diff --git a/cmd/tools/modules/scripting/scripting.v b/cmd/tools/modules/scripting/scripting.v
index 017a4eea54..4d07c56928 100644
--- a/cmd/tools/modules/scripting/scripting.v
+++ b/cmd/tools/modules/scripting/scripting.v
@@ -95,7 +95,7 @@ pub fn chdir(path string) {
 	}
 }
 
-pub fn mkdir(path string) ? {
+pub fn mkdir(path string) ! {
 	verbose_trace_strong(modfn(@MOD, @FN), 'mkdir ${path}')
 	os.mkdir(path) or {
 		verbose_trace(modfn(@MOD, @FN), '## failed.')
@@ -103,7 +103,7 @@ pub fn mkdir(path string) ? {
 	}
 }
 
-pub fn mkdir_all(path string) ? {
+pub fn mkdir_all(path string) ! {
 	verbose_trace_strong(modfn(@MOD, @FN), 'mkdir -p ${path}')
 	os.mkdir_all(path) or {
 		verbose_trace(modfn(@MOD, @FN), '## failed.')
@@ -123,7 +123,7 @@ pub fn rmrf(path string) {
 }
 
 // execute a command, and return a result, or an error, if it failed in any way.
-pub fn exec(cmd string) ?os.Result {
+pub fn exec(cmd string) !os.Result {
 	verbose_trace_strong(modfn(@MOD, @FN), cmd)
 	x := os.execute(cmd)
 	if x.exit_code != 0 {
diff --git a/cmd/tools/modules/testing/common.v b/cmd/tools/modules/testing/common.v
index bfbbafd971..d053eb08e7 100644
--- a/cmd/tools/modules/testing/common.v
+++ b/cmd/tools/modules/testing/common.v
@@ -721,7 +721,7 @@ pub fn get_test_details(file string) TestDetails {
 	return res
 }
 
-pub fn find_started_process(pname string) ?string {
+pub fn find_started_process(pname string) !string {
 	for line in testing.all_processes {
 		if line.contains(pname) {
 			return line
diff --git a/cmd/tools/vbump.v b/cmd/tools/vbump.v
index c47dc704ae..952441e29c 100644
--- a/cmd/tools/vbump.v
+++ b/cmd/tools/vbump.v
@@ -169,7 +169,7 @@ Try ${tool_name} -h for more help...')
 	}
 }
 
-fn validate_options(options Options) ? {
+fn validate_options(options Options) ! {
 	if options.patch && options.major {
 		return error('Cannot specify both --patch and --major.')
 	}
diff --git a/cmd/tools/vfmt.v b/cmd/tools/vfmt.v
index f2564208e3..73104ea5f5 100644
--- a/cmd/tools/vfmt.v
+++ b/cmd/tools/vfmt.v
@@ -213,7 +213,7 @@ fn (mut foptions FormatOptions) find_diff_cmd() string {
 	return foptions.diff_cmd
 }
 
-fn (mut foptions FormatOptions) post_process_file(file string, formatted_file_path string) ? {
+fn (mut foptions FormatOptions) post_process_file(file string, formatted_file_path string) ! {
 	if formatted_file_path.len == 0 {
 		return
 	}
@@ -307,7 +307,7 @@ fn file_to_mod_name_and_is_module_file(file string) (string, bool) {
 	return mod_name, is_module_file
 }
 
-fn read_source_lines(file string) ?[]string {
+fn read_source_lines(file string) ![]string {
 	source_lines := os.read_lines(file) or { return error('can not read ${file}') }
 	return source_lines
 }
diff --git a/cmd/tools/vpm.v b/cmd/tools/vpm.v
index 839e8b92cc..6523b27c4a 100644
--- a/cmd/tools/vpm.v
+++ b/cmd/tools/vpm.v
@@ -414,7 +414,7 @@ fn vpm_update(m []string) {
 	}
 }
 
-fn get_outdated() ?[]string {
+fn get_outdated() ![]string {
 	module_names := get_installed_modules()
 	mut outdated := []string{}
 	for name in module_names {
@@ -720,7 +720,7 @@ fn verbose_println(s string) {
 	}
 }
 
-fn get_mod_by_url(name string) ?Mod {
+fn get_mod_by_url(name string) !Mod {
 	if purl := urllib.parse(name) {
 		verbose_println('purl: ${purl}')
 		mod := Mod{
@@ -733,7 +733,7 @@ fn get_mod_by_url(name string) ?Mod {
 	return error('invalid url: ${name}')
 }
 
-fn get_module_meta_info(name string) ?Mod {
+fn get_module_meta_info(name string) !Mod {
 	if mod := get_mod_by_url(name) {
 		return mod
 	}
diff --git a/cmd/tools/vrepl.v b/cmd/tools/vrepl.v
index 370d96b1e9..d55160e431 100644
--- a/cmd/tools/vrepl.v
+++ b/cmd/tools/vrepl.v
@@ -576,7 +576,7 @@ fn cleanup_files(file string) {
 	}
 }
 
-fn repl_run_vfile(file string) ?os.Result {
+fn repl_run_vfile(file string) !os.Result {
 	$if trace_repl_temp_files ? {
 		eprintln('>> repl_run_vfile file: ${file}')
 	}
diff --git a/cmd/tools/vsymlink.v b/cmd/tools/vsymlink.v
index e41bbd7c84..199605d1bc 100644
--- a/cmd/tools/vsymlink.v
+++ b/cmd/tools/vsymlink.v
@@ -152,7 +152,7 @@ fn warn_and_exit(err string) {
 }
 
 // get the system environment registry handle
-fn get_reg_sys_env_handle() ?voidptr {
+fn get_reg_sys_env_handle() !voidptr {
 	$if windows { // wrap for cross-compile compat
 		// open the registry key
 		reg_key_path := 'Environment'
@@ -166,7 +166,7 @@ fn get_reg_sys_env_handle() ?voidptr {
 }
 
 // get a value from a given $key
-fn get_reg_value(reg_env_key voidptr, key string) ?string {
+fn get_reg_value(reg_env_key voidptr, key string) !string {
 	$if windows {
 		// query the value (shortcut the sizing step)
 		reg_value_size := u32(4095) // this is the max length (not for the registry, but for the system %PATH%)
@@ -180,7 +180,7 @@ fn get_reg_value(reg_env_key voidptr, key string) ?string {
 }
 
