diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index fb87869741..771ba4fb75 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -1490,34 +1490,6 @@ pub fn (s string) fields() []string {
 	return s.replace('\t', ' ').split(' ')
 }
 
-pub fn (s string) map(func fn(byte) byte) string {
-	unsafe {
-		mut res := malloc(s.len + 1)
-		for i in 0..s.len {
-			res[i] = func(s[i])
-		}
-		return tos(res, s.len)
-	}
-}
-
-pub fn (s string) filter(func fn(b byte) bool) string {
-	mut new_len := 0
-	mut buf := malloc(s.len + 1)
-	for i in 0 .. s.len {
-		mut b := s[i]
-		if func(b) {
-			unsafe {
-				buf[new_len] = b
-			}
-			new_len++
-		}
-	}
-	unsafe {
-		buf[new_len] = 0
-		return buf.vstring_with_len(new_len)
-	}
-}
-
 // Allows multi-line strings to be formatted in a way that removes white-space
 // before a delimeter. by default `|` is used.
 // Note: the delimiter has to be a byte at this time. That means surrounding
diff --git a/vlib/builtin/string_test.v b/vlib/builtin/string_test.v
index d5f3497911..324f7384eb 100644
--- a/vlib/builtin/string_test.v
+++ b/vlib/builtin/string_test.v
@@ -816,41 +816,10 @@ fn test_double_quote_inter() {
 	assert '${a} ${b}' == "1 2"
 }
 
-fn test_string_map() {
-	$if windows {
-		// TODO
-		return
-	}
-	original := 'Hello'
-	println('original.len = $original.len')
-	a := original.map(fn (b byte) byte {
-		return b + 1
-	})
-	expected := 'Ifmmp'
-	println('a[0] = ' + a[0].str())
-	println('a[1] = ' + a[1].str())
-	println('a[2] = ' + a[2].str())
-	println('a[3] = ' + a[3].str())
-	println('a[4] = ' + a[4].str())
-	println('a.len = $a.len')
-	assert a.len == expected.len
-	assert a == expected
-
-	assert 'foo'.map(foo) == r'\ee'
-}
-
 fn foo(b byte) byte {
 	return b - 10
 }
 
-fn test_string_filter() {
-	foo := 'V is awesome!!!!'.filter(fn (b byte) bool {
-		return b != `!`
-	})
-	assert foo == 'V is awesome'
-	assert 'Alexander'.filter(filter) == 'Alexnder'
-}
-
 fn filter(b byte) bool {
 	return b != `a`
 }
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index f23fffd59b..648f6a53d3 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -1050,15 +1050,15 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_e
 	match arg_expr {
 		ast.AnonFn {
 			if arg_expr.decl.params.len > 1 {
-				c.error('function needs exactly 1 argument', call_expr.pos)
+				c.error('function needs exactly 1 argument', arg_expr.decl.pos)
 			} else if is_map &&
 				(arg_expr.decl.return_type != elem_typ || arg_expr.decl.params[0].typ != elem_typ) {
 				c.error('type mismatch, should use `fn(a $elem_sym.source_name) $elem_sym.source_name {...}`',
-					call_expr.pos)
+					arg_expr.decl.pos)
 			} else if !is_map &&
 				(arg_expr.decl.return_type != table.bool_type || arg_expr.decl.params[0].typ != elem_typ) {
 				c.error('type mismatch, should use `fn(a $elem_sym.source_name) bool {...}`',
-					call_expr.pos)
+					arg_expr.decl.pos)
 			}
 		}
 		ast.Ident {
@@ -1071,11 +1071,11 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_e
 					c.error('function needs exactly 1 argument', call_expr.pos)
 				} else if is_map && (func.return_type != elem_typ || func.params[0].typ != elem_typ) {
 					c.error('type mismatch, should use `fn(a $elem_sym.source_name) $elem_sym.source_name {...}`',
-						call_expr.pos)
+						arg_expr.pos)
 				} else if !is_map &&
 					(func.return_type != table.bool_type || func.params[0].typ != elem_typ) {
 					c.error('type mismatch, should use `fn(a $elem_sym.source_name) bool {...}`',
-						call_expr.pos)
+						arg_expr.pos)
 				}
 			}
 		}
@@ -1110,13 +1110,15 @@ pub fn (mut c Checker) call_method(mut call_expr ast.CallExpr) table.Type {
 		is_sort := method_name == 'sort'
 		if is_filter_map || is_sort {
 			array_info := left_type_sym.info as table.Array
-			mut scope := c.file.scope.innermost(call_expr.pos.pos)
+			args_pos := call_expr.pos.pos + call_expr.name.len
+			mut scope := c.file.scope.innermost(args_pos)
 			if is_filter_map {
-				scope.update_var_type('it', array_info.elem_type)
+				// position of `it` doesn't matter
+				scope_register_it(mut scope, call_expr.pos, array_info.elem_type)
 			} else if is_sort {
 				c.fail_if_immutable(call_expr.left)
-				scope.update_var_type('a', array_info.elem_type)
-				scope.update_var_type('b', array_info.elem_type)
+				// position of `a` and `b` doesn't matter, they're the same
+				scope_register_ab(mut scope, call_expr.pos, array_info.elem_type)
 				// Verify `.sort(a < b)`
 				if call_expr.args.len > 0 {
 					if call_expr.args[0].expr !is ast.InfixExpr {
@@ -2251,11 +2253,28 @@ pub fn (mut c Checker) assign_stmt(mut assign_stmt ast.AssignStmt) {
 	}
 }
 
