syntax/typedjson: expose shfmt's "typed JSON" as Go APIs

Create a new package with docs, copy the code, and expose the API.
The following changes were made to the API:

* Name the main APIs Encode and Decode.
* Added options structs, to give us flexibility in the future.
* The "pretty" option is now an Indent string, like encoding/json.

Note that we now require "Type" keys to come first in each object.
This is not required right now, nor is it enforced by our decoder yet,
but it will be necessary to implement a performant decoder.
This is because we can only decode fields once we know the type;
if the "Type" key comes later, we must decode twice or buffer tokens.

The typed JSON encoding also changes slightly: since we want Encode and
Decode to work on syntax.Node rather than syntax.File, the root node
needs to include a "Type" JSON key as well.

This also makes shfmt's --to-json behave nicer on empty files;
rather than emitting `{}` lacking any information at all,
it now emits an empty file node in tthe form of `{"Type":"File"}`.

Finally, we apply some minor refactors to the code and tests.

Fixes #885.

cmd/shfmt/main.go

@@ -24,6 +24,7 @@ import (
 
 	"mvdan.cc/sh/v3/fileutil"
 	"mvdan.cc/sh/v3/syntax"
+	"mvdan.cc/sh/v3/syntax/typedjson"
 )
 
 // TODO: this flag business screams generics. try again with Go 1.18+.
@@ -444,7 +445,7 @@ func formatBytes(src []byte, path string, fileLang syntax.LangVariant) error {
 	var node syntax.Node
 	var err error
 	if fromJSON.val {
-		node, err = readJSON(bytes.NewReader(src))
+		node, err = typedjson.Decode(bytes.NewReader(src))
 		if err != nil {
 			return err
 		}
@@ -462,7 +463,9 @@ func formatBytes(src []byte, path string, fileLang syntax.LangVariant) error {
 	}
 	if toJSON.val {
 		// must be standard input; fine to return
-		return writeJSON(out, node, true)
+		// TODO: change the default behavior to be compact,
+		// and allow using --to-json=pretty or --to-json=indent.
+		return typedjson.EncodeOptions{Indent: "\t"}.Encode(out, node)
 	}
 	writeBuf.Reset()
 	printer.Print(&writeBuf, node)

cmd/shfmt/testdata/scripts/tojson.txt

@@ -17,11 +17,14 @@ cmp stdout comment.sh.json
 
 -- empty.sh --
 -- empty.sh.json --
-{}
+{
+	"Type": "File"
+}
 -- simple.sh --
 foo
 -- simple.sh.json --
 {
+	"Type": "File",
 	"Pos": {
 		"Offset": 0,
 		"Line": 1,
@@ -109,6 +112,7 @@ foo
 ((2))
 -- arithmetic.sh.json --
 {
+	"Type": "File",
 	"Pos": {
 		"Offset": 0,
 		"Line": 1,
@@ -205,6 +209,7 @@ foo
 #
 -- comment.sh.json --
 {
+	"Type": "File",
 	"Pos": {
 		"Offset": 0,
 		"Line": 1,

cmd/shfmt/json.go → syntax/typedjson/json.go

@@ -1,7 +1,20 @@
 // Copyright (c) 2017, Daniel Martí <mvdan@mvdan.cc>
 // See LICENSE for licensing information
 
-package main
+// Package typedjson allows encoding and decoding shell syntax trees as JSON.
+// The decoding process needs to know what syntax node types to decode into,
+// so the "typed JSON" requires "Type" keys in some syntax tree node objects:
+//
+//   - The root node
+//   - Any node represented as an interface field in the parent Go type
+//
+// The types of all other nodes can be inferred from context alone.
+//
+// For the sake of efficiency and simplicity, the "Type" key
+// described above must be first in each JSON object.
+package typedjson
+
+// TODO: encoding and decoding nodes other than File is untested.
 
 import (
 	"encoding/json"
@@ -12,32 +25,50 @@ import (
 	"mvdan.cc/sh/v3/syntax"
 )
 