 // sets the value for the given $key to the given  $value
-fn set_reg_value(reg_key voidptr, key string, value string) ?bool {
+fn set_reg_value(reg_key voidptr, key string, value string) !bool {
 	$if windows {
 		if C.RegSetValueExW(reg_key, key.to_wide(), 0, C.REG_EXPAND_SZ, value.to_wide(),
 			value.len * 2) != 0 {
@@ -193,7 +193,7 @@ fn set_reg_value(reg_key voidptr, key string, value string) ?bool {
 
 // Broadcasts a message to all listening windows (explorer.exe in particular)
 // letting them know that the system environment has changed and should be reloaded
-fn send_setting_change_msg(message_data string) ?bool {
+fn send_setting_change_msg(message_data string) !bool {
 	$if windows {
 		if C.SendMessageTimeoutW(os.hwnd_broadcast, os.wm_settingchange, 0, unsafe { &u32(message_data.to_wide()) },
 			os.smto_abortifhung, 5000, 0) == 0 {
diff --git a/examples/js_dom_draw_bechmark_chart/chart/main.v b/examples/js_dom_draw_bechmark_chart/chart/main.v
index 6076c4845a..b4ae061322 100644
--- a/examples/js_dom_draw_bechmark_chart/chart/main.v
+++ b/examples/js_dom_draw_bechmark_chart/chart/main.v
@@ -69,7 +69,7 @@ fn new_app() &App {
 }
 
 ['/'; get]
-pub fn (mut app App) controller_get_all_task() ?vweb.Result {
+pub fn (mut app App) controller_get_all_task() !vweb.Result {
 	v_version := version.full_v_version(true)
 	orm_stmt_kinds := ['insert', 'select', 'update']
 
diff --git a/examples/vweb_fullstack/src/product_service.v b/examples/vweb_fullstack/src/product_service.v
index e844adcd72..15f55bf4e8 100644
--- a/examples/vweb_fullstack/src/product_service.v
+++ b/examples/vweb_fullstack/src/product_service.v
@@ -25,7 +25,7 @@ fn (mut app App) service_add_product(product_name string, user_id int) ! {
 	}
 }
 
-fn (mut app App) service_get_all_products_from(user_id int) ?[]Product {
+fn (mut app App) service_get_all_products_from(user_id int) ![]Product {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
diff --git a/examples/vweb_fullstack/src/user_services.v b/examples/vweb_fullstack/src/user_services.v
index 71d76c91a2..30e675f3ca 100644
--- a/examples/vweb_fullstack/src/user_services.v
+++ b/examples/vweb_fullstack/src/user_services.v
@@ -30,7 +30,7 @@ fn (mut app App) service_add_user(username string, password string) ! {
 	}
 }
 
-fn (mut app App) service_get_all_user() ?[]User {
+fn (mut app App) service_get_all_user() ![]User {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
@@ -47,7 +47,7 @@ fn (mut app App) service_get_all_user() ?[]User {
 	return results
 }
 
-fn (mut app App) service_get_user(id int) ?User {
+fn (mut app App) service_get_user(id int) !User {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
diff --git a/examples/vweb_orm_jwt/src/user_services.v b/examples/vweb_orm_jwt/src/user_services.v
index 4b6fce98f7..d2006c193f 100644
--- a/examples/vweb_orm_jwt/src/user_services.v
+++ b/examples/vweb_orm_jwt/src/user_services.v
@@ -3,7 +3,7 @@ module main
 import crypto.bcrypt
 import databases
 
-fn (mut app App) service_add_user(username string, password string) ?User {
+fn (mut app App) service_add_user(username string, password string) !User {
 	mut db := databases.create_db_connection() or {
 		eprintln(err)
 		return err
@@ -38,7 +38,7 @@ fn (mut app App) service_add_user(username string, password string) ?User {
 	return result
 }
 
-fn (mut app App) service_get_user_by_id(user_id int) ?User {
+fn (mut app App) service_get_user_by_id(user_id int) !User {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
@@ -55,7 +55,7 @@ fn (mut app App) service_get_user_by_id(user_id int) ?User {
 	return results
 }
 
-fn (mut app App) service_get_all_user() ?[]User {
+fn (mut app App) service_get_all_user() ![]User {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
@@ -72,7 +72,7 @@ fn (mut app App) service_get_all_user() ?[]User {
 	return results
 }
 
-fn (mut app App) service_get_by_username(username string) ?User {
+fn (mut app App) service_get_by_username(username string) !User {
 	mut db := databases.create_db_connection() or {
 		println(err)
 		return err
diff --git a/vlib/builtin/int.v b/vlib/builtin/int.v
index 480570d726..4236d8568d 100644
--- a/vlib/builtin/int.v
+++ b/vlib/builtin/int.v
@@ -562,8 +562,8 @@ pub fn (b []u8) bytestr() string {
 // from utf8 to utf32 and return the result as a rune
 // it will produce an error if there are more than
 // four bytes in the array.
-pub fn (b []u8) byterune() ?rune {
-	r := b.utf8_to_utf32()?
+pub fn (b []u8) byterune() !rune {
+	r := b.utf8_to_utf32()!
 	return rune(r)
 }
 
diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 9c50aa800d..02f1d8c833 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -910,7 +910,7 @@ pub fn (s string) substr(start int, end int) string {
 // version of `substr()` that is used in `a[start..end] or {`
 // return an error when the index is out of range
 [direct_array_access]
-pub fn (s string) substr_with_check(start int, end int) ?string {
+pub fn (s string) substr_with_check(start int, end int) !string {
 	if start > end || start > s.len || end > s.len || start < 0 || end < 0 {
 		return error('substr(${start}, ${end}) out of bounds (len=${s.len})')
 	}
@@ -1606,7 +1606,7 @@ fn (s string) at(idx int) byte {
 // return an error when the index is out of range
 fn (s string) at_with_check(idx int) ?u8 {
 	if idx < 0 || idx >= s.len {
-		return error('string index out of range')
+		return none
 	}
 	unsafe {
 		return s.str[idx]
diff --git a/vlib/builtin/utf8.v b/vlib/builtin/utf8.v
index 847feb6768..91d38c57b4 100644
--- a/vlib/builtin/utf8.v
+++ b/vlib/builtin/utf8.v
@@ -98,7 +98,7 @@ pub fn (_rune string) utf32_code() int {
 
