cgen: impliment gen_str_for_fn_type() (#7353)

vlib/v/checker/check_types.v

@@ -331,6 +331,9 @@ pub fn (c &Checker) get_default_fmt(ftyp table.Type, typ table.Type) byte {
 				return `s`
 			}
 		}
+		if sym.kind == .function {
+			return `s`
+		}
 		if ftyp in [table.string_type, table.bool_type] ||
 			sym.kind in
 			[.enum_, .array, .array_fixed, .struct_, .map, .multi_return, .sum_type, .none_] || ftyp.has_flag(.optional) ||

vlib/v/gen/auto_str_methods.v

@@ -40,6 +40,9 @@ fn (mut g Gen) gen_str_for_type(typ table.Type) string {
 			table.Enum {
 				g.gen_str_for_enum(sym.info, styp, str_fn_name)
 			}
+			table.FnType {
+				g.gen_str_for_fn_type(sym.info, styp, str_fn_name)
+			}
 			table.Struct {
 				g.gen_str_for_struct(sym.info, styp, str_fn_name)
 			}
@@ -526,6 +529,26 @@ fn (mut g Gen) gen_str_for_union_sum_type(info table.SumType, styp string, str_f
 	g.auto_str_funcs.writeln('}')
 }
 
+fn (mut g Gen) fn_decl_str(info table.FnType) string {
+	mut fn_str := 'fn ('
+	for i, arg in info.func.params {
+		if i > 0 {
+			fn_str += ', '
+		}
+		fn_str += util.strip_main_name(g.table.get_type_name(arg.typ))
+	}
+	fn_str += ')'
+	if info.func.return_type != table.void_type {
+		fn_str += ' ${util.strip_main_name(g.table.get_type_name(info.func.return_type))}'
+	}
+	return fn_str
+}
+
+fn (mut g Gen) gen_str_for_fn_type(info table.FnType, styp string, str_fn_name string) {
+	g.type_definitions.writeln('static string ${str_fn_name}(); // auto')
+	g.auto_str_funcs.writeln('static string ${str_fn_name}() { return _SLIT("${g.fn_decl_str(info)}");}')
+}
+
 [inline]
 fn styp_to_str_fn_name(styp string) string {
 	return styp.replace_each(['*', '', '.', '__']) + '_str'

vlib/v/gen/cgen.v

@@ -2741,19 +2741,7 @@ fn (mut g Gen) typeof_expr(node ast.TypeOf) {
 		g.write('_SLIT("[$fixed_info.size]${util.strip_main_name(typ_name)}")')
 	} else if sym.kind == .function {
 		info := sym.info as table.FnType
-		fn_info := info.func
-		mut repr := 'fn ('
-		for i, arg in fn_info.params {
-			if i > 0 {
-				repr += ', '
-			}
-			repr += util.strip_main_name(g.table.get_type_name(arg.typ))
-		}
-		repr += ')'
-		if fn_info.return_type != table.void_type {
-			repr += ' ${util.strip_main_name(g.table.get_type_name(fn_info.return_type))}'
-		}
-		g.write('_SLIT("$repr")')
+		g.write('_SLIT("${g.fn_decl_str(info)}")')
 	} else if node.expr_type.has_flag(.variadic) {
 		g.write('_SLIT("...${util.strip_main_name(sym.name)}")')
 	} else {

vlib/v/tests/string_interpolation_function_test.v

@@ -0,0 +1,12 @@
+fn show(a string) string {
+	return a
+}
+
+fn test_function_interpolation() {
+	f := fn()(string, bool) {return 'aaa', true}
+	println(f)
+	assert '$f' == 'fn () (string, bool)'
+
+	println(show)
+	assert '$show' == 'fn (string) string'
+}