Source: rule.go in package golang.org/x/mod/modfile

Package modfile implements a parser and formatter for go.mod files. The go.mod syntax is described in https://golang.org/cmd/go/#hdr-The_go_mod_file. The Parse and ParseLax functions both parse a go.mod file and return an abstract syntax tree. ParseLax ignores unknown statements and may be used to parse go.mod files that may have been developed with newer versions of Go. The File struct returned by Parse and ParseLax represent an abstract go.mod file. File has several methods like AddNewRequire and DropReplace that can be used to programmatically edit a file. The Format function formats a File back to a byte slice which can be written to a file.

package modfile

import (
	"errors"
	"fmt"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
	"unicode"

	"golang.org/x/mod/internal/lazyregexp"
	"golang.org/x/mod/module"
	"golang.org/x/mod/semver"
)

A File is the parsed, interpreted form of a go.mod file.

type File struct {
	Module  *Module
	Go      *Go
	Require []*Require
	Exclude []*Exclude
	Replace []*Replace
	Retract []*Retract

	Syntax *FileSyntax
}

A Module is the module statement.

type Module struct {
	Mod    module.Version
	Syntax *Line
}

A Go is the go statement.

type Go struct {
	Version string // "1.23"
	Syntax  *Line
}

A Require is a single require statement.

type Require struct {
	Mod      module.Version
	Indirect bool // has "// indirect" comment
	Syntax   *Line
}

An Exclude is a single exclude statement.

type Exclude struct {
	Mod    module.Version
	Syntax *Line
}

A Replace is a single replace statement.

type Replace struct {
	Old    module.Version
	New    module.Version
	Syntax *Line
}

A Retract is a single retract statement.

type Retract struct {
	VersionInterval
	Rationale string
	Syntax    *Line
}

A VersionInterval represents a range of versions with upper and lower bounds. Intervals are closed: both bounds are included. When Low is equal to High, the interval may refer to a single version ('v1.2.3') or an interval ('[v1.2.3, v1.2.3]'); both have the same representation.

type VersionInterval struct {
	Low, High string
}

func (f *File) AddModuleStmt(path string) error {
	if f.Syntax == nil {
		f.Syntax = new(FileSyntax)
	}
	if f.Module == nil {
		f.Module = &Module{
			Mod:    module.Version{Path: path},
			Syntax: f.Syntax.addLine(nil, "module", AutoQuote(path)),
		}
	} else {
		f.Module.Mod.Path = path
		f.Syntax.updateLine(f.Module.Syntax, "module", AutoQuote(path))
	}
	return nil
}

func (f *File) AddComment(text string) {
	if f.Syntax == nil {
		f.Syntax = new(FileSyntax)
	}
	f.Syntax.Stmt = append(f.Syntax.Stmt, &CommentBlock{
		Comments: Comments{
			Before: []Comment{
				{
					Token: text,
				},
			},
		},
	})
}

type VersionFixer func(path, version string) (string, error)

Parse parses the data, reported in errors as being from file, into a File struct. It applies fix, if non-nil, to canonicalize all module versions found.

func Parse(file string, data []byte, fix VersionFixer) (*File, error) {
	return parseToFile(file, data, fix, true)
}

ParseLax is like Parse but ignores unknown statements. It is used when parsing go.mod files other than the main module, under the theory that most statement types we add in the future will only apply in the main module, like exclude and replace, and so we get better gradual deployments if old go commands simply ignore those statements when found in go.mod files in dependencies.

func ParseLax(file string, data []byte, fix VersionFixer) (*File, error) {
	return parseToFile(file, data, fix, false)
}

func parseToFile(file string, data []byte, fix VersionFixer, strict bool) (*File, error) {
	fs, err := parse(file, data)
	if err != nil {
		return nil, err
	}
	f := &File{
		Syntax: fs,
	}

	var errs ErrorList
	for _, x := range fs.Stmt {
		switch x := x.(type) {
		case *Line:
			f.add(&errs, nil, x, x.Token[0], x.Token[1:], fix, strict)