 // convert array of utf8 bytes to single utf32 value
 // will error if more than 4 bytes are submitted
-pub fn (_bytes []u8) utf8_to_utf32() ?rune {
+pub fn (_bytes []u8) utf8_to_utf32() !rune {
 	if _bytes.len == 0 {
 		return 0
 	}
diff --git a/vlib/crypto/bcrypt/bcrypt.v b/vlib/crypto/bcrypt/bcrypt.v
index b396552dcf..5d74de215c 100644
--- a/vlib/crypto/bcrypt/bcrypt.v
+++ b/vlib/crypto/bcrypt/bcrypt.v
@@ -43,14 +43,14 @@ const magic_cipher_data = [u8(0x4f), 0x72, 0x70, 0x68, 0x65, 0x61, 0x6e, 0x42, 0
 	0x6c, 0x64, 0x65, 0x72, 0x53, 0x63, 0x72, 0x79, 0x44, 0x6f, 0x75, 0x62, 0x74]
 
 // generate_from_password return a bcrypt string from Hashed struct.
-pub fn generate_from_password(password []u8, cost int) ?string {
+pub fn generate_from_password(password []u8, cost int) !string {
 	mut p := new_from_password(password, cost) or { return error('Error: ${err}') }
 	x := p.hash_u8()
 	return x.bytestr()
 }
 
 // compare_hash_and_password compares a bcrypt hashed password with its possible hashed version.
-pub fn compare_hash_and_password(password []u8, hashed_password []u8) ? {
+pub fn compare_hash_and_password(password []u8, hashed_password []u8) ! {
 	mut p := new_from_hash(hashed_password) or { return error('Error: ${err}') }
 	p.salt << `=`
 	p.salt << `=`
@@ -76,7 +76,7 @@ pub fn generate_salt() string {
 }
 
 // new_from_password converting from password to a Hashed struct with bcrypt.
-fn new_from_password(password []u8, cost int) ?&Hashed {
+fn new_from_password(password []u8, cost int) !&Hashed {
 	mut cost_ := cost
 	if cost < bcrypt.min_cost {
 		cost_ = bcrypt.default_cost
@@ -98,7 +98,7 @@ fn new_from_password(password []u8, cost int) ?&Hashed {
 }
 
 // new_from_hash converting from hashed data to a Hashed struct.
-fn new_from_hash(hashed_secret []u8) ?&Hashed {
+fn new_from_hash(hashed_secret []u8) !&Hashed {
 	mut tmp := hashed_secret.clone()
 	if tmp.len < bcrypt.min_hash_size {
 		return error('hash to short')
@@ -118,7 +118,7 @@ fn new_from_hash(hashed_secret []u8) ?&Hashed {
 }
 
 // bcrypt hashing passwords.
-fn bcrypt(password []u8, cost int, salt []u8) ?[]u8 {
+fn bcrypt(password []u8, cost int, salt []u8) ![]u8 {
 	mut cipher_data := []u8{len: 72 - bcrypt.magic_cipher_data.len, init: 0}
 	cipher_data << bcrypt.magic_cipher_data
 
@@ -135,7 +135,7 @@ fn bcrypt(password []u8, cost int, salt []u8) ?[]u8 {
 }
 
 // expensive_blowfish_setup generate a Blowfish cipher, given key, cost and salt.
-fn expensive_blowfish_setup(key []u8, cost u32, salt []u8) ?&blowfish.Blowfish {
+fn expensive_blowfish_setup(key []u8, cost u32, salt []u8) !&blowfish.Blowfish {
 	csalt := base64.decode(salt.bytestr())
 	// Bug compatibility with C bcrypt implementations, which use the trailing NULL in the key string during expansion.
 	// See https://cs.opensource.google/go/x/crypto/+/master:bcrypt/bcrypt.go;l=226
@@ -180,7 +180,7 @@ fn (mut h Hashed) hash_u8() []u8 {
 }
 
 // decode_version decode bcrypt version.
-fn (mut h Hashed) decode_version(sbytes []u8) ?int {
+fn (mut h Hashed) decode_version(sbytes []u8) !int {
 	if sbytes[0] != `$` {
 		return error("bcrypt hashes must start with '$'")
 	}
@@ -197,7 +197,7 @@ fn (mut h Hashed) decode_version(sbytes []u8) ?int {
 }
 
 // decode_cost extracts the value of cost and returns the next index in the array.
-fn (mut h Hashed) decode_cost(sbytes []u8) ?int {
+fn (mut h Hashed) decode_cost(sbytes []u8) !int {
 	cost := sbytes[0..2].bytestr().int()
 	check_cost(cost) or { return err }
 	h.cost = cost
@@ -205,7 +205,7 @@ fn (mut h Hashed) decode_cost(sbytes []u8) ?int {
 }
 
 // check_cost check for reasonable quantities.
-fn check_cost(cost int) ? {
+fn check_cost(cost int) ! {
 	if cost < bcrypt.min_cost || cost > bcrypt.max_cost {
 		return error('invalid cost')
 	}
diff --git a/vlib/crypto/blowfish/blowfish.v b/vlib/crypto/blowfish/blowfish.v
index af1ea99849..c1e7650055 100644
--- a/vlib/crypto/blowfish/blowfish.v
+++ b/vlib/crypto/blowfish/blowfish.v
@@ -8,7 +8,7 @@ pub mut:
 
 // new_cipher creates and returns a new Blowfish cipher.
 // The key argument should be the Blowfish key, from 1 to 56 bytes.
-pub fn new_cipher(key []u8) ?Blowfish {
+pub fn new_cipher(key []u8) !Blowfish {
 	mut bf := Blowfish{}
 	unsafe { vmemcpy(&bf.p[0], &p[0], int(sizeof(bf.p))) }
 	unsafe { vmemcpy(&bf.s[0], &s[0], int(sizeof(bf.s))) }
@@ -21,7 +21,7 @@ pub fn new_cipher(key []u8) ?Blowfish {
 }
 
