diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 0d645761ce..e7cd4eaf00 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -159,6 +159,7 @@ pub:
 	name        string
 	field_names []string
 	methods     []FnDecl
+	pos token.Position
 }
 
 pub struct StructInitField {
diff --git a/vlib/v/ast/str.v b/vlib/v/ast/str.v
index 635c9ad0cb..76bbb64ad8 100644
--- a/vlib/v/ast/str.v
+++ b/vlib/v/ast/str.v
@@ -40,9 +40,13 @@ pub fn (node &FnDecl) str(t &table.Table) string {
 	f.write('fn ${receiver}${name}(')
 	for i, arg in node.args {
 		// skip receiver
+		// if (node.is_method || node.is_interface) && i == 0 {
 		if node.is_method && i == 0 {
 			continue
 		}
+		if arg.is_hidden {
+			continue
+		}
 		is_last_arg := i == node.args.len - 1
 		should_add_type := is_last_arg || node.args[i + 1].typ != arg.typ || (node.is_variadic &&
 			i == node.args.len - 2)
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 93f2c54799..596b46fffa 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -90,7 +90,8 @@ pub fn (mut c Checker) check_files(ast_files []ast.File) {
 		return
 	}
 	if !has_main_mod_file {
-		c.error('projet must include a `main` module or be a shared library (compile with `v -shared`)', token.Position{})
+		c.error('projet must include a `main` module or be a shared library (compile with `v -shared`)',
+			token.Position{})
 	} else if !has_main_fn {
 		c.error('function `main` must be declared in the main module', token.Position{})
 	}
@@ -886,7 +887,8 @@ fn (mut c Checker) type_implements(typ, inter_typ table.Type, pos token.Position
 	for imethod in inter_sym.methods {
 		if method := typ_sym.find_method(imethod.name) {
 			if !imethod.is_same_method_as(method) {
-				c.error('`$styp` incorrectly implements method `$imethod.name` of interface `$inter_sym.name`, expected `${c.table.fn_to_str(imethod)}`', pos)
+				c.error('`$styp` incorrectly implements method `$imethod.name` of interface `$inter_sym.name`, expected `${c.table.fn_to_str(imethod)}`',
+					pos)
 			}
 			continue
 		}
@@ -1474,6 +1476,16 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 		}
 		// ast.HashStmt {}
 		ast.Import {}
+		ast.InterfaceDecl {
+			if !it.name[0].is_capital() {
+				pos := token.Position{
+					line_nr: it.pos.line_nr
+					pos: it.pos.pos + 'interface'.len
+					len: it.name.len
+				}
+				c.error('interface name must begin with capital letter', pos)
+			}
+		}
 		ast.Module {
 			c.mod = it.name
 			c.is_builtin_mod = it.name == 'builtin'
@@ -1561,8 +1573,8 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 		ast.CastExpr {
 			it.expr_type = c.expr(it.expr)
 			sym := c.table.get_type_symbol(it.expr_type)
-			if it.typ == table.string_type && !(sym.kind in [.byte, .byteptr] ||
-				sym.kind == .array && sym.name == 'array_byte') {
+			if it.typ == table.string_type && !(sym.kind in [.byte, .byteptr] || sym.kind ==
+				.array && sym.name == 'array_byte') {
 				type_name := c.table.type_to_str(it.expr_type)
 				c.error('cannot cast type `$type_name` to string', it.pos)
 			}
diff --git a/vlib/v/parser/struct.v b/vlib/v/parser/struct.v
index a358ef1c8f..62d8ba12f5 100644
--- a/vlib/v/parser/struct.v
+++ b/vlib/v/parser/struct.v
@@ -6,6 +6,7 @@ module parser
 import v.ast
 import v.table
 import v.token
+import v.util
 
 fn (mut p Parser) struct_decl() ast.StructDecl {
 	start_pos := p.tok.position()
@@ -255,6 +256,7 @@ fn (mut p Parser) struct_init(short_syntax bool) ast.StructInit {
 }
 
