diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 1b0598ef87..35f229e635 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -245,6 +245,7 @@ pub:
 pub mut:
 	name               string
 	is_method          bool
+	is_mut             bool // !
 	args               []CallArg
 	expected_arg_types []table.Type
 	language           table.Language
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 9529be3789..2be05fe81a 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -799,6 +799,7 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 		}
 		if method.args[0].is_mut {
 			c.fail_if_immutable(call_expr.left)
+			call_expr.is_mut = true
 		}
 		if method.return_type == table.void_type && method.ctdefine.len > 0 && method.ctdefine !in
 			c.pref.compile_defines {
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index afe355ece0..c073989c08 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -470,7 +470,7 @@ pub fn (mut f Fmt) struct_decl(node ast.StructDecl) {
 		}
 		if field.has_default_expr {
 			f.write(' = ')
-			f.struct_field_expr( field.default_expr )
+			f.struct_field_expr(field.default_expr)
 		}
 		// f.write('// $field.pos.line_nr')
 		if field.comment.text != '' && field.comment.pos.line_nr == field.pos.line_nr {
@@ -487,7 +487,6 @@ pub fn (mut f Fmt) struct_decl(node ast.StructDecl) {
 	f.writeln('}\n')
 }
 
-
 pub fn (mut f Fmt) struct_field_expr(fexpr ast.Expr) {
 	mut is_pe_amp_ce := false
 	mut ce := ast.CastExpr{}
@@ -633,7 +632,6 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 					ktyp = minfo.key_type
 					vtyp = minfo.value_type
 				}
-
 				f.write('map[')
 				f.write(f.type_to_str(ktyp))
 				f.write(']')
@@ -912,6 +910,9 @@ pub fn (mut f Fmt) call_expr(node ast.CallExpr) {
 		f.write('.' + node.name + '(')
 		f.call_args(node.args)
 		f.write(')')
+		if node.is_mut {
+			// f.write('!')
+		}
 		f.or_expr(node.or_block)
 	} else {
 		f.write_language_prefix(node.language)
@@ -1033,12 +1034,8 @@ pub fn (mut f Fmt) mark_module_as_used(name string) {
 
 fn (mut f Fmt) write_language_prefix(lang table.Language) {
 	match lang {
-		.c {
-			f.write('C.')
-		}
-		.js {
-			f.write('JS.')
-		}
+		.c { f.write('C.') }
+		.js { f.write('JS.') }
 		else {}
 	}
 }
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 7331b994e0..c31b384d20 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -1000,6 +1000,10 @@ fn (mut p Parser) dot_expr(left ast.Expr) ast.Expr {
 		// p.close_scope()
 		// }
 	}
+	// ! in mutable methods
+	if p.tok.kind == .not && p.peek_tok.kind == .lpar {
+		p.next()
+	}
 	// Method call
 	// TODO move to fn.v call_expr()
 	if p.tok.kind == .lpar {
@@ -1030,14 +1034,12 @@ fn (mut p Parser) dot_expr(left ast.Expr) ast.Expr {
 			or_stmts = p.parse_block_no_scope()
 			p.close_scope()
 		}
-		if p.tok.kind == .not {
-			p.next()
-		}
+		// `foo()?`
 		if p.tok.kind == .question {
-			// `foo()?`
 			p.next()
 			or_kind = .propagate
 		}
+		//
 		end_pos := p.tok.position()
 		pos := token.Position{
 			line_nr: name_pos.line_nr