-fn (mut c Checker) open_scope(mut parent ast.Scope, start_pos int) &ast.Scope {
-	mut s := ast.new_scope(parent, start_pos)
-	s.end_pos = parent.end_pos
-	parent.children << s
-	return s
+fn scope_register_it(mut s ast.Scope, pos token.Position, typ table.Type) {
+	s.register('it', ast.Var{
+		name: 'it'
+		pos: pos
+		typ: typ
+		is_used: true
+	})
+}
+
+fn scope_register_ab(mut s ast.Scope, pos token.Position, typ table.Type) {
+	s.register('a', ast.Var{
+		name: 'a'
+		pos: pos
+		typ: typ
+		is_used: true
+	})
+	s.register('b', ast.Var{
+		name: 'b'
+		pos: pos
+		typ: typ
+		is_used: true
+	})
 }
 
 fn (mut c Checker) check_array_init_para_type(para string, expr ast.Expr, pos token.Position) {
diff --git a/vlib/v/checker/tests/array_filter_anon_fn_err_a.out b/vlib/v/checker/tests/array_filter_anon_fn_err_a.out
index d2d90953f4..a8c77d9d0c 100644
--- a/vlib/v/checker/tests/array_filter_anon_fn_err_a.out
+++ b/vlib/v/checker/tests/array_filter_anon_fn_err_a.out
@@ -1,6 +1,6 @@
-vlib/v/checker/tests/array_filter_anon_fn_err_a.vv:2:23: error: function needs exactly 1 argument
+vlib/v/checker/tests/array_filter_anon_fn_err_a.vv:2:24: error: function needs exactly 1 argument
     1 | fn main() {
     2 |     a := [1,2,3,4].filter(fn(a int, b int) bool { return a > 0 })
-      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+      |                           ~~
     3 |     println(a)
-    4 | }
+    4 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_filter_anon_fn_err_b.out b/vlib/v/checker/tests/array_filter_anon_fn_err_b.out
index fd574ab084..b08034a71d 100644
--- a/vlib/v/checker/tests/array_filter_anon_fn_err_b.out
+++ b/vlib/v/checker/tests/array_filter_anon_fn_err_b.out
@@ -1,6 +1,6 @@
-vlib/v/checker/tests/array_filter_anon_fn_err_b.vv:2:23: error: type mismatch, should use `fn(a int) bool {...}`
+vlib/v/checker/tests/array_filter_anon_fn_err_b.vv:2:24: error: type mismatch, should use `fn(a int) bool {...}`
     1 | fn main() {
     2 |     a := [1,2,3,4].filter(fn(a string) bool { return a.len > 0 })
-      |                          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+      |                           ~~
     3 |     println(a)
-    4 | }
+    4 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_filter_fn_err_a.out b/vlib/v/checker/tests/array_filter_fn_err_a.out
index 1e82166d3a..257203f80b 100644
--- a/vlib/v/checker/tests/array_filter_fn_err_a.out
+++ b/vlib/v/checker/tests/array_filter_fn_err_a.out
@@ -1,7 +1,7 @@
-vlib/v/checker/tests/array_filter_fn_err_a.vv:5:23: error: function needs exactly 1 argument
+vlib/v/checker/tests/array_filter_fn_err_a.vv:5:17: error: function needs exactly 1 argument
     3 | }
     4 | fn main() {
     5 |     a := [1,2,3,4].filter(fil)
-      |                          ~~~~~
+      |                    ~~~~~~~~~~~
     6 |     println(a)
-    7 | }
+    7 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_filter_fn_err_b.out b/vlib/v/checker/tests/array_filter_fn_err_b.out
index ecc2480bc1..9e5a8917a9 100644
--- a/vlib/v/checker/tests/array_filter_fn_err_b.out
+++ b/vlib/v/checker/tests/array_filter_fn_err_b.out
@@ -1,7 +1,7 @@
-vlib/v/checker/tests/array_filter_fn_err_b.vv:5:23: error: type mismatch, should use `fn(a int) bool {...}`
+vlib/v/checker/tests/array_filter_fn_err_b.vv:5:24: error: type mismatch, should use `fn(a int) bool {...}`
     3 | }
     4 | fn main() {
     5 |     a := [1,2,3,4].filter(fil)
-      |                          ~~~~~
+      |                           ~~~
     6 |     println(a)
-    7 | }
+    7 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_map_anon_fn_err_a.out b/vlib/v/checker/tests/array_map_anon_fn_err_a.out
index 0243eccb46..156a7d711b 100644
--- a/vlib/v/checker/tests/array_map_anon_fn_err_a.out
+++ b/vlib/v/checker/tests/array_map_anon_fn_err_a.out
@@ -1,6 +1,6 @@
-vlib/v/checker/tests/array_map_anon_fn_err_a.vv:2:20: error: function needs exactly 1 argument