		case *LineBlock:
			if len(x.Token) > 1 {
				if strict {
					errs = append(errs, Error{
						Filename: file,
						Pos:      x.Start,
						Err:      fmt.Errorf("unknown block type: %s", strings.Join(x.Token, " ")),
					})
				}
				continue
			}
			switch x.Token[0] {
			default:
				if strict {
					errs = append(errs, Error{
						Filename: file,
						Pos:      x.Start,
						Err:      fmt.Errorf("unknown block type: %s", strings.Join(x.Token, " ")),
					})
				}
				continue
			case "module", "require", "exclude", "replace", "retract":
				for _, l := range x.Line {
					f.add(&errs, x, l, x.Token[0], l.Token, fix, strict)
				}
			}
		}
	}

	if len(errs) > 0 {
		return nil, errs
	}
	return f, nil
}

var GoVersionRE = lazyregexp.New(`^([1-9][0-9]*)\.(0|[1-9][0-9]*)$`)

If strict is false, this module is a dependency. We ignore all unknown directives as well as main-module-only directives like replace and exclude. It will work better for forward compatibility if we can depend on modules that have unknown statements (presumed relevant only when acting as the main module) and simply ignore those statements.

	if !strict {
		switch verb {

want these even for dependency go.mods

		default:
			return
		}
	}

	wrapModPathError := func(modPath string, err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      line.Start,
			ModPath:  modPath,
			Verb:     verb,
			Err:      err,
		})
	}
	wrapError := func(err error) {
		*errs = append(*errs, Error{
			Filename: f.Syntax.Name,
			Pos:      line.Start,
			Err:      err,
		})
	}
	errorf := func(format string, args ...interface{}) {
		wrapError(fmt.Errorf(format, args...))
	}

	switch verb {
	default:
		errorf("unknown directive: %s", verb)

	case "go":
		if f.Go != nil {
			errorf("repeated go statement")
			return
		}
		if len(args) != 1 {
			errorf("go directive expects exactly one argument")
			return
		} else if !GoVersionRE.MatchString(args[0]) {
			errorf("invalid go version '%s': must match format 1.23", args[0])
			return
		}

		f.Go = &Go{Syntax: line}
		f.Go.Version = args[0]

	case "module":
		if f.Module != nil {
			errorf("repeated module statement")
			return
		}
		f.Module = &Module{Syntax: line}
		if len(args) != 1 {
			errorf("usage: module module/path")
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		f.Module.Mod = module.Version{Path: s}

	case "require", "exclude":
		if len(args) != 2 {
			errorf("usage: %s module/path v1.2.3", verb)
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		v, err := parseVersion(verb, s, &args[1], fix)
		if err != nil {
			wrapError(err)
			return
		}
		pathMajor, err := modulePathMajor(s)
		if err != nil {
			wrapError(err)
			return
		}
		if err := module.CheckPathMajor(v, pathMajor); err != nil {
			wrapModPathError(s, err)
			return
		}
		if verb == "require" {
			f.Require = append(f.Require, &Require{
				Mod:      module.Version{Path: s, Version: v},
				Syntax:   line,
				Indirect: isIndirect(line),
			})
		} else {
			f.Exclude = append(f.Exclude, &Exclude{
				Mod:    module.Version{Path: s, Version: v},
				Syntax: line,
			})
		}

	case "replace":
		arrow := 2
		if len(args) >= 2 && args[1] == "=>" {
			arrow = 1
		}
		if len(args) < arrow+2 || len(args) > arrow+3 || args[arrow] != "=>" {
			errorf("usage: %s module/path [v1.2.3] => other/module v1.4\n\t or %s module/path [v1.2.3] => ../local/directory", verb, verb)
			return
		}
		s, err := parseString(&args[0])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		pathMajor, err := modulePathMajor(s)
		if err != nil {
			wrapModPathError(s, err)
			return
		}
		var v string
		if arrow == 2 {
			v, err = parseVersion(verb, s, &args[1], fix)
			if err != nil {
				wrapError(err)
				return
			}
			if err := module.CheckPathMajor(v, pathMajor); err != nil {
				wrapModPathError(s, err)
				return
			}
		}
		ns, err := parseString(&args[arrow+1])
		if err != nil {
			errorf("invalid quoted string: %v", err)
			return
		}
		nv := ""
		if len(args) == arrow+2 {
			if !IsDirectoryPath(ns) {
				errorf("replacement module without version must be directory path (rooted or starting with ./ or ../)")
				return
			}
			if filepath.Separator == '/' && strings.Contains(ns, `\`) {
				errorf("replacement directory appears to be Windows path (on a non-windows system)")
				return
			}
		}
		if len(args) == arrow+3 {
			nv, err = parseVersion(verb, ns, &args[arrow+2], fix)
			if err != nil {
				wrapError(err)
				return
			}
			if IsDirectoryPath(ns) {
				errorf("replacement module directory path %q cannot have version", ns)
				return
			}
		}
		f.Replace = append(f.Replace, &Replace{
			Old:    module.Version{Path: s, Version: v},
			New:    module.Version{Path: ns, Version: nv},
			Syntax: line,
		})