 // new_salted_cipher returns a new Blowfish cipher that folds a salt into its key schedule.
-pub fn new_salted_cipher(key []u8, salt []u8) ?Blowfish {
+pub fn new_salted_cipher(key []u8, salt []u8) !Blowfish {
 	if salt.len == 0 {
 		return new_cipher(key)
 	}
diff --git a/vlib/io/util/util.v b/vlib/io/util/util.v
index f8252cf6bb..d6ae7a40e0 100644
--- a/vlib/io/util/util.v
+++ b/vlib/io/util/util.v
@@ -76,7 +76,7 @@ fn random_number() string {
 	return s.substr(1, s.len)
 }
 
-fn prefix_and_suffix(pattern string) ?(string, string) {
+fn prefix_and_suffix(pattern string) !(string, string) {
 	mut pat := pattern
 	if pat.contains(os.path_separator) {
 		return error('pattern cannot contain path separators (${os.path_separator}).')
diff --git a/vlib/os/filepath.v b/vlib/os/filepath.v
index d617354486..06c2308e6b 100644
--- a/vlib/os/filepath.v
+++ b/vlib/os/filepath.v
@@ -131,7 +131,7 @@ pub fn norm_path(path string) string {
 
 // existing_path returns the existing part of the given `path`.
 // An error is returned if there is no existing part of the given `path`.
-pub fn existing_path(path string) ?string {
+pub fn existing_path(path string) !string {
 	err := error('path does not exist')
 	if path.len == 0 {
 		return err
diff --git a/vlib/picoev/picoev.v b/vlib/picoev/picoev.v
index 993904f65e..bfb63b3921 100644
--- a/vlib/picoev/picoev.v
+++ b/vlib/picoev/picoev.v
@@ -77,7 +77,7 @@ mut:
 }
 
 [inline]
-fn setup_sock(fd int) ? {
+fn setup_sock(fd int) ! {
 	flag := 1
 	if C.setsockopt(fd, C.IPPROTO_TCP, C.TCP_NODELAY, &flag, sizeof(int)) < 0 {
 		return error('setup_sock.setup_sock failed')
diff --git a/vlib/regex/regex_opt.v b/vlib/regex/regex_opt.v
index 3e366f517e..e686a0a595 100644
--- a/vlib/regex/regex_opt.v
+++ b/vlib/regex/regex_opt.v
@@ -3,7 +3,7 @@ module regex
 import strings
 
 // compile_opt compile RE pattern string
-pub fn (mut re RE) compile_opt(pattern string) ? {
+pub fn (mut re RE) compile_opt(pattern string) ! {
 	re_err, err_pos := re.impl_compile(pattern)
 
 	if re_err != compile_ok {
@@ -34,7 +34,7 @@ pub fn new() RE {
 }
 
 // regex_opt create new RE object from RE pattern string
-pub fn regex_opt(pattern string) ?RE {
+pub fn regex_opt(pattern string) !RE {
 	// init regex
 	mut re := RE{}
 	re.prog = []Token{len: pattern.len + 1} // max program length, can not be longer then the pattern
@@ -47,7 +47,7 @@ pub fn regex_opt(pattern string) ?RE {
 	re.group_data = []int{len: re.group_max, init: -1}
 
 	// compile the pattern
-	re.compile_opt(pattern)?
+	re.compile_opt(pattern)!
 
 	return re
 }
diff --git a/vlib/semver/parse.v b/vlib/semver/parse.v
index 0b2c6b1527..3db05fd636 100644
--- a/vlib/semver/parse.v
+++ b/vlib/semver/parse.v
@@ -53,7 +53,7 @@ fn (ver RawVersion) is_missing(typ int) bool {
 	return typ >= ver.raw_ints.len - 1
 }
 
-fn (raw_ver RawVersion) coerce() ?Version {
+fn (raw_ver RawVersion) coerce() !Version {
 	ver := raw_ver.complete()
 	if !is_valid_number(ver.raw_ints[semver.ver_major]) {
 		return error('Invalid major version: ${ver.raw_ints}[ver_major]')
diff --git a/vlib/semver/range.v b/vlib/semver/range.v
index 2d2c398976..71e328ea1a 100644
--- a/vlib/semver/range.v
+++ b/vlib/semver/range.v
@@ -69,7 +69,7 @@ fn (c Comparator) satisfies(ver Version) bool {
 	return false
 }
 
-fn parse_range(input string) ?Range {
+fn parse_range(input string) !Range {
 	raw_comparator_sets := input.split(semver.comparator_set_sep)
 	mut comparator_sets := []ComparatorSet{}
 	for raw_comp_set in raw_comparator_sets {
@@ -84,7 +84,7 @@ fn parse_range(input string) ?Range {
 	return Range{comparator_sets}
 }
 
-fn parse_comparator_set(input string) ?ComparatorSet {
+fn parse_comparator_set(input string) !ComparatorSet {
 	raw_comparators := input.split(semver.comparator_sep)
 	if raw_comparators.len > 2 {
 		return &InvalidComparatorFormatError{
diff --git a/vlib/semver/semver.v b/vlib/semver/semver.v
index b7bdba123b..aa121ad6cd 100644
--- a/vlib/semver/semver.v
+++ b/vlib/semver/semver.v
@@ -38,7 +38,7 @@ pub fn (err InvalidVersionFormatError) msg() string {
 
 // * Constructor.
 // from returns a `Version` structure parsed from `input` `string`.
-pub fn from(input string) ?Version {
+pub fn from(input string) !Version {
 	if input.len == 0 {
 		return &EmptyInputError{}
 	}
@@ -114,7 +114,7 @@ import semver
 v := semver.coerce('1.3-RC1-b2') or { semver.Version{} }
 assert v.satisfies('>1.0 <2.0') == true // 1.3.0
 */
-pub fn coerce(input string) ?Version {
+pub fn coerce(input string) !Version {
 	return coerce_version(input)
 }
 
diff --git a/vlib/semver/util.v b/vlib/semver/util.v
index ebaa172b82..8cab5ef7dc 100644
--- a/vlib/semver/util.v
+++ b/vlib/semver/util.v
@@ -8,7 +8,7 @@ fn is_version_valid(input string) bool {
 }
 
 [inline]
-fn coerce_version(input string) ?Version {
+fn coerce_version(input string) !Version {
 	raw_ver := parse(input)
 	ver := raw_ver.coerce() or { return error('Invalid version for input "${input}"') }
 	return ver
diff --git a/vlib/time/time.v b/vlib/time/time.v
index cba513f97c..329dd59f30 100644
--- a/vlib/time/time.v
+++ b/vlib/time/time.v
@@ -285,7 +285,7 @@ pub fn is_leap_year(year int) bool {
 }
 
