 					return err
@@ -807,7 +801,7 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, path encPath, for
 					return err
 				}
 			default:
-				return &EncodeError{Path: apath.elem(i).String(), Err: errors.New("heterogeneous array, expected table")}
+				return &EncodeError{Path: apath.elem(i).segments(), Err: errors.New("heterogeneous array, expected table")}
 			}
 			subs[i] = sub
 		}
@@ -826,7 +820,7 @@ func addArrayValue(doc *tomlDoc, name string, v reflect.Value, path encPath, for
 		}
 		val, err := normaliseValue(ev)
 		if err != nil {
-			return &EncodeError{Path: apath.elem(i).String(), Err: err}
+			return &EncodeError{Path: apath.elem(i).segments(), Err: err}
 		}
 		items[i] = val
 	}
@@ -867,7 +861,7 @@ func marshalerOf(v reflect.Value) (Marshaler, bool) {
 func resolveElement(v reflect.Value, path encPath) (reflect.Value, error) {
 	ev := followPtr(v)
 	if !ev.IsValid() {
-		return ev, &EncodeError{Path: path.String(), Err: errors.New("nil element")}
+		return ev, &EncodeError{Path: path.segments(), Err: errors.New("nil element")}
 	}
 	m, ok := marshalerOf(ev)
 	if !ok {
@@ -875,14 +869,14 @@ func resolveElement(v reflect.Value, path encPath) (reflect.Value, error) {
 	}
 	mv, err := m.MarshalTOML()
 	if err != nil {
-		return reflect.Value{}, &EncodeError{Path: path.String(), Err: err}
+		return reflect.Value{}, &EncodeError{Path: path.segments(), Err: err}
 	}
 	if mv == nil {
-		return reflect.Value{}, &EncodeError{Path: path.String(), Err: errNilMarshalTOML}
+		return reflect.Value{}, &EncodeError{Path: path.segments(), Err: errNilMarshalTOML}
 	}
 	ev = followPtr(reflect.ValueOf(mv))
 	if !ev.IsValid() {
-		return ev, &EncodeError{Path: path.String(), Err: errors.New("nil element")}
+		return ev, &EncodeError{Path: path.segments(), Err: errors.New("nil element")}
 	}
 	return ev, nil
 }

encode_test.go

@@ -402,8 +402,8 @@ func TestMarshalRejectsNilMarshalerResult(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "f" {
+	if ee.Path.String() != "f" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "f")
+		t.Fatalf("Path = %v, want f", ee.Path)
 	}
 
 	// Inside a value array the nil result used to reach reflection as a zero
@@ -1339,8 +1339,8 @@ func TestEncodeErrorCarriesPath(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "server.port" {
+	if ee.Path.String() != "server.port" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "server.port")
+		t.Fatalf("Path = %v, want server.port", ee.Path)
 	}
 	if ee.Err == nil || ee.Err.Error() != "bad timestamp" {
 		t.Fatalf("Err = %v", ee.Err)
@@ -1359,8 +1359,8 @@ func TestEncodeErrorTopLevelPathHasNoLeadingDot(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "port" {
+	if ee.Path.String() != "port" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "port")
+		t.Fatalf("Path = %v, want port", ee.Path)
 	}
 	if err.Error() != "interpres: port: bad timestamp" {
 		t.Fatalf("message = %q", err.Error())
@@ -1382,8 +1382,8 @@ func TestEncodeErrorHeterogeneousArrayPath(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "items[0]" {
+	if ee.Path.String() != "items[0]" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "items[0]")
+		t.Fatalf("Path = %v, want items[0]", ee.Path)
 	}
 }
 
@@ -1541,8 +1541,8 @@ func TestMarshalTextErrorCarriesPath(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "inner.f" {
+	if ee.Path.String() != "inner.f" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "inner.f")
+		t.Fatalf("Path = %v, want inner.f", ee.Path)
 	}
 }
 