	case "retract":
		rationale := parseRetractRationale(block, line)
		vi, err := parseVersionInterval(verb, &args, fix)
		if err != nil {
			if strict {
				wrapError(err)
				return

Only report errors parsing intervals in the main module. We may support additional syntax in the future, such as open and half-open intervals. Those can't be supported now, because they break the go.mod parser, even in lax mode.

				return
			}
		}

In the future, there may be additional information after the version.

			errorf("unexpected token after version: %q", args[0])
			return
		}
		retract := &Retract{
			VersionInterval: vi,
			Rationale:       rationale,
			Syntax:          line,
		}
		f.Retract = append(f.Retract, retract)
	}
}

isIndirect reports whether line has a "// indirect" comment, meaning it is in go.mod only for its effect on indirect dependencies, so that it can be dropped entirely once the effective version of the indirect dependency reaches the given minimum version.

func isIndirect(line *Line) bool {
	if len(line.Suffix) == 0 {
		return false
	}
	f := strings.Fields(strings.TrimPrefix(line.Suffix[0].Token, string(slashSlash)))
	return (len(f) == 1 && f[0] == "indirect" || len(f) > 1 && f[0] == "indirect;")
}

setIndirect sets line to have (or not have) a "// indirect" comment.

func setIndirect(line *Line, indirect bool) {
	if isIndirect(line) == indirect {
		return
	}

Adding comment.

New comment.

			line.Suffix = []Comment{{Token: "// indirect", Suffix: true}}
			return
		}

		com := &line.Suffix[0]
		text := strings.TrimSpace(strings.TrimPrefix(com.Token, string(slashSlash)))

Empty comment.

			com.Token = "// indirect"
			return
		}

Insert at beginning of existing comment.

		com.Token = "// indirect; " + text
		return
	}

Removing comment.

	f := strings.Fields(line.Suffix[0].Token)

Remove whole comment.

		line.Suffix = nil
		return
	}

Remove comment prefix.

	com := &line.Suffix[0]
	i := strings.Index(com.Token, "indirect;")
	com.Token = "//" + com.Token[i+len("indirect;"):]
}

IsDirectoryPath reports whether the given path should be interpreted as a directory path. Just like on the go command line, relative paths and rooted paths are directory paths; the rest are module paths.

Because go.mod files can move from one system to another, we check all known path syntaxes, both Unix and Windows.

	return strings.HasPrefix(ns, "./") || strings.HasPrefix(ns, "../") || strings.HasPrefix(ns, "/") ||
		strings.HasPrefix(ns, `.\`) || strings.HasPrefix(ns, `..\`) || strings.HasPrefix(ns, `\`) ||
		len(ns) >= 2 && ('A' <= ns[0] && ns[0] <= 'Z' || 'a' <= ns[0] && ns[0] <= 'z') && ns[1] == ':'
}

MustQuote reports whether s must be quoted in order to appear as a single token in a go.mod line.

func MustQuote(s string) bool {
	for _, r := range s {
		switch r {
		case ' ', '"', '\'', '`':
			return true

		case '(', ')', '[', ']', '{', '}', ',':
			if len(s) > 1 {
				return true
			}

		default:
			if !unicode.IsPrint(r) {
				return true
			}
		}
	}
	return s == "" || strings.Contains(s, "//") || strings.Contains(s, "/*")
}

