diff --git a/cmd/tools/check_os_api_parity.v b/cmd/tools/check_os_api_parity.v
index 4d3054cfb0..cb5f32c30c 100644
--- a/cmd/tools/check_os_api_parity.v
+++ b/cmd/tools/check_os_api_parity.v
@@ -105,7 +105,7 @@ fn (app App) gen_api_for_module_in_os(mod_name string, os_name string) string {
 		for s in f.stmts {
 			if s is ast.FnDecl {
 				if s.is_pub {
-					fn_signature := s.stringify(b.table, mod_name)
+					fn_signature := s.stringify(b.table, mod_name, map[string]string)
 					fn_mod := s.modname()
 					if fn_mod == mod_name {
 						fline := '$fn_mod: $fn_signature'
diff --git a/vlib/v/ast/str.v b/vlib/v/ast/str.v
index 7855aa457b..9f6a022f59 100644
--- a/vlib/v/ast/str.v
+++ b/vlib/v/ast/str.v
@@ -19,7 +19,7 @@ pub fn (node &FnDecl) modname() string {
 }
 
 // These methods are used only by vfmt, vdoc, and for debugging.
-pub fn (node &FnDecl) stringify(t &table.Table, cur_mod string) string {
+pub fn (node &FnDecl) stringify(t &table.Table, cur_mod string, m2a map[string]string) string {
 	mut f := strings.new_builder(30)
 	if node.is_pub {
 		f.write('pub ')
@@ -84,6 +84,9 @@ pub fn (node &FnDecl) stringify(t &table.Table, cur_mod string) string {
 			}
 		}
 		s = util.no_cur_mod(s, cur_mod)
+		for mod, alias in m2a {
+			s = s.replace(mod, alias)
+		}
 		if should_add_type {
 			if !is_type_only {
 				f.write(' ')
@@ -99,9 +102,11 @@ pub fn (node &FnDecl) stringify(t &table.Table, cur_mod string) string {
 	}
 	f.write(')')
 	if node.return_type != table.void_type {
-		// typ := t.type_to_str(node.typ)
-		// if typ.starts_with('
-		f.write(' ' + util.no_cur_mod(t.type_to_str(node.return_type), cur_mod))
+		mut rs := util.no_cur_mod(t.type_to_str(node.return_type), cur_mod)
+		for mod, alias in m2a {
+			rs = rs.replace(mod, alias)
+		}
+		f.write(' ' + rs)
 	}
 	return f.str()
 }
diff --git a/vlib/v/builder/builder.v b/vlib/v/builder/builder.v
index 40ae8ee253..3332eb7e85 100644
--- a/vlib/v/builder/builder.v
+++ b/vlib/v/builder/builder.v
@@ -331,7 +331,7 @@ fn (b &Builder) print_warnings_and_errors() {
 				for stmt in file.stmts {
 					if stmt is ast.FnDecl {
 						if stmt.name == fn_name {
-							fheader := stmt.stringify(b.table, 'main')
+							fheader := stmt.stringify(b.table, 'main', map[string]string{})
 							redefines << FunctionRedefinition{
 								fpath: file.path
 								fline: stmt.pos.line_nr
diff --git a/vlib/v/doc/utils.v b/vlib/v/doc/utils.v
index 38d322157a..4c4acc5b97 100644
--- a/vlib/v/doc/utils.v
+++ b/vlib/v/doc/utils.v
@@ -85,7 +85,7 @@ pub fn (mut d Doc) stmt_signature(stmt ast.Stmt) string {
 			return 'module $stmt.name'
 		}
 		ast.FnDecl {
-			return stmt.stringify(d.table, d.fmt.cur_mod)
+			return stmt.stringify(d.table, d.fmt.cur_mod, d.fmt.mod2alias)
 		}
 		else {
 			d.fmt.out = strings.new_builder(1000)
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index 7e36e4e132..abe779df5a 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -705,7 +705,7 @@ pub fn (mut f Fmt) interface_decl(node ast.InterfaceDecl) {
 	f.comments_after_last_field(node.pre_comments)
 	for method in node.methods {
 		f.write('\t')
-		f.write(method.stringify(f.table, f.cur_mod).after('fn '))
+		f.write(method.stringify(f.table, f.cur_mod, f.mod2alias).after('fn '))
 		f.comments(method.comments, inline: true, has_nl: false, level: .indent)
 		f.writeln('')
 	}
@@ -1251,7 +1251,7 @@ pub fn (mut f Fmt) fn_decl(node ast.FnDecl) {
 	// println('$it.name find_comment($it.pos.line_nr)')
 	// f.find_comment(it.pos.line_nr)
 	f.attrs(node.attrs)
-	f.write(node.stringify(f.table, f.cur_mod)) // `Expr` instead of `ast.Expr` in mod ast
+	f.write(node.stringify(f.table, f.cur_mod, f.mod2alias)) // `Expr` instead of `ast.Expr` in mod ast
 	if node.language == .v {
 		if !node.no_body {
 			f.writeln(' {')
diff --git a/vlib/v/fmt/tests/fn_js_object_keep.vv b/vlib/v/fmt/tests/fn_js_object_keep.vv
deleted file mode 100644
index 3a2f172499..0000000000
--- a/vlib/v/fmt/tests/fn_js_object_keep.vv
+++ /dev/null
@@ -1,7 +0,0 @@
-fn (v JS.String) toString() JS.String
-
-fn JS.Math.abs(f64) f64
-
-fn main() {
-	JS.Math.abs(0)
-}
diff --git a/vlib/v/fmt/tests/language_prefixes_keep.vv b/vlib/v/fmt/tests/language_prefixes_keep.vv
new file mode 100644
index 0000000000..a0a25c5994
--- /dev/null
+++ b/vlib/v/fmt/tests/language_prefixes_keep.vv
@@ -0,0 +1,13 @@
+fn (v JS.String) toString() JS.String
+
+fn (v JS.String) toMultiRet() (JS.String, int)
+
+fn JS.Math.abs(f64) f64
+
+fn main() {
+	JS.Math.abs(0)
+}
+
+fn object_ref_optional() ?&C.File {
+	return error('')
+}
diff --git a/vlib/v/fmt/tests/module_alias_keep.vv b/vlib/v/fmt/tests/module_alias_keep.vv
new file mode 100644
index 0000000000..6057d39ad5
--- /dev/null
+++ b/vlib/v/fmt/tests/module_alias_keep.vv
@@ -0,0 +1,17 @@
+import time
+import semver as sv
+
+fn foo(f time.Time) time.Time {
+	f2 := time.Time{}
+	return f
+}
+
+fn bar(b sv.Version) sv.Version {
+	b2 := sv.Version{}
+	return b
+}
+
+fn bar_multi_return(b sv.Version) (sv.Version, int) {
+	b2 := sv.Version{}
+	return b, 0
+}