 // days_in_month returns a number of days in a given month.
-pub fn days_in_month(month int, year int) ?int {
+pub fn days_in_month(month int, year int) !int {
 	if month > 12 || month < 1 {
 		return error('Invalid month: ${month}')
 	}
diff --git a/vlib/v/builder/builder.v b/vlib/v/builder/builder.v
index d20b6bee47..a3dc5c4efc 100644
--- a/vlib/v/builder/builder.v
+++ b/vlib/v/builder/builder.v
@@ -368,7 +368,7 @@ pub fn module_path(mod string) string {
 
 // TODO: try to merge this & util.module functions to create a
 // reliable multi use function. see comments in util/module.v
-pub fn (b &Builder) find_module_path(mod string, fpath string) ?string {
+pub fn (b &Builder) find_module_path(mod string, fpath string) !string {
 	// support @VROOT/v.mod relative paths:
 	mut mcache := vmod.get_cache()
 	vmod_file_location := mcache.get_by_file(fpath)
diff --git a/vlib/v/checker/check_types.v b/vlib/v/checker/check_types.v
index 0138804100..a555f30ffd 100644
--- a/vlib/v/checker/check_types.v
+++ b/vlib/v/checker/check_types.v
@@ -194,7 +194,7 @@ fn (c Checker) check_multiple_ptr_match(got ast.Type, expected ast.Type, param a
 	return true
 }
 
-fn (mut c Checker) check_expected_call_arg(got ast.Type, expected_ ast.Type, language ast.Language, arg ast.CallArg) ? {
+fn (mut c Checker) check_expected_call_arg(got ast.Type, expected_ ast.Type, language ast.Language, arg ast.CallArg) ! {
 	if got == 0 {
 		return error('unexpected 0 type')
 	}
diff --git a/vlib/v/checker/return.v b/vlib/v/checker/return.v
index 9d17312fb4..bf4c2d16aa 100644
--- a/vlib/v/checker/return.v
+++ b/vlib/v/checker/return.v
@@ -96,6 +96,12 @@ fn (mut c Checker) return_stmt(mut node ast.Return) {
 	option_type_idx := c.table.type_idxs['_option']
 	result_type_idx := c.table.type_idxs['_result']
 	got_types_0_idx := got_types[0].idx()
+	if exp_is_option && got_types_0_idx == ast.error_type_idx {
+		c.warn('Option and Result types have been split, use `!Foo` to return errors',
+			node.pos)
+	} else if exp_is_result && got_types_0_idx == ast.none_type_idx {
+		c.warn('Option and Result types have been split, use `?Foo` to return none', node.pos)
+	}
 	if (exp_is_option
 		&& got_types_0_idx in [ast.none_type_idx, ast.error_type_idx, option_type_idx])
 		|| (exp_is_result && got_types_0_idx in [ast.error_type_idx, result_type_idx]) {
diff --git a/vlib/v/checker/tests/error_fn_with_0_args.out b/vlib/v/checker/tests/error_fn_with_0_args.out
index c4aceee5d7..df6b2375d8 100644
--- a/vlib/v/checker/tests/error_fn_with_0_args.out
+++ b/vlib/v/checker/tests/error_fn_with_0_args.out
@@ -1,5 +1,5 @@
 vlib/v/checker/tests/error_fn_with_0_args.vv:2:9: error: expected 1 arguments, but got 0
-    1 | fn abc() ? {
+    1 | fn abc() ! {
     2 |     return error()
       |            ~~~~~~~
     3 | }
diff --git a/vlib/v/checker/tests/error_fn_with_0_args.vv b/vlib/v/checker/tests/error_fn_with_0_args.vv
index ebbfe4715b..e5a8a82e54 100644
--- a/vlib/v/checker/tests/error_fn_with_0_args.vv
+++ b/vlib/v/checker/tests/error_fn_with_0_args.vv
@@ -1,3 +1,3 @@
-fn abc() ? {
+fn abc() ! {
 	return error()
 }
diff --git a/vlib/v/checker/tests/expression_should_return_an_option.vv b/vlib/v/checker/tests/expression_should_return_an_option.vv
index 05ad45a39e..334a2be55b 100644
--- a/vlib/v/checker/tests/expression_should_return_an_option.vv
+++ b/vlib/v/checker/tests/expression_should_return_an_option.vv
@@ -1,4 +1,4 @@
-fn return_option(fail bool) ?string {
+fn return_option(fail bool) !string {
 	if fail {
 		return error('nope')
 	}
diff --git a/vlib/v/checker/tests/invalid_parameter_name_err.out b/vlib/v/checker/tests/invalid_parameter_name_err.out
index b8940b18fc..db6ec6fa05 100644
--- a/vlib/v/checker/tests/invalid_parameter_name_err.out
+++ b/vlib/v/checker/tests/invalid_parameter_name_err.out
@@ -1,14 +1,14 @@
 vlib/v/checker/tests/invalid_parameter_name_err.vv:4:24: error: invalid use of reserved type `char` as a parameter name
     2 | }
     3 |
-    4 | fn (mut b Buffer) put8(char u8) ? {
+    4 | fn (mut b Buffer) put8(char u8) ! {
       |                        ~~~~
     5 |     return error('-')
     6 | }
 vlib/v/checker/tests/invalid_parameter_name_err.vv:10:6: error: invalid use of reserved type `char` as a parameter name
     8 | interface Writer {
     9 | mut:
-   10 |     put(char u8)?
+   10 |     put(char u8) !
       |         ~~~~
    11 | }
    12 |
diff --git a/vlib/v/checker/tests/invalid_parameter_name_err.vv b/vlib/v/checker/tests/invalid_parameter_name_err.vv
index 39054879a1..a74b41c601 100644
--- a/vlib/v/checker/tests/invalid_parameter_name_err.vv
+++ b/vlib/v/checker/tests/invalid_parameter_name_err.vv
@@ -1,13 +1,13 @@
 struct Buffer {
 }
 