@@ -1920,8 +1920,8 @@ func TestMarshalerElementErrorCarriesPath(t *testing.T) {
 	if !ok {
 		t.Fatalf("expected an *EncodeError, got %T: %v", err, err)
 	}
-	if ee.Path != "items[1]" {
+	if ee.Path.String() != "items[1]" {
-		t.Fatalf("Path = %q, want %q", ee.Path, "items[1]")
+		t.Fatalf("Path = %v, want items[1]", ee.Path)
 	}
 }
 

interpres.go

@@ -68,37 +68,65 @@ func (e *SyntaxError) SourceLine(src []byte) string {
 	return string(src[start:end]) + "\n" + strings.Repeat(" ", off-start) + "^"
 }
 
+// A Path names a value in a document, one segment per level from the root:
+// a key contributes its name and an array element its bracketed index, so the
+// path of the weight field of the first item is the segments
+// ["items", "[0]", "weight"]. String renders the TOML notation,
+// "items[0].weight".
+type Path []string
+
+// String renders the path the way a TOML document writes it: keys join with
+// dots and an index attaches to the previous segment in brackets.
+func (p Path) String() string {
+	var b strings.Builder
+	for _, s := range p {
+		if strings.HasPrefix(s, "[") {
+			b.WriteString(s)
+			continue
+		}
+		if b.Len() > 0 {
+			b.WriteByte('.')
+		}
+		b.WriteString(s)
+	}
+	return b.String()
+}
+
 // A DecodeError wraps a decoding failure with the key path at which it
-// happened. Path lists one segment per level from the document root, the
+// happened. Read the path programmatically with errors.AsType:
-// outermost key first: a key contributes its name and an array element its
-// bracketed index, so the path of the weight field in the first item reads
-// ["items", "[0]", "weight"]. The rendered message is unchanged by the type;
-// read it programmatically with errors.AsType:
 //
 //	if de, ok := errors.AsType[*interpres.DecodeError](err); ok {
-//		fmt.Println(de.Path, de.Err)
+//		fmt.Println(de.Path.String(), de.Err)
 //	}
 type DecodeError struct {
 	// Path is the key path from the document root, outermost key first.
-	Path []string
+	Path Path
 	// Err is the failure at that path.
 	Err error
 }
 
-func (e *DecodeError) Error() string { return e.Path[0] + ": " + e.Err.Error() }
+func (e *DecodeError) Error() string {
+	msg := strings.TrimPrefix(e.Err.Error(), "interpres: ")
+	if p := e.Path.String(); p != "" {
+		return "interpres: " + p + ": " + msg
+	}
+	return "interpres: " + msg
+}
 
 // Unwrap returns the failure the path points at.
 func (e *DecodeError) Unwrap() error { return e.Err }
 
 // newDecodeError wraps err with one path segment. The rest of the path comes
 // from the DecodeError err already carries, if any: the decoder wraps each
-// key and index on its way down, so the innermost wrap holds the deepest
+// key and index on its way down, so the wrap flattens that inner error's
-// segments and each outer wrap prepends one.
+// segments onto the front and keeps the failure it pointed at, leaving one
+// path and one failure to render.
 func newDecodeError(key string, err error) *DecodeError {
-	path := make([]string, 0, 4)
+	path := make(Path, 0, 4)
 	path = append(path, key)
 	if de, ok := errors.AsType[*DecodeError](err); ok {
 		path = append(path, de.Path...)
+		err = de.Err
 	}
 	return &DecodeError{Path: path, Err: err}
 }
@@ -110,12 +138,18 @@ func newDecodeError(key string, err error) *DecodeError {
 // unchanged by the type; read it programmatically with errors.AsType.
 type EncodeError struct {
 	// Path is the key path of the failing value.
-	Path string
+	Path Path
 	// Err is the failure at that path.
 	Err error
 }
 
-func (e *EncodeError) Error() string { return "interpres: " + e.Path + ": " + e.Err.Error() }
+func (e *EncodeError) Error() string {
+	msg := strings.TrimPrefix(e.Err.Error(), "interpres: ")
+	if p := e.Path.String(); p != "" {
+		return "interpres: " + p + ": " + msg
+	}
+	return "interpres: " + msg
+}
 
 // Unwrap returns the failure the path points at.
 func (e *EncodeError) Unwrap() error { return e.Err }