AutoQuote returns s or, if quoting is required for s to appear in a go.mod, the quotation of s.

func AutoQuote(s string) string {
	if MustQuote(s) {
		return strconv.Quote(s)
	}
	return s
}

func parseVersionInterval(verb string, args *[]string, fix VersionFixer) (VersionInterval, error) {
	toks := *args
	if len(toks) == 0 || toks[0] == "(" {
		return VersionInterval{}, fmt.Errorf("expected '[' or version")
	}
	if toks[0] != "[" {
		v, err := parseVersion(verb, "", &toks[0], fix)
		if err != nil {
			return VersionInterval{}, err
		}
		*args = toks[1:]
		return VersionInterval{Low: v, High: v}, nil
	}
	toks = toks[1:]

	if len(toks) == 0 {
		return VersionInterval{}, fmt.Errorf("expected version after '['")
	}
	low, err := parseVersion(verb, "", &toks[0], fix)
	if err != nil {
		return VersionInterval{}, err
	}
	toks = toks[1:]

	if len(toks) == 0 || toks[0] != "," {
		return VersionInterval{}, fmt.Errorf("expected ',' after version")
	}
	toks = toks[1:]

	if len(toks) == 0 {
		return VersionInterval{}, fmt.Errorf("expected version after ','")
	}
	high, err := parseVersion(verb, "", &toks[0], fix)
	if err != nil {
		return VersionInterval{}, err
	}
	toks = toks[1:]

	if len(toks) == 0 || toks[0] != "]" {
		return VersionInterval{}, fmt.Errorf("expected ']' after version")
	}
	toks = toks[1:]

	*args = toks
	return VersionInterval{Low: low, High: high}, nil
}

func parseString(s *string) (string, error) {
	t := *s
	if strings.HasPrefix(t, `"`) {
		var err error
		if t, err = strconv.Unquote(t); err != nil {
			return "", err
		}

Other quotes are reserved both for possible future expansion and to avoid confusion. For example if someone types 'x' we want that to be a syntax error and not a literal x in literal quotation marks.

		return "", fmt.Errorf("unquoted string cannot contain quote")
	}
	*s = AutoQuote(t)
	return t, nil
}

parseRetractRationale extracts the rationale for a retract directive from the surrounding comments. If the line does not have comments and is part of a block that does have comments, the block's comments are used.

func parseRetractRationale(block *LineBlock, line *Line) string {
	comments := line.Comment()
	if block != nil && len(comments.Before) == 0 && len(comments.Suffix) == 0 {
		comments = block.Comment()
	}
	groups := [][]Comment{comments.Before, comments.Suffix}
	var lines []string
	for _, g := range groups {
		for _, c := range g {
			if !strings.HasPrefix(c.Token, "//") {
				continue // blank line
			}
			lines = append(lines, strings.TrimSpace(strings.TrimPrefix(c.Token, "//")))
		}
	}
	return strings.Join(lines, "\n")
}

type ErrorList []Error

func (e ErrorList) Error() string {
	errStrs := make([]string, len(e))
	for i, err := range e {
		errStrs[i] = err.Error()
	}
	return strings.Join(errStrs, "\n")
}

type Error struct {
	Filename string
	Pos      Position
	Verb     string
	ModPath  string
	Err      error
}

func (e *Error) Error() string {
	var pos string

Don't print LineRune if it's 1 (beginning of line). It's always 1 except in scanner errors, which are rare.

		pos = fmt.Sprintf("%s:%d:%d: ", e.Filename, e.Pos.Line, e.Pos.LineRune)
	} else if e.Pos.Line > 0 {
		pos = fmt.Sprintf("%s:%d: ", e.Filename, e.Pos.Line)
	} else if e.Filename != "" {
		pos = fmt.Sprintf("%s: ", e.Filename)
	}

	var directive string
	if e.ModPath != "" {
		directive = fmt.Sprintf("%s %s: ", e.Verb, e.ModPath)
	} else if e.Verb != "" {
		directive = fmt.Sprintf("%s: ", e.Verb)
	}