-fn (mut b Buffer) put8(char u8) ? {
+fn (mut b Buffer) put8(char u8) ! {
 	return error('-')
 }
 
 interface Writer {
 mut:
-	put(char u8)?
+	put(char u8) !
 }
 
 fn main() {
diff --git a/vlib/v/checker/tests/option_fn_return_error.out b/vlib/v/checker/tests/option_fn_return_error.out
new file mode 100644
index 0000000000..180b0ae3f7
--- /dev/null
+++ b/vlib/v/checker/tests/option_fn_return_error.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/option_fn_return_error.vv:2:2: warning: Option and Result types have been split, use `!Foo` to return errors
+    1 | fn func() ?int {
+    2 |     return error('Some error')
+      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~
+    3 | }
+    4 |
diff --git a/vlib/v/checker/tests/option_fn_return_error.vv b/vlib/v/checker/tests/option_fn_return_error.vv
new file mode 100644
index 0000000000..a7be95cd02
--- /dev/null
+++ b/vlib/v/checker/tests/option_fn_return_error.vv
@@ -0,0 +1,11 @@
+fn func() ?int {
+	return error('Some error')
+}
+
+fn main() {
+	mut a := ?int(5)
+	a = 1
+	a = none
+	a = func()
+	println(a)
+}
diff --git a/vlib/v/checker/tests/option_or_block_returns_value_of_incompatible_type.vv b/vlib/v/checker/tests/option_or_block_returns_value_of_incompatible_type.vv
index ec75da47f1..11cf1f0e27 100644
--- a/vlib/v/checker/tests/option_or_block_returns_value_of_incompatible_type.vv
+++ b/vlib/v/checker/tests/option_or_block_returns_value_of_incompatible_type.vv
@@ -1,4 +1,4 @@
-fn test_option(fail bool) ?string {
+fn test_option(fail bool) !string {
 	if fail {
 		return error('false')
 	}
diff --git a/vlib/v/checker/tests/option_propagate_nested.out b/vlib/v/checker/tests/option_propagate_nested.out
index 3174c43a10..ecd4a02f72 100644
--- a/vlib/v/checker/tests/option_propagate_nested.out
+++ b/vlib/v/checker/tests/option_propagate_nested.out
@@ -12,17 +12,17 @@ Details: vlib/v/checker/tests/option_propagate_nested.vv:9:14: details: prepend
       |              ~~~~~~
    10 |     s := ret(raise()?)
    11 |     return s
-vlib/v/checker/tests/option_propagate_nested.vv:28:21: error: to propagate the option call, `aa_propagate` must return an option
+vlib/v/checker/tests/option_propagate_nested.vv:28:21: error: to propagate the result call, `aa_propagate` must return a result
    26 |
    27 | fn (mut s St) aa_propagate() {
-   28 |     f := retf(s.raise()?)
+   28 |     f := retf(s.raise()!)
       |                        ^
    29 |     s.z = 7.5
    30 |     println(f)
-Details: vlib/v/checker/tests/option_propagate_nested.vv:27:30: details: prepend ? before the declaration of the return type of `aa_propagate`
+Details: vlib/v/checker/tests/option_propagate_nested.vv:27:30: details: prepend ! before the declaration of the return type of `aa_propagate`
    25 | }
    26 |
    27 | fn (mut s St) aa_propagate() {
       |                              ^
-   28 |     f := retf(s.raise()?)
-   29 |     s.z = 7.5
\ No newline at end of file
+   28 |     f := retf(s.raise()!)
+   29 |     s.z = 7.5
diff --git a/vlib/v/checker/tests/option_propagate_nested.vv b/vlib/v/checker/tests/option_propagate_nested.vv
index 07c299b55a..b320d4d4ea 100644
--- a/vlib/v/checker/tests/option_propagate_nested.vv
+++ b/vlib/v/checker/tests/option_propagate_nested.vv
@@ -16,7 +16,7 @@ mut:
 	z f64
 }
 
-fn (mut s St) raise() ?f64 {
+fn (mut s St) raise() !f64 {
 	return error('some error')
 }
 
@@ -25,7 +25,7 @@ fn retf(f f64) f64 {
 }
 
 fn (mut s St) aa_propagate() {
-	f := retf(s.raise()?)
+	f := retf(s.raise()!)
 	s.z = 7.5
 	println(f)
 }
diff --git a/vlib/v/checker/tests/run/noreturn_fn_can_be_used_instead_of_panic.vv b/vlib/v/checker/tests/run/noreturn_fn_can_be_used_instead_of_panic.vv
index 10cf22c37d..901c3c97f0 100644
--- a/vlib/v/checker/tests/run/noreturn_fn_can_be_used_instead_of_panic.vv
+++ b/vlib/v/checker/tests/run/noreturn_fn_can_be_used_instead_of_panic.vv
@@ -1,4 +1,4 @@
-fn abc() ?int {
+fn abc() !int {
 	return error('oh no')
 }
 
diff --git a/vlib/v/checker/tests/run/noreturn_method_can_be_used_instead_of_panic.vv b/vlib/v/checker/tests/run/noreturn_method_can_be_used_instead_of_panic.vv
index 9b6b7bcab6..10cdcb1fad 100644
--- a/vlib/v/checker/tests/run/noreturn_method_can_be_used_instead_of_panic.vv
+++ b/vlib/v/checker/tests/run/noreturn_method_can_be_used_instead_of_panic.vv
@@ -7,7 +7,7 @@ fn (mut t Test) zz_exit() {
 	exit(0)
 }
 
-fn option() ?int {
+fn option() !int {
 	return error('oh no')
 }
 
diff --git a/vlib/v/checker/tests/store_string_err.vv b/vlib/v/checker/tests/store_string_err.vv
index 7f8b3ff29a..22dabdded6 100644
--- a/vlib/v/checker/tests/store_string_err.vv
+++ b/vlib/v/checker/tests/store_string_err.vv
@@ -1,4 +1,4 @@
-fn return_err() ?int {
+fn return_err() !int {
 	return error('')
 }
 
diff --git a/vlib/v/doc/doc.v b/vlib/v/doc/doc.v
index a5bc7835b2..33b691cd2b 100644
--- a/vlib/v/doc/doc.v
+++ b/vlib/v/doc/doc.v
@@ -49,7 +49,7 @@ pub enum Platform {
 }
 
