flag: add overridable --help and --version by default

vlib/flag/default_flag_options_test.v

@@ -0,0 +1,35 @@
+import os
+
+const source = 'vlib/flag/testdata/simplest_flag_program.v'
+
+const simple_flag_app_executable = os.real_path(os.join_path(os.cache_dir(), 'simple_flag_app.exe'))
+
+fn testsuite_begin() {
+	os.chdir(@VMODROOT)
+	os.rm(simple_flag_app_executable) or {}
+	res := os.execute('${@VEXE} -o $simple_flag_app_executable $source')
+	assert res.exit_code == 0
+	assert os.execute(simple_flag_app_executable).exit_code == 0
+}
+
+fn testsuite_end() {
+	os.rm(simple_flag_app_executable) or {}
+	assert true
+}
+
+fn check_program(opts string, extension string) {
+	result := source.replace('.v', extension)
+	res := os.execute('$simple_flag_app_executable $opts')
+	lines := os.read_lines(result) or { panic(err) }
+	assert res.exit_code == 0
+	assert res.output.split_into_lines() == lines
+}
+
+fn test_default_builtin_flag_options() {
+	check_program('', '.out')
+	check_program(' -- --help', '.dashdash.help.out')
+	check_program(' -- --version', '.dashdash.version.out')
+	check_program(' -h', '.help.out')
+	check_program(' --help', '.help.out')
+	check_program(' --version', '.version.out')
+}

vlib/flag/flag.v

@@ -62,6 +62,8 @@ pub:
 	idx_dashdash       int      // the index of a `--`, -1 if there is not any
 	all_after_dashdash []string // all options after `--` are ignored, and will be passed to the application unmodified
 pub mut:
+	default_help_label      string = 'display this help and exit'
+	default_version_label   string = 'output version information and exit'
 	args                    []string // the current list of processed args
 	max_free_args           int
 	flags                   []Flag // registered flags
@@ -507,12 +509,41 @@ pub fn (fs FlagParser) usage() string {
 	return use.join('\n').replace('- ,', '   ')
 }
 
+fn (mut fs FlagParser) find_existing_flag(fname string) ?Flag {
+	for f in fs.flags {
+		if f.name == fname {
+			return f
+		}
+	}
+	return error('no such flag')
+}
+
+fn (mut fs FlagParser) handle_builtin_options() {
+	mut show_version := false
+	mut show_help := false
+	fs.find_existing_flag('help') or {
+		show_help = fs.bool('help', `h`, false, fs.default_help_label)
+	}
+	fs.find_existing_flag('version') or {
+		show_version = fs.bool('version', 0, false, fs.default_version_label)
+	}
+	if show_help {
+		println(fs.usage())
+		exit(0)
+	}
+	if show_version {
+		println('$fs.application_name $fs.application_version')
+		exit(0)
+	}
+}
+
 // finalize - return all remaining arguments (non options).
 // Call .finalize() after all arguments are defined.
 // The remaining arguments are returned in the same order they are
 // defined on the command line. If additional flags are found, i.e.
 // (things starting with '--' or '-'), it returns an error.
-pub fn (fs FlagParser) finalize() ?[]string {
+pub fn (mut fs FlagParser) finalize() ?[]string {
+	fs.handle_builtin_options()
 	mut remaining := fs.args.clone()
 	if !fs.allow_unknown_args {
 		for a in remaining {

vlib/flag/testdata/simplest_flag_program.dashdash.help.out

@@ -0,0 +1 @@
+[vlib/flag/testdata/simplest_flag_program.v:13] rest_of_args: ['--help']

vlib/flag/testdata/simplest_flag_program.dashdash.version.out

@@ -0,0 +1 @@
+[vlib/flag/testdata/simplest_flag_program.v:13] rest_of_args: ['--version']

vlib/flag/testdata/simplest_flag_program.help.out

@@ -0,0 +1,7 @@
+abc 0.0.1
+-----------------------------------------------
+Usage: abc [options] [ARGS]
+
+Options:
+  -h, --help                display this help and exit
+  --version                 output version information and exit

vlib/flag/testdata/simplest_flag_program.out

@@ -0,0 +1 @@
+[vlib/flag/testdata/simplest_flag_program.v:13] rest_of_args: []

vlib/flag/testdata/simplest_flag_program.v

@@ -0,0 +1,14 @@
+import os
+import flag
+
+fn main() {
+	mut fp := flag.new_flag_parser(os.args)
+	fp.application('abc')
+	fp.version('0.0.1')
+	fp.skip_executable()
+	rest_of_args := fp.finalize() or {
+		eprintln(err)
+		exit(1)
+	}
+	dump(rest_of_args)
+}

vlib/flag/testdata/simplest_flag_program.version.out

@@ -0,0 +1 @@
+abc 0.0.1