	return pos + directive + e.Err.Error()
}

func (e *Error) Unwrap() error { return e.Err }

func parseVersion(verb string, path string, s *string, fix VersionFixer) (string, error) {
	t, err := parseString(s)
	if err != nil {
		return "", &Error{
			Verb:    verb,
			ModPath: path,
			Err: &module.InvalidVersionError{
				Version: *s,
				Err:     err,
			},
		}
	}
	if fix != nil {
		var err error
		t, err = fix(path, t)
		if err != nil {
			if err, ok := err.(*module.ModuleError); ok {
				return "", &Error{
					Verb:    verb,
					ModPath: path,
					Err:     err.Err,
				}
			}
			return "", err
		}
	}
	if v := module.CanonicalVersion(t); v != "" {
		*s = v
		return *s, nil
	}
	return "", &Error{
		Verb:    verb,
		ModPath: path,
		Err: &module.InvalidVersionError{
			Version: t,
			Err:     errors.New("must be of the form v1.2.3"),
		},
	}
}

func modulePathMajor(path string) (string, error) {
	_, major, ok := module.SplitPathVersion(path)
	if !ok {
		return "", fmt.Errorf("invalid module path")
	}
	return major, nil
}

func (f *File) Format() ([]byte, error) {
	return Format(f.Syntax), nil
}

Cleanup cleans up the file f after any edit operations. To avoid quadratic behavior, modifications like DropRequire clear the entry but do not remove it from the slice. Cleanup cleans out all the cleared entries.

func (f *File) Cleanup() {
	w := 0
	for _, r := range f.Require {
		if r.Mod.Path != "" {
			f.Require[w] = r
			w++
		}
	}
	f.Require = f.Require[:w]

	w = 0
	for _, x := range f.Exclude {
		if x.Mod.Path != "" {
			f.Exclude[w] = x
			w++
		}
	}
	f.Exclude = f.Exclude[:w]

	w = 0
	for _, r := range f.Replace {
		if r.Old.Path != "" {
			f.Replace[w] = r
			w++
		}
	}
	f.Replace = f.Replace[:w]

	w = 0
	for _, r := range f.Retract {
		if r.Low != "" || r.High != "" {
			f.Retract[w] = r
			w++
		}
	}
	f.Retract = f.Retract[:w]

	f.Syntax.Cleanup()
}

func (f *File) AddGoStmt(version string) error {
	if !GoVersionRE.MatchString(version) {
		return fmt.Errorf("invalid language version string %q", version)
	}
	if f.Go == nil {
		var hint Expr
		if f.Module != nil && f.Module.Syntax != nil {
			hint = f.Module.Syntax
		}
		f.Go = &Go{
			Version: version,
			Syntax:  f.Syntax.addLine(hint, "go", version),
		}
	} else {
		f.Go.Version = version
		f.Syntax.updateLine(f.Go.Syntax, "go", version)
	}
	return nil
}

func (f *File) AddRequire(path, vers string) error {
	need := true
	for _, r := range f.Require {
		if r.Mod.Path == path {
			if need {
				r.Mod.Version = vers
				f.Syntax.updateLine(r.Syntax, "require", AutoQuote(path), vers)
				need = false
			} else {
				f.Syntax.removeLine(r.Syntax)
				*r = Require{}
			}
		}
	}

	if need {
		f.AddNewRequire(path, vers, false)
	}
	return nil
}

func (f *File) AddNewRequire(path, vers string, indirect bool) {
	line := f.Syntax.addLine(nil, "require", AutoQuote(path), vers)
	setIndirect(line, indirect)
	f.Require = append(f.Require, &Require{module.Version{Path: path, Version: vers}, indirect, line})
}

func (f *File) SetRequire(req []*Require) {
	need := make(map[string]string)
	indirect := make(map[string]bool)
	for _, r := range req {
		need[r.Mod.Path] = r.Mod.Version
		indirect[r.Mod.Path] = r.Indirect
	}

	for _, r := range f.Require {
		if v, ok := need[r.Mod.Path]; ok {
			r.Mod.Version = v
			r.Indirect = indirect[r.Mod.Path]
		} else {
			*r = Require{}
		}
	}