 // copy of pref.os_from_string
-pub fn platform_from_string(platform_str string) ?Platform {
+pub fn platform_from_string(platform_str string) !Platform {
 	match platform_str {
 		'all', 'cross' { return .cross }
 		'linux' { return .linux }
@@ -177,7 +177,7 @@ pub fn new(input_path string) Doc {
 // stmt reads the data of an `ast.Stmt` node and returns a `DocNode`.
 // An option error is thrown if the symbol is not exposed to the public
 // (when `pub_only` is enabled) or the content's of the AST node is empty.
-pub fn (mut d Doc) stmt(stmt ast.Stmt, filename string) ?DocNode {
+pub fn (mut d Doc) stmt(stmt ast.Stmt, filename string) !DocNode {
 	mut name := d.stmt_name(stmt)
 	if name in d.common_symbols {
 		return error('already documented')
@@ -421,7 +421,7 @@ pub fn (mut d Doc) file_ast_with_pos(file_ast ast.File, pos int) map[string]DocN
 
 // generate is a `Doc` method that will start documentation
 // process based on a file path provided.
-pub fn (mut d Doc) generate() ? {
+pub fn (mut d Doc) generate() ! {
 	// get all files
 	d.base_path = if os.is_dir(d.base_path) {
 		d.base_path
@@ -451,7 +451,7 @@ pub fn (mut d Doc) generate() ? {
 
 // file_asts has the same function as the `file_ast` function but
 // accepts an array of `ast.File` and throws an error if necessary.
-pub fn (mut d Doc) file_asts(file_asts []ast.File) ? {
+pub fn (mut d Doc) file_asts(file_asts []ast.File) ! {
 	mut fname_has_set := false
 	d.orig_mod_name = file_asts[0].mod.name
 	for i, file_ast in file_asts {
@@ -506,7 +506,7 @@ pub fn (mut d Doc) file_asts(file_asts []ast.File) ? {
 
 // generate documents a certain file directory and returns an
 // instance of `Doc` if it is successful. Otherwise, it will  throw an error.
-pub fn generate(input_path string, pub_only bool, with_comments bool, platform Platform, filter_symbol_names ...string) ?Doc {
+pub fn generate(input_path string, pub_only bool, with_comments bool, platform Platform, filter_symbol_names ...string) !Doc {
 	if platform == .js {
 		return error('vdoc: Platform `${platform}` is not supported.')
 	}
@@ -519,19 +519,19 @@ pub fn generate(input_path string, pub_only bool, with_comments bool, platform P
 	} else {
 		unsafe { pref.OS(int(platform)) }
 	}
-	doc.generate()?
+	doc.generate()!
 	return doc
 }
 
 // generate_with_pos has the same function as the `generate` function but
 // accepts an offset-based position and enables the comments by default.
-pub fn generate_with_pos(input_path string, filename string, pos int) ?Doc {
+pub fn generate_with_pos(input_path string, filename string, pos int) !Doc {
 	mut doc := new(input_path)
 	doc.pub_only = false
 	doc.with_comments = true
 	doc.with_pos = true
 	doc.filename = filename
 	doc.pos = pos
-	doc.generate()?
+	doc.generate()!
 	return doc
 }
diff --git a/vlib/v/doc/module.v b/vlib/v/doc/module.v
index f6e4151698..72176a0d9e 100644
--- a/vlib/v/doc/module.v
+++ b/vlib/v/doc/module.v
@@ -13,7 +13,7 @@ import v.pref
 // Note: calling this is expensive, so keep the result, instead of recomputing it.
 // TODO: turn this to a Doc method, so that the new_vdoc_preferences call here can
 // be removed.
-fn get_parent_mod(input_dir string) ?string {
+fn get_parent_mod(input_dir string) !string {
 	// windows root path is C: or D:
 	if input_dir.len == 2 && input_dir[1] == `:` {
 		return error('root folder reached')
@@ -56,7 +56,7 @@ fn get_parent_mod(input_dir string) ?string {
 
 // lookup_module_with_path looks up the path of a given module name.
 // Throws an error if the module was not found.
-pub fn lookup_module_with_path(mod string, base_path string) ?string {
+pub fn lookup_module_with_path(mod string, base_path string) !string {
 	vexe := pref.vexe_path()
 	vroot := os.dir(vexe)
 	mod_path := mod.replace('.', os.path_separator)
@@ -79,12 +79,12 @@ pub fn lookup_module_with_path(mod string, base_path string) ?string {
 
 // lookup_module returns the result of the `lookup_module_with_path`
 // but with the current directory as the provided base lookup path.
-pub fn lookup_module(mod string) ?string {
+pub fn lookup_module(mod string) !string {
 	return lookup_module_with_path(mod, os.dir('.'))
 }
 
 // generate_from_mod generates a documentation from a specific module.
-pub fn generate_from_mod(module_name string, pub_only bool, with_comments bool) ?Doc {
-	mod_path := lookup_module(module_name)?
+pub fn generate_from_mod(module_name string, pub_only bool, with_comments bool) !Doc {
+	mod_path := lookup_module(module_name)!
 	return generate(mod_path, pub_only, with_comments, .auto)
 }
diff --git a/vlib/v/doc/node.v b/vlib/v/doc/node.v
index 068f2f1f30..954360fb3d 100644
--- a/vlib/v/doc/node.v
+++ b/vlib/v/doc/node.v
@@ -1,6 +1,6 @@
 module doc
 
-pub fn (nodes []DocNode) find(symname string) ?DocNode {
+pub fn (nodes []DocNode) find(symname string) !DocNode {
 	for node in nodes {
 		if node.name != symname {
 			continue
diff --git a/vlib/v/gen/c/comptime.v b/vlib/v/gen/c/comptime.v
index c5e12c4236..8f462df1bd 100644
--- a/vlib/v/gen/c/comptime.v
+++ b/vlib/v/gen/c/comptime.v
@@ -835,7 +835,7 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 	g.writeln('}// \$for')
 }
 