-func writeJSON(w io.Writer, node syntax.Node, pretty bool) error {
+// Encode is a shortcut for EncodeOptions.Encode, with the default options.
+func Encode(w io.Writer, node syntax.Node) error {
+	return EncodeOptions{}.Encode(w, node)
+}
+
+// EncodeOptions allows configuring how syntax nodes are encoded.
+type EncodeOptions struct {
+	Indent string // e.g. "\t"
+
+	// Allows us to add options later.
+}
+
+// Encode writes node to w in its typed JSON form,
+// as described in the package documentation.
+func (opts EncodeOptions) Encode(w io.Writer, node syntax.Node) error {
 	val := reflect.ValueOf(node)
-	encVal, _ := encode(val)
+	encVal, tname := encodeValue(val)
+	if tname == "" {
+		panic("node did not contain a named type?")
+	}
+	encVal.Elem().Field(0).SetString(tname)
 	enc := json.NewEncoder(w)
-	if pretty {
-		enc.SetIndent("", "\t")
+	if opts.Indent != "" {
+		enc.SetIndent("", opts.Indent)
 	}
 	return enc.Encode(encVal.Interface())
 }
 
-func encode(val reflect.Value) (reflect.Value, string) {
+func encodeValue(val reflect.Value) (reflect.Value, string) {
 	switch val.Kind() {
 	case reflect.Ptr:
-		elem := val.Elem()
-		if !elem.IsValid() {
+		if val.IsNil() {
 			break
 		}
-		return encode(elem)
+		return encodeValue(val.Elem())
 	case reflect.Interface:
 		if val.IsNil() {
 			break
 		}
-		enc, tname := encode(val.Elem())
-		if tname != "" {
-			enc.Elem().Field(0).SetString(tname)
+		enc, tname := encodeValue(val.Elem())
+		if tname == "" {
+			panic("interface did not contain a named type?")
 		}
+		enc.Elem().Field(0).SetString(tname)
 		return enc, ""
 	case reflect.Struct:
 		// Construct a new struct with an optional Type, Pos and End,
@@ -71,7 +102,7 @@ func encode(val reflect.Value) (reflect.Value, string) {
 			if ftyp.Type == exportedPosType {
 				encodePos(enc.Field(i), fval)
 			} else {
-				encElem, _ := encode(fval)
+				encElem, _ := encodeValue(fval)
 				if encElem.IsValid() {
 					enc.Field(i).Set(encElem)
 				}
@@ -88,7 +119,7 @@ func encode(val reflect.Value) (reflect.Value, string) {
 		enc := reflect.MakeSlice(anySliceType, n, n)
 		for i := 0; i < n; i++ {
 			elem := val.Index(i)
-			encElem, _ := encode(elem)
+			encElem, _ := encodeValue(elem)
 			enc.Index(i).Set(encElem)
 		}
 		return enc, ""
@@ -161,19 +192,32 @@ func decodePos(val reflect.Value, enc map[string]interface{}) {
 	val.Set(reflect.ValueOf(syntax.NewPos(offset, line, column)))
 }
 
-func readJSON(r io.Reader) (syntax.Node, error) {
+// Decode is a shortcut for DecodeOptions.Decode, with the default options.
+func Decode(r io.Reader) (syntax.Node, error) {
+	return DecodeOptions{}.Decode(r)
+}
+
+// DecodeOptions allows configuring how syntax nodes are encoded.
+type DecodeOptions struct {
+	// Empty for now; allows us to add options later.
+}
+
+// Decode writes node to w in its typed JSON form,
+// as described in the package documentation.
+func (opts DecodeOptions) Decode(r io.Reader) (syntax.Node, error) {
 	var enc interface{}
 	if err := json.NewDecoder(r).Decode(&enc); err != nil {
 		return nil, err
 	}
-	node := &syntax.File{}
-	if err := decode(reflect.ValueOf(node), enc); err != nil {
+	node := new(syntax.Node)
+	if err := decodeValue(reflect.ValueOf(node).Elem(), enc); err != nil {
 		return nil, err
 	}
-	return node, nil
+	return *node, nil
 }
 
 var nodeByName = map[string]reflect.Type{
+	"File": reflect.TypeOf((*syntax.File)(nil)).Elem(),
 	"Word": reflect.TypeOf((*syntax.Word)(nil)).Elem(),
 
 	"Lit":       reflect.TypeOf((*syntax.Lit)(nil)).Elem(),
@@ -215,7 +259,7 @@ var nodeByName = map[string]reflect.Type{
 	"CStyleLoop": reflect.TypeOf((*syntax.CStyleLoop)(nil)).Elem(),
 }
 