	var newStmts []Expr
	for _, stmt := range f.Syntax.Stmt {
		switch stmt := stmt.(type) {
		case *LineBlock:
			if len(stmt.Token) > 0 && stmt.Token[0] == "require" {
				var newLines []*Line
				for _, line := range stmt.Line {
					if p, err := parseString(&line.Token[0]); err == nil && need[p] != "" {
						if len(line.Comments.Before) == 1 && len(line.Comments.Before[0].Token) == 0 {
							line.Comments.Before = line.Comments.Before[:0]
						}
						line.Token[1] = need[p]
						delete(need, p)
						setIndirect(line, indirect[p])
						newLines = append(newLines, line)
					}
				}
				if len(newLines) == 0 {
					continue // drop stmt
				}
				stmt.Line = newLines
			}

		case *Line:
			if len(stmt.Token) > 0 && stmt.Token[0] == "require" {
				if p, err := parseString(&stmt.Token[1]); err == nil && need[p] != "" {
					stmt.Token[2] = need[p]
					delete(need, p)
					setIndirect(stmt, indirect[p])
				} else {
					continue // drop stmt
				}
			}
		}
		newStmts = append(newStmts, stmt)
	}
	f.Syntax.Stmt = newStmts

	for path, vers := range need {
		f.AddNewRequire(path, vers, indirect[path])
	}
	f.SortBlocks()
}

func (f *File) DropRequire(path string) error {
	for _, r := range f.Require {
		if r.Mod.Path == path {
			f.Syntax.removeLine(r.Syntax)
			*r = Require{}
		}
	}
	return nil
}

func (f *File) AddExclude(path, vers string) error {
	var hint *Line
	for _, x := range f.Exclude {
		if x.Mod.Path == path && x.Mod.Version == vers {
			return nil
		}
		if x.Mod.Path == path {
			hint = x.Syntax
		}
	}

	f.Exclude = append(f.Exclude, &Exclude{Mod: module.Version{Path: path, Version: vers}, Syntax: f.Syntax.addLine(hint, "exclude", AutoQuote(path), vers)})
	return nil
}

func (f *File) DropExclude(path, vers string) error {
	for _, x := range f.Exclude {
		if x.Mod.Path == path && x.Mod.Version == vers {
			f.Syntax.removeLine(x.Syntax)
			*x = Exclude{}
		}
	}
	return nil
}

func (f *File) AddReplace(oldPath, oldVers, newPath, newVers string) error {
	need := true
	old := module.Version{Path: oldPath, Version: oldVers}
	new := module.Version{Path: newPath, Version: newVers}
	tokens := []string{"replace", AutoQuote(oldPath)}
	if oldVers != "" {
		tokens = append(tokens, oldVers)
	}
	tokens = append(tokens, "=>", AutoQuote(newPath))
	if newVers != "" {
		tokens = append(tokens, newVers)
	}

	var hint *Line
	for _, r := range f.Replace {
		if r.Old.Path == oldPath && (oldVers == "" || r.Old.Version == oldVers) {

Found replacement for old; update to use new.

				r.New = new
				f.Syntax.updateLine(r.Syntax, tokens...)
				need = false
				continue

Already added; delete other replacements for same.

			f.Syntax.removeLine(r.Syntax)
			*r = Replace{}
		}
		if r.Old.Path == oldPath {
			hint = r.Syntax
		}
	}
	if need {
		f.Replace = append(f.Replace, &Replace{Old: old, New: new, Syntax: f.Syntax.addLine(hint, tokens...)})
	}
	return nil
}

func (f *File) DropReplace(oldPath, oldVers string) error {
	for _, r := range f.Replace {
		if r.Old.Path == oldPath && r.Old.Version == oldVers {
			f.Syntax.removeLine(r.Syntax)
			*r = Replace{}
		}
	}
	return nil
}

func (f *File) AddRetract(vi VersionInterval, rationale string) error {
	r := &Retract{
		VersionInterval: vi,
	}
	if vi.Low == vi.High {
		r.Syntax = f.Syntax.addLine(nil, "retract", AutoQuote(vi.Low))
	} else {
		r.Syntax = f.Syntax.addLine(nil, "retract", "[", AutoQuote(vi.Low), ",", AutoQuote(vi.High), "]")
	}
	if rationale != "" {
		for _, line := range strings.Split(rationale, "\n") {
			com := Comment{Token: "// " + line}
			r.Syntax.Comment().Before = append(r.Syntax.Comment().Before, com)
		}
	}
	return nil
}