-fn (mut g Gen) comptime_if_to_ifdef(name string, is_comptime_option bool) ?string {
+fn (mut g Gen) comptime_if_to_ifdef(name string, is_comptime_option bool) !string {
 	match name {
 		// platforms/os-es:
 		'windows' {
@@ -991,5 +991,5 @@ fn (mut g Gen) comptime_if_to_ifdef(name string, is_comptime_option bool) ?strin
 			return error('bad os ifdef name "${name}"') // should never happen, caught in the checker
 		}
 	}
-	return none
+	return error('none')
 }
diff --git a/vlib/v/gen/c/index.v b/vlib/v/gen/c/index.v
index 09d4bd1b23..99a7f78165 100644
--- a/vlib/v/gen/c/index.v
+++ b/vlib/v/gen/c/index.v
@@ -72,7 +72,7 @@ fn (mut g Gen) index_range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 				tmp_opt = g.new_tmp_var()
 				cur_line = g.go_before_stmt(0)
 				g.out.write_string(util.tabs(g.indent))
-				opt_elem_type := g.typ(ast.string_type.set_flag(.option))
+				opt_elem_type := g.typ(ast.string_type.set_flag(.result))
 				g.write('${opt_elem_type} ${tmp_opt} = string_substr_with_check(')
 			} else {
 				g.write('string_substr(')
@@ -165,7 +165,7 @@ fn (mut g Gen) index_range_expr(node ast.IndexExpr, range ast.RangeExpr) {
 
 	if gen_or {
 		if !node.is_option {
-			g.or_block(tmp_opt, node.or_expr, ast.string_type)
+			g.or_block(tmp_opt, node.or_expr, ast.string_type.set_flag(.result))
 		}
 
 		g.write('\n${cur_line}*(string*)&${tmp_opt}.data')
diff --git a/vlib/v/pkgconfig/main.v b/vlib/v/pkgconfig/main.v
index 267a2e7f48..5c3f5a253d 100644
--- a/vlib/v/pkgconfig/main.v
+++ b/vlib/v/pkgconfig/main.v
@@ -34,7 +34,7 @@ struct MainOptions {
 	args              []string
 }
 
-fn desc(mod string) ?string {
+fn desc(mod string) !string {
 	options := Options{
 		only_description: true
 	}
@@ -42,7 +42,7 @@ fn desc(mod string) ?string {
 	return pc.description
 }
 
-pub fn main(args []string) ?&Main {
+pub fn main(args []string) !&Main {
 	mut fp := flag.new_flag_parser(args)
 	fp.application('pkgconfig')
 	fp.version(version)
@@ -72,7 +72,7 @@ pub fn main(args []string) ?&Main {
 	return m
 }
 
-pub fn (mut m Main) run() ?string {
+pub fn (mut m Main) run() !string {
 	options := Options{
 		debug: m.opt.debug
 	}
@@ -94,7 +94,7 @@ pub fn (mut m Main) run() ?string {
 			res += pcdep.description
 		}
 		if unsafe { pc != 0 } {
-			pc.extend(pcdep)?
+			pc.extend(pcdep)!
 		} else {
 			pc = pcdep
 		}
diff --git a/vlib/v/pkgconfig/pkgconfig.v b/vlib/v/pkgconfig/pkgconfig.v
index 090b5fae77..7c89b8374e 100644
--- a/vlib/v/pkgconfig/pkgconfig.v
+++ b/vlib/v/pkgconfig/pkgconfig.v
@@ -140,7 +140,7 @@ fn (mut pc PkgConfig) parse(file string) bool {
 	return true
 }
 
-fn (mut pc PkgConfig) resolve(pkgname string) ?string {
+fn (mut pc PkgConfig) resolve(pkgname string) !string {
 	if pkgname.ends_with('.pc') {
 		if os.exists(pkgname) {
 			return pkgname
@@ -171,7 +171,7 @@ pub fn (mut pc PkgConfig) atleast(v string) bool {
 	return v0.gt(v1)
 }
 
-pub fn (mut pc PkgConfig) extend(pcdep &PkgConfig) ?string {
+pub fn (mut pc PkgConfig) extend(pcdep &PkgConfig) !string {
 	for flag in pcdep.cflags {
 		if pc.cflags.index(flag) == -1 {
 			pc.cflags << flag
@@ -187,19 +187,19 @@ pub fn (mut pc PkgConfig) extend(pcdep &PkgConfig) ?string {
 			pc.libs_private << lib
 		}
 	}
-	return none
+	return error('')
 }
 
-fn (mut pc PkgConfig) load_requires() ? {
+fn (mut pc PkgConfig) load_requires() ! {
 	for dep in pc.requires {
-		pc.load_require(dep)?
+		pc.load_require(dep)!
 	}
 	for dep in pc.requires_private {
-		pc.load_require(dep)?
+		pc.load_require(dep)!
 	}
 }
 
-fn (mut pc PkgConfig) load_require(dep string) ? {
+fn (mut pc PkgConfig) load_require(dep string) ! {
 	if dep in pc.loaded {
 		return
 	}
@@ -218,9 +218,9 @@ fn (mut pc PkgConfig) load_require(dep string) ? {
 		return error('required file "${depfile}" could not be parsed')
 	}
 	if !pc.options.norecurse {
-		pcdep.load_requires()?
+		pcdep.load_requires()!
 	}
-	pc.extend(pcdep)?
+	pc.extend(pcdep) or {}
 }
 
 fn (mut pc PkgConfig) add_path(path string) {
@@ -267,7 +267,7 @@ fn (mut pc PkgConfig) load_paths() {
 	}
 }
 
-pub fn load(pkgname string, options Options) ?&PkgConfig {
+pub fn load(pkgname string, options Options) !&PkgConfig {
 	mut pc := &PkgConfig{
 		modname: pkgname
 		options: options
@@ -278,7 +278,7 @@ pub fn load(pkgname string, options Options) ?&PkgConfig {
 		return error('file "${file}" could not be parsed')
 	}
 	if !options.norecurse {
-		pc.load_requires()?
+		pc.load_requires()!
 	}
 	return pc
 }
diff --git a/vlib/v/tests/string_index_or_test.v b/vlib/v/tests/string_index_or_test.v
index ed87bd9626..e445e105c9 100644
--- a/vlib/v/tests/string_index_or_test.v
+++ b/vlib/v/tests/string_index_or_test.v
@@ -48,5 +48,5 @@ fn test_propagation() {
 	x := get_propagate(s, 2) or { '${err}' }
 	y := get_propagate(s, 5) or { '${err}' }
 	assert x == 'got `c`'
-	assert y == 'string index out of range'
+	assert y == 'none'
 }