-func decode(val reflect.Value, enc interface{}) error {
+func decodeValue(val reflect.Value, enc interface{}) error {
 	switch enc := enc.(type) {
 	case map[string]interface{}:
 		if val.Kind() == reflect.Ptr && val.IsNil() {
@@ -246,14 +290,14 @@ func decode(val reflect.Value, enc interface{}) error {
 				decodePos(fval, fv.(map[string]interface{}))
 				continue
 			}
-			if err := decode(fval, fv); err != nil {
+			if err := decodeValue(fval, fv); err != nil {
 				return err
 			}
 		}
 	case []interface{}:
 		for _, encElem := range enc {
 			elem := reflect.New(val.Type().Elem()).Elem()
-			if err := decode(elem, encElem); err != nil {
+			if err := decodeValue(elem, encElem); err != nil {
 				return err
 			}
 			val.Set(reflect.Append(val, elem))

syntax/typedjson/json_test.go

@@ -0,0 +1,86 @@
+// Copyright (c) 2017, Daniel Martí <mvdan@mvdan.cc>
+// See LICENSE for licensing information
+
+package typedjson_test
+
+import (
+	"bytes"
+	"flag"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+
+	"mvdan.cc/sh/v3/syntax"
+	"mvdan.cc/sh/v3/syntax/typedjson"
+)
+
+var update = flag.Bool("u", false, "update output files")
+
+func TestRoundtrip(t *testing.T) {
+	t.Parallel()
+
+	dir := filepath.Join("testdata", "roundtrip")
+	shellPaths, err := filepath.Glob(filepath.Join(dir, "*.sh"))
+	qt.Assert(t, err, qt.IsNil)
+	for _, shellPath := range shellPaths {
+		shellPath := shellPath // do not reuse the range var
+		name := strings.TrimSuffix(filepath.Base(shellPath), ".sh")
+		jsonPath := filepath.Join(dir, name+".json")
+		t.Run(name, func(t *testing.T) {
+			shellInput, err := os.ReadFile(shellPath)
+			qt.Assert(t, err, qt.IsNil)
+			jsonInput, err := os.ReadFile(jsonPath)
+			if !*update { // allow it to not exist
+				qt.Assert(t, err, qt.IsNil)
+			}
+			sb := new(strings.Builder)
+
+			// Parse the shell source and check that it is well formatted.
+			parser := syntax.NewParser(syntax.KeepComments(true))
+			node, err := parser.Parse(bytes.NewReader(shellInput), "")
+			qt.Assert(t, err, qt.IsNil)
+
+			printer := syntax.NewPrinter()
+			sb.Reset()
+			err = printer.Print(sb, node)
+			qt.Assert(t, err, qt.IsNil)
+			qt.Assert(t, sb.String(), qt.Equals, string(shellInput))
+
+			// Validate writing the pretty JSON.
+			sb.Reset()
+			encOpts := typedjson.EncodeOptions{Indent: "\t"}
+			err = encOpts.Encode(sb, node)
+			qt.Assert(t, err, qt.IsNil)
+			got := sb.String()
+			if *update {
+				err := os.WriteFile(jsonPath, []byte(got), 0o666)
+				qt.Assert(t, err, qt.IsNil)
+			} else {
+				qt.Assert(t, got, qt.Equals, string(jsonInput))
+			}
+
+			// Ensure we don't use the originally parsed node again.
+			node = nil
+
+			// Validate reading the pretty JSON and check that it formats the same.
+			node2, err := typedjson.Decode(bytes.NewReader(jsonInput))
+			qt.Assert(t, err, qt.IsNil)
+
+			sb.Reset()
+			err = printer.Print(sb, node2)
+			qt.Assert(t, err, qt.IsNil)
+			qt.Assert(t, sb.String(), qt.Equals, string(shellInput))
+
+			// Validate that emitting the JSON again produces the same result.
+			sb.Reset()
+			err = encOpts.Encode(sb, node2)
+			qt.Assert(t, err, qt.IsNil)
+			got = sb.String()
+			qt.Assert(t, got, qt.Equals, string(jsonInput))
+			t.Parallel()
+		})
+	}
+}

cmd/shfmt/testdata/json.json → syntax/typedjson/testdata/roundtrip/file.json

@@ -1,4 +1,5 @@
 {
+	"Type": "File",
 	"Pos": {
 		"Offset": 0,
 		"Line": 1,