func (f *File) DropRetract(vi VersionInterval) error {
	for _, r := range f.Retract {
		if r.VersionInterval == vi {
			f.Syntax.removeLine(r.Syntax)
			*r = Retract{}
		}
	}
	return nil
}

func (f *File) SortBlocks() {
	f.removeDups() // otherwise sorting is unsafe

	for _, stmt := range f.Syntax.Stmt {
		block, ok := stmt.(*LineBlock)
		if !ok {
			continue
		}
		less := lineLess
		if block.Token[0] == "retract" {
			less = lineRetractLess
		}
		sort.SliceStable(block.Line, func(i, j int) bool {
			return less(block.Line[i], block.Line[j])
		})
	}
}

removeDups removes duplicate exclude and replace directives. Earlier exclude directives take priority. Later replace directives take priority. require directives are not de-duplicated. That's left up to higher-level logic (MVS). retract directives are not de-duplicated since comments are meaningful, and versions may be retracted multiple times.

func (f *File) removeDups() {
	kill := make(map[*Line]bool)

Remove duplicate excludes.

	haveExclude := make(map[module.Version]bool)
	for _, x := range f.Exclude {
		if haveExclude[x.Mod] {
			kill[x.Syntax] = true
			continue
		}
		haveExclude[x.Mod] = true
	}
	var excl []*Exclude
	for _, x := range f.Exclude {
		if !kill[x.Syntax] {
			excl = append(excl, x)
		}
	}
	f.Exclude = excl

Remove duplicate replacements. Later replacements take priority over earlier ones.

	haveReplace := make(map[module.Version]bool)
	for i := len(f.Replace) - 1; i >= 0; i-- {
		x := f.Replace[i]
		if haveReplace[x.Old] {
			kill[x.Syntax] = true
			continue
		}
		haveReplace[x.Old] = true
	}
	var repl []*Replace
	for _, x := range f.Replace {
		if !kill[x.Syntax] {
			repl = append(repl, x)
		}
	}
	f.Replace = repl

Duplicate require and retract directives are not removed.

Drop killed statements from the syntax tree.

	var stmts []Expr
	for _, stmt := range f.Syntax.Stmt {
		switch stmt := stmt.(type) {
		case *Line:
			if kill[stmt] {
				continue
			}
		case *LineBlock:
			var lines []*Line
			for _, line := range stmt.Line {
				if !kill[line] {
					lines = append(lines, line)
				}
			}
			stmt.Line = lines
			if len(lines) == 0 {
				continue
			}
		}
		stmts = append(stmts, stmt)
	}
	f.Syntax.Stmt = stmts
}

lineLess returns whether li should be sorted before lj. It sorts lexicographically without assigning any special meaning to tokens.

func lineLess(li, lj *Line) bool {
	for k := 0; k < len(li.Token) && k < len(lj.Token); k++ {
		if li.Token[k] != lj.Token[k] {
			return li.Token[k] < lj.Token[k]
		}
	}
	return len(li.Token) < len(lj.Token)
}

lineRetractLess returns whether li should be sorted before lj for lines in a "retract" block. It treats each line as a version interval. Single versions are compared as if they were intervals with the same low and high version. Intervals are sorted in descending order, first by low version, then by high version, using semver.Compare.

func lineRetractLess(li, lj *Line) bool {
	interval := func(l *Line) VersionInterval {
		if len(l.Token) == 1 {
			return VersionInterval{Low: l.Token[0], High: l.Token[0]}
		} else if len(l.Token) == 5 && l.Token[0] == "[" && l.Token[2] == "," && l.Token[4] == "]" {
			return VersionInterval{Low: l.Token[1], High: l.Token[3]}

Line in unknown format. Treat as an invalid version.

			return VersionInterval{}
		}
	}
	vii := interval(li)
	vij := interval(lj)
	if cmp := semver.Compare(vii.Low, vij.Low); cmp != 0 {
		return cmp > 0
	}
	return semver.Compare(vii.High, vij.High) > 0