+vlib/v/checker/tests/array_map_anon_fn_err_a.vv:2:21: error: function needs exactly 1 argument
     1 | fn main() {
     2 |     a := [1,2,3,4].map(fn(a int, b int) int {return a + b})
-      |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+      |                        ~~
     3 |     println(a)
-    4 | }
+    4 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_map_anon_fn_err_b.out b/vlib/v/checker/tests/array_map_anon_fn_err_b.out
index 41fd5292fa..409d866d1a 100644
--- a/vlib/v/checker/tests/array_map_anon_fn_err_b.out
+++ b/vlib/v/checker/tests/array_map_anon_fn_err_b.out
@@ -1,6 +1,6 @@
-vlib/v/checker/tests/array_map_anon_fn_err_b.vv:2:20: error: type mismatch, should use `fn(a int) int {...}`
+vlib/v/checker/tests/array_map_anon_fn_err_b.vv:2:21: error: type mismatch, should use `fn(a int) int {...}`
     1 | fn main() {
     2 |     a := [1,2,3,4].map(fn(a string) string { return a })
-      |                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+      |                        ~~
     3 |     println(a)
-    4 | }
+    4 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_map_fn_err_a.out b/vlib/v/checker/tests/array_map_fn_err_a.out
index 808aff808c..1b3269caf1 100644
--- a/vlib/v/checker/tests/array_map_fn_err_a.out
+++ b/vlib/v/checker/tests/array_map_fn_err_a.out
@@ -1,7 +1,7 @@
-vlib/v/checker/tests/array_map_fn_err_a.vv:5:20: error: function needs exactly 1 argument
+vlib/v/checker/tests/array_map_fn_err_a.vv:5:17: error: function needs exactly 1 argument
     3 | }
     4 | fn main() {
     5 |     a := [1,2,3,4].map(add)
-      |                       ~~~~~
+      |                    ~~~~~~~~
     6 |     println(a)
-    7 | }
+    7 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/array_map_fn_err_b.out b/vlib/v/checker/tests/array_map_fn_err_b.out
index 00a8bdbb77..bbdbcbad0c 100644
--- a/vlib/v/checker/tests/array_map_fn_err_b.out
+++ b/vlib/v/checker/tests/array_map_fn_err_b.out
@@ -1,7 +1,7 @@
-vlib/v/checker/tests/array_map_fn_err_b.vv:5:20: error: type mismatch, should use `fn(a int) int {...}`
+vlib/v/checker/tests/array_map_fn_err_b.vv:5:21: error: type mismatch, should use `fn(a int) int {...}`
     3 | }
     4 | fn main() {
     5 |     a := [1,2,3,4].map(add)
-      |                       ~~~~~
+      |                        ~~~
     6 |     println(a)
-    7 | }
+    7 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/filter_on_non_arr_err.out b/vlib/v/checker/tests/filter_on_non_arr_err.out
new file mode 100644
index 0000000000..2f0deabdd4
--- /dev/null
+++ b/vlib/v/checker/tests/filter_on_non_arr_err.out
@@ -0,0 +1,6 @@
+vlib/v/checker/tests/filter_on_non_arr_err.vv:2:14: error: unknown method: `string.filter`.
+Did you mean `after`?
+    1 | fn main() {
+    2 |     _ := 'test'.filter(it == `t`)
+      |                 ~~~~~~~~~~~~~~~~~
+    3 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/filter_on_non_arr_err.vv b/vlib/v/checker/tests/filter_on_non_arr_err.vv
new file mode 100644
index 0000000000..ee1bb7cca2
--- /dev/null
+++ b/vlib/v/checker/tests/filter_on_non_arr_err.vv
@@ -0,0 +1,3 @@
+fn main() {
+	_ := 'test'.filter(it == `t`)
+}
\ No newline at end of file
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 1b7c051f83..522c1e3b71 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -1276,21 +1276,13 @@ fn (mut p Parser) dot_expr(left ast.Expr) ast.Expr {
 	if p.tok.kind == .dollar {
 		return p.comptime_method_call(left)
 	}
-	mut name_pos := p.tok.position()
+	name_pos := p.tok.position()
 	field_name := p.check_name()
 	is_filter := field_name in ['filter', 'map']
 	if is_filter {
 		p.open_scope()
-		name_pos = p.tok.position()
-		p.scope_register_it()
-		// wrong tok position when using defer
-		// defer {
-		// p.close_scope()
-		// }
 	} else if field_name == 'sort' {
 		p.open_scope()
-		name_pos = p.tok.position()
-		p.scope_register_ab()
 	}
 	// ! in mutable methods
 	if p.tok.kind == .not && p.peek_tok.kind == .lpar {