 fn (mut p Parser) interface_decl() ast.InterfaceDecl {
+	start_pos := p.tok.position()
 	is_pub := p.tok.kind == .key_pub
 	if is_pub {
 		p.next()
@@ -278,11 +280,15 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	for p.tok.kind != .rcbr && p.tok.kind != .eof {
 		line_nr := p.tok.line_nr
 		name := p.check_name()
+		if util.contains_capital(name) {
+			p.error('interface methods cannot contain uppercase letters, use snake_case instead')
+		}
 		// field_names << name
 		args2, _ := p.fn_args()
 		mut args := [table.Arg{
 			name: 'x'
 			typ: typ
+			is_hidden: true
 		}]
 		args << args2
 		mut method := ast.FnDecl{
@@ -306,5 +312,6 @@ fn (mut p Parser) interface_decl() ast.InterfaceDecl {
 	return ast.InterfaceDecl{
 		name: interface_name
 		methods: methods
+		pos: start_pos
 	}
 }
diff --git a/vlib/v/table/table.v b/vlib/v/table/table.v
index 37ba4018a3..4970335921 100644
--- a/vlib/v/table/table.v
+++ b/vlib/v/table/table.v
@@ -33,9 +33,10 @@ pub mut:
 
 pub struct Arg {
 pub:
-	name   string
-	is_mut bool
-	typ    Type
+	name      string
+	is_mut    bool
+	typ       Type
+	is_hidden bool // interface first arg
 }
 
 pub struct Var {
diff --git a/vlib/v/tests/interface_test.v b/vlib/v/tests/interface_test.v
index 2c874971a9..68807e860b 100644
--- a/vlib/v/tests/interface_test.v
+++ b/vlib/v/tests/interface_test.v
@@ -1,4 +1,3 @@
-
 struct Dog {
 	breed string
 }
@@ -46,11 +45,10 @@ fn (d Dog) name_detailed(pet_name string) string {
 }
 
 // do not add to Dog the utility function 'str', as a sample
-
-
 fn test_todo() {
-	if true {}
-	else {}
+	if true {
+	} else {
+	}
 }
 
 fn perform_speak(a Animal) {
@@ -58,24 +56,30 @@ fn perform_speak(a Animal) {
 	assert true
 	name := a.name()
 	assert name == 'Dog' || name == 'Cat'
-	//if a is Dog {
-		//assert name == 'Dog'
-	//}
+	// if a is Dog {
+	// assert name == 'Dog'
+	// }
 	println(a.name())
 }
 
 fn test_perform_speak() {
-	dog := Dog{breed: 'Labrador Retriever'}
+	dog := Dog{
+		breed: 'Labrador Retriever'
+	}
 	perform_speak(dog)
-	cat := Cat{breed: 'Persian'}
+	cat := Cat{
+		breed: 'Persian'
+	}
 	perform_speak(cat)
-	perform_speak(Cat{breed: 'Persian'})
+	perform_speak(Cat{
+		breed: 'Persian'
+	})
 	handle_animals([dog, cat])
 	/*
 	f := Foo {
 		speaker: dog
 	}
-*/
+	*/
 }
 
 fn perform_name_detailed(a Animal) {
@@ -85,22 +89,24 @@ fn perform_name_detailed(a Animal) {
 }
 
 fn test_perform_name_detailed() {
-	dog := Dog{breed: 'Labrador Retriever'}
+	dog := Dog{
+		breed: 'Labrador Retriever'
+	}
 	println('Test on Dog: $dog ...')
 	perform_name_detailed(dog)
-
 	cat := Cat{}
 	println('Test on Cat: $cat ...')
 	perform_speak(cat)
-
 	println('Test on another Cat: ...')
-	perform_speak(Cat{breed: 'Persian'})
-
+	perform_speak(Cat{
+		breed: 'Persian'
+	})
 	println('Test (dummy/empty) on array of animals ...')
 	handle_animals([dog, cat])
 }
 
-fn handle_animals(a []Animal) {}
+fn handle_animals(a []Animal) {
+}
 
 interface Register {
 	register()
@@ -110,9 +116,11 @@ struct RegTest {
 	a int
 }
 
-fn (f RegTest) register() {}
+fn (f RegTest) register() {
+}
 
-fn handle_reg(r Register) {}
+fn handle_reg(r Register) {
+}
 
 fn test_register() {
 	f := RegTest{}
@@ -125,9 +133,8 @@ interface Speaker2 {
 	speak()
 }
 
-
 struct Foo {
-	animal Animal
+	animal  Animal
 	animals []Animal
 }
 
@@ -140,7 +147,9 @@ interface Animal {
 fn test_interface_array() {
 	println('Test on array of animals ...')
 	mut animals := []Animal{}
-	animals = [ Cat{}, Dog{breed: 'Labrador Retriever'} ]
+	animals = [Cat{}, Dog{
+		breed: 'Labrador Retriever'
+	}]
 	animals << Cat{}
 	assert true
 	// TODO .str